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'[EE]: Switching low-voltage AC'
2002\02\18@114448 by Dale Botkin

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Hi,

I'm working on a light controller for my yard lights.  My old power supply
is a nice 12VAC transformer with a timer and photocell that lets you
select several modes, including on all night, on for 1-12 hours after
dusk, always off, or always on.  The new lights came with a new xformer
but no fancy stuff, just an on/off mechanical timer.  Unfortunately it has
no indication of its current rating and it's running pretty hot - I'd like
to use the new transformer but don't want to lose all the cool features.
So of course I'm building a nice PIC-controlled add-on!  8-)  I plan to
add the same thing to the deck lights in back of the house once I get it
working.

My question relates to how to switch the 12VAC to the lights on and off.
I'm thinking a sensitive-gate (logic) TRIAC on each leg of the AC line to
the lights, with both gates controlled by the PIC.  From the looks of what
I have seen in various Web resources about TRIACs, it looks like I could
use one instead of two, but it just seems to me that for safety's sake
using two would be like using a DPST switch and completely remove any
potential from the lights when turned off.  Or should I even worry about
that?

Ebmarrassingly, I have never used a TRIAC before, but it looks from the
data sheet like I can turn the AC on and off with nothing more than a
logic HI to the gate(s).  Am I reading that right?  Is there a better way
to do this?  I guess I could use a SSR, but a couple of TRIACs will be a
lot cheaper and a LOT smaller.

Comments?  Suggestions?  Am I making this harder than it needs to be?

Dale
--
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2002\02\18@120210 by Spehro Pefhany

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At 10:42 AM 2/18/02 -0600, you wrote:

Hi, Dale:-

>My question relates to how to switch the 12VAC to the lights on and off.
>I'm thinking a sensitive-gate (logic) TRIAC on each leg of the AC line to
>the lights, with both gates controlled by the PIC.  From the looks of what
>I have seen in various Web resources about TRIACs, it looks like I could
>use one instead of two, but it just seems to me that for safety's sake
>using two would be like using a DPST switch and completely remove any
>potential from the lights when turned off.  Or should I even worry about
>that?

You don't need two, the 12VAC is isolated and two won't help. The voltage
losses will be significant enough from just one. Why don't you use a
relay? No voltage loss to speak of, and at 2 operations per day, a relay
rated at 50,000 operations (tungsten load) should last the rest of your life.

(Do be sure to get one with a suitable 'tungsten' rating, otherwise
there's a risk of contacts welding "on" or short life)

It will also run cool. A 20A triac switch will dissipate ~20W (that won't
go to your lights, but you'll have to get rid of it in a heat sink, even
on a hot day). 20A @12V is only 240W, not a lot of exterior lighting.
If you do insist on using a triac, get a 40A or higher one (de-rate) and
you'll probably have to put a driver in there for the gate, maybe 50mA,
because logic-level triacs are only low current. If you *want* to be
fancy rather than just having your lights go on and off, make yourself
a hybrid SS/mechanical switch, but at those rep rates it's not required.

>Ebmarrassingly, I have never used a TRIAC before, but it looks from the
>data sheet like I can turn the AC on and off with nothing more than a
>logic HI to the gate(s).  Am I reading that right?  Is there a better way
>to do this?  I guess I could use a SSR, but a couple of TRIACs will be a
>lot cheaper and a LOT smaller.

They work better if you drive them with a negative gate current. Some
are not rated or guaranteed to operate properly in Quadrant IV
(positive gate/negative MT)

Best regards,

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2002\02\18@121714 by Alan B. Pearce

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>You don't need two, the 12VAC is isolated and two won't help. The voltage
>losses will be significant enough from just one. Why don't you use a
>relay? No voltage loss to speak of, and at 2 operations per day, a relay
>rated at 50,000 operations (tungsten load) should last the rest of your
life.
>
>(Do be sure to get one with a suitable 'tungsten' rating, otherwise
>there's a risk of contacts welding "on" or short life)

You could always use a triac to soft start the lamp, then use relay contacts
across the triac when close to 100% current. Bit complex perhaps, but would
cut the losses, while being gentler on relay contacts.

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2002\02\18@123350 by Spehro Pefhany

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At 05:14 PM 2/18/02 +0000, you wrote:

>You could always use a triac to soft start the lamp, then use relay contacts
>across the triac when close to 100% current. Bit complex perhaps, but would
>cut the losses, while being gentler on relay contacts.

A fancier version of the hybrid relay! Sure, why not. ;-)

The dimming effect would be pleasant. You could also have a very low
brightness mode (energy/light saving) for late-night.

Best regards,

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2002\02\18@124244 by Michael Rigby-Jones

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{Quote hidden}

A low value resistor with a relay accross if would be a little simpler.

But if we are going complex, how about a bridge rectifier in series with the
lamp, and a FET or bipolar on the output of the bridge rectifier.

Mike

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2002\02\18@132310 by Dale Botkin

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On Mon, 18 Feb 2002, Michael Rigby-Jones wrote:

> But if we are going complex, how about a bridge rectifier in series with the
> lamp, and a FET or bipolar on the output of the bridge rectifier.

Interesting point -- I could then use a MOSFET and PWM to start the lights
soft, then cut 'em back to a soft glow late at night.  Now I realize how
little I know about light bulbs -- these are the little 8W wedge-base
landscape light bulbs intended for 12V operation.  Is there any problem
using DC instead of AC, and will running them on a low brightness with PWM
shorten their life, I wonder?

Dale

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2002\02\18@133319 by Spehro Pefhany

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At 12:21 PM 2/18/02 -0600, you wrote:
>On Mon, 18 Feb 2002, Michael Rigby-Jones wrote:
>
> > But if we are going complex, how about a bridge rectifier in series
> with the
> > lamp, and a FET or bipolar on the output of the bridge rectifier.
>
>Interesting point -- I could then use a MOSFET and PWM to start the lights
>soft, then cut 'em back to a soft glow late at night.

The triac and phase control will be more efficient, and probably cheaper,
and won't look any different.

(Two series diode losses + 1 transistor loss) > one triac loss

>  Now I realize how
>little I know about light bulbs -- these are the little 8W wedge-base
>landscape light bulbs intended for 12V operation.  Is there any problem
>using DC instead of AC, and will running them on a low brightness with PWM
>shorten their life, I wonder?

Wedge base? Ours are automotive style bayonet base (tail-lamps)
Anyway, the DC thing is pretty much a myth. If they are halogen they
should be operated at full power for a while periodically to get the
envelope temperature up. Easy to program with your PIC. ;-)

Best regards,

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2002\02\18@141648 by Dale Botkin

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On Mon, 18 Feb 2002, Spehro Pefhany wrote:

> >Interesting point -- I could then use a MOSFET and PWM to start the lights
> >soft, then cut 'em back to a soft glow late at night.
>
> The triac and phase control will be more efficient, and probably cheaper,
> and won't look any different.
>
> (Two series diode losses + 1 transistor loss) > one triac loss

True... but then I get into zero-crossing detection, etc...  dimming is
not a primary requirement here.  I just wanted to avoid using a relay for
space & cost reasons (mostly space).  If I can get dimming as a side
benefit, so much the better.

{Quote hidden}

I don't think they're halogen, will have to look.  Well, maybe some are.
I have mostly the round path lights and a few spot lights for the bushes,
all 8W bulbs but the spots may be halogen.  At any rate, they'd be run
full power for at least several hours a night.  By "DC thing is pretty
much a myth"  do you mean you think they'll run as well on DC as on AC?
I can't imagine why they wouldn't.

Dale

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2002\02\18@142444 by Jim

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One question, two parts -

Do you have any neighbors who are hams or
listen to LW or MW (known as "the AM Broadcast
band" here in the states) broadcasts?

Any sort of SCR or FET based PWM should sound *swell*
on those lower bands!

I have an SCR wall switch that works well as a
broad-band noise source - from LW though 80
Meters (4 MHz).

Jim


{Original Message removed}

2002\02\18@144355 by Spehro Pefhany

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At 01:14 PM 2/18/02 -0600, you wrote:
>  By "DC thing is pretty
>much a myth"  do you mean you think they'll run as well on DC as on AC?
>I can't imagine why they wouldn't.

DC causes tungsten atoms to migrate in one direction along crystal
defect lines. The result is 'notching' and premature failure. It is more of
a factor in low-current (< 50mA) bulbs.

Here is data from one manufacturer where they claim an average reduction
of 50% in life. But if you put your bridge and MOSFET in there, the lights
will last longer on account of them being dimmer (12th power relationship
with voltage, so a 3.5% change in RMS voltage will have the same effect).

http://www.htl.co.jp/pro/kogata/tokusei_e.html

Best regards,

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2002\02\18@150054 by Dale Botkin

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On Mon, 18 Feb 2002, Jim wrote:

> One question, two parts -
>
> Do you have any neighbors who are hams or
> listen to LW or MW (known as "the AM Broadcast
> band" here in the states) broadcasts?

Just me (n0xas) on HF, and my wife listening to the AM talk stations.

> Any sort of SCR or FET based PWM should sound *swell*
> on those lower bands!

I have thought of that, and will play around with it until the RF is
minimal.  I figure it should be controllable with a relatively "soft"
turn-on/off, most likely achieved with some capacitance in parallel with
the lamps.  I use a fairly sensitive DC receiver with a pretty wide front
end (Oak Hills Sprint) for 40m QRP, so I'm somewhat sensitive to those
issues.  8-)

Dale

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2002\02\18@154732 by Olin Lathrop

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> But if we are going complex, how about a bridge rectifier in series with
the
> lamp, and a FET or bipolar on the output of the bridge rectifier.

That also has the advantage of allowing higher efficiency by allowing higher
voltage.  Instead of 12V, you can use 20-50V DC.  Since the voltage drops
across the diodes will be the same but a smaller fraction of the overall
voltage, efficiency goes up.  Of course you have to trust your code to
always stay below a certain PWM on percentage, else poof!


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, @spam@olinKILLspamspamembedinc.com, http://www.embedinc.com

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2002\02\18@154747 by Olin Lathrop

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> Interesting point -- I could then use a MOSFET and PWM to start the lights
> soft, then cut 'em back to a soft glow late at night.  Now I realize how
> little I know about light bulbs -- these are the little 8W wedge-base
> landscape light bulbs intended for 12V operation.  Is there any problem
> using DC instead of AC,

No.  LEBs (Light Emitting Bulbs <g>) work by resistively heating the
fillament so that at least some small fraction of its black body radition
makes it to the visible region.

> and will running them on a low brightness with PWM
> shorten their life, I wonder?

No, it will extend it, as long as the PWM frequency is fast enough so that
the fillament doesn't significantly cool down between pulses.  A few 100 Hz
is fine.  Incandescent bulbs like to have their voltage changed slowly to
minimize fillament stress.

One thing to watch out for is the startup surge.  Since the fillament
temperature changes drastically from off to on, so does its resistance.
When first turned on, an incandescent bulb can draw 10x or more of its
steady state on current for a short time.  The fillaments hate that, which
is why soft starting extends life, but also why you have to design the
switching circuit carefully.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, KILLspamolinKILLspamspamembedinc.com, http://www.embedinc.com

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2002\02\18@162052 by Andre Abelian

picon face
Dale,


>Hi,

>I'm working on a light controller for my yard lights.  My old power
supply
>is a nice 12VAC transformer with a timer and photocell that lets you
>select several modes, including on all night, on for 1-12 hours after
>dusk, always off, or always on.  The new lights came with a new xformer
>but no fancy stuff, just an on/off mechanical timer.  Unfortunately it
has
>no indication of its current rating and it's running pretty hot - I'd
like
>to use the new transformer but don't want to lose all the cool
features.
>So of course I'm building a nice PIC-controlled add-on!  8-)  I plan to
>add the same thing to the deck lights in back of the house once I get
it
>working.


Do you want to control each lamp separately or all at once ?


>My question relates to how to switch the 12VAC to the lights on and
off.
>I'm thinking a sensitive-gate (logic) TRIAC on each leg of the AC line
to
>the lights, with both gates controlled by the PIC.  From the looks of
what
>I have seen in various Web resources about TRIACs, it looks like I
could
>use one instead of two, but it just seems to me that for safety's sake
>using two would be like using a DPST switch and completely remove any
>potential from the lights when turned off.  Or should I even worry
about
>that?

>Ebmarrassingly, I have never used a TRIAC before, but it looks from the
>data sheet like I can turn the AC on and off with nothing more than a
>logic HI to the gate(s).  Am I reading that right?  Is there a better
way
>to do this?  I guess I could use a SSR, but a couple of TRIACs will be
a
>lot cheaper and a LOT smaller.

>Comments?  Suggestions?  Am I making this harder than it needs to be?

Turning on and off with triac is easy but if you want to dim it
Then you need to detect zero crossing on each phase and call
Delay up to 7.3ms

>Dale

Andre Abelian

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2002\02\18@171216 by Dale Botkin

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On Mon, 18 Feb 2002, Olin Lathrop wrote:

> One thing to watch out for is the startup surge.  Since the fillament
> temperature changes drastically from off to on, so does its resistance.
> When first turned on, an incandescent bulb can draw 10x or more of its
> steady state on current for a short time.  The fillaments hate that, which
> is why soft starting extends life, but also why you have to design the
> switching circuit carefully.

Good to have you back, Olin, and thanks for responding.  So let's assume
I'm going to use a PWM rate of a couple hundred Hz.  Would something
simple, like going from 0/256 to 256/256 PWM duty cycle over a period of a
second or so, be sufficient, or am I missing something?

Dale

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2002\02\18@172432 by Dale Botkin

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On Mon, 18 Feb 2002, Andre Abelian wrote:

> Do you want to control each lamp separately or all at once ?

All at once.  There's low-voltage 2-conductor stranded wire run around the
landscaping, and the lights tap into this for their power, all in parallel
along the run.

> Turning on and off with triac is easy but if you want to dim it
> Then you need to detect zero crossing on each phase and call
> Delay up to 7.3ms

Dimming isn't a reall *requirement*, but of course the more I think about
it the more reasons I can think of to include it!  8-)

Dale

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2002\02\18@180632 by Dave Dilatush

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Dale Botkin wrote...

>My question relates to how to switch the 12VAC to the lights on and off.
>I'm thinking a sensitive-gate (logic) TRIAC...

TRIACs.  Ugh.  Voltage drop.  Lost power.  Unclean.

>Ebmarrassingly, I have never used a TRIAC before...

Be thankful, not embarrassed.

>Is there a better way to do this?

Yup.

No one ever said you can't use MOSFETs on AC; you just have to be a
little clever, is all.  Usually, that means you end up having to use two
of them in series-- one for each half-cycle-- because of the intrinsic
body diode that shunts the drain and source.

The circuit below uses no voltage-dropping, power-wasting TRIAC, no
likewise bridge rectifier, no relay^H^H^H^H^H "electromechanical logic",
and allows you to control your lamps either ON/OFF with a logic level
from a PIC (might want to boost that up to a little higher than 5V,
unless you use logic level FETs), or control lamp brightness with the
PIC's PWM.

          0--------------------------0

       FROM 12V                  TO LAMP
       XFORMER                    STRING

          0------+           +-------0
                 |           |
                 |           |
                 |           |
                 |           |
                 D           D
             Q1    G---+---G    Q2
            Nch  S     |     S  Nch
                 |     |     |
GATE DRV  0------------+     |
                 |           |
                 |           |
PIC GND   0------+-----------+

Voila...

Dave

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2002\02\18@190156 by Spehro Pefhany

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At 11:05 PM 2/18/02 +0000, you wrote:
{Quote hidden}

Very nice, Dave! A couple of IRLZ544's maybe. They would have
a total drop of about 1.1V at 20A, so about as good as a triac.
At 10A, they'd beat it by a mile.

You should be able to get half-wave power using a diode from the
top transformer wire to a PIC-GND referenced supply.

Best regards,

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2002\02\18@194728 by Dave Dilatush

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Spehro Pefhany wrote...

>Very nice, Dave! A couple of IRLZ544's maybe. They would have
>a total drop of about 1.1V at 20A, so about as good as a triac.
>At 10A, they'd beat it by a mile.

I've got some old NSC NDP706's in my junk drawer.  TO-220, rated at 75A,
Rds(on) something like 0.05 ohms.  They'd be great...

>You should be able to get half-wave power using a diode from the
>top transformer wire to a PIC-GND referenced supply.

Dang, why didn't I think of that...

OK, here's Rev. 2.0, with a half-wave rectifier & filter for supplying
power to a 7805 regulator:

           0-----------+--------------0
                       |          FROM 12V       |          TO LAMP
        XFORMER        |           STRING
                       |
           0------+    |      +-------0
                  |    |      |
                  |    |      |    D1
                  |    +----------->|-----+-----0 +17V to
                  |           |  1N4003   |       7805 Reg.
                  D           D           |                Q1    G---+---G    Q2       |+
             Nch  S     |     S  Nch      C1 470 uF
                  |     |     |           |-  35 WV
 GATE DRV  0------------+     |           |
                  |           |           |
                  |           |           |
 PIC GND   0------+-----------+-----------+


Cheers,

Dave

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2002\02\18@194732 by Dale Botkin

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On Mon, 18 Feb 2002, Dave Dilatush wrote:

> Dale Botkin wrote...
>
> >My question relates to how to switch the 12VAC to the lights on and off.
> >I'm thinking a sensitive-gate (logic) TRIAC...
>
> TRIACs.  Ugh.  Voltage drop.  Lost power.  Unclean.

Do I sense you don't like TRIACs?  8-)

{Quote hidden}

Voila indeed.  Thanks, Dave, this is an awesome idea.  I had thought of
anti-parallel MOSFETs, but not anything nearly as elegant.  I'm so used to
using bridge rectifiers on a transformer, I just didn't think of making
one side ground.  As a bonus, it looks like I could use one more pin and
turn on just one MOSFET to drive the lights with half-wave rectified DC.
Hmmm.

I took a quick look at Digikey - 2SK2614 looks like a winner.  50V/20A,
less than .08 Ohm Rds(on), 4V gate drive and a tad over a buck each.  I
can drive the gates with RA4 and a pull-up.

Dale

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2002\02\18@202042 by Dave Dilatush

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Dale Botkin wrote...

>Do I sense you don't like TRIACs?  8-)

Moi???  I love them.  Dearly.  I must, for I can't see any other reason
why I've been keeping a whole drawerful of TRIACs & SCRs around all
these years.  
I have SC141B's, SC146's and TR146's in TO-220; I have SC260M3's, big
stud-mounted bruisers, almost as impressive-looking as vacuum tubes; I
have 2N6394's; I have teeny little C6B's in TO-5 cases...

I must have two hundred of the dang things, and I've NEVER found a use
for them.  Can't seem to ever find a use for the unijunction transistors
or PUTs, either.

But I do love them.

>Voila indeed.  Thanks, Dave, this is an awesome idea.  I had thought of
>anti-parallel MOSFETs, but not anything nearly as elegant.  I'm so used to
>using bridge rectifiers on a transformer, I just didn't think of making
>one side ground.  As a bonus, it looks like I could use one more pin and
>turn on just one MOSFET to drive the lights with half-wave rectified DC.
>Hmmm.

Yes, that should work fine.

>I took a quick look at Digikey - 2SK2614 looks like a winner.  50V/20A,
>less than .08 Ohm Rds(on), 4V gate drive and a tad over a buck each.  I
>can drive the gates with RA4 and a pull-up.

Ummm... Want some TRIACs?

Dave

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2002\02\18@210708 by Olin Lathrop

face picon face
> Good to have you back, Olin, and thanks for responding.  So let's assume
> I'm going to use a PWM rate of a couple hundred Hz.  Would something
> simple, like going from 0/256 to 256/256 PWM duty cycle over a period of a
> second or so, be sufficient, or am I missing something?

A second or so is usually fine.  A bit longer wouldn't hurt unless there is
a reason the light needs to come on quickly.  Just keep in mind that the
earlier pulses are going to draw hefty currents, which is another reason for
keeping them short and starting slowly.

By the way, you could increase efficiency by using a higher voltage and
reducing the maximum PWM duty cycle accordingly.

********************************************************************
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(978) 742-9014, spamBeGoneolinspamBeGonespamembedinc.com, http://www.embedinc.com

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2002\02\18@212603 by Tony Nixon

flavicon
picon face
> > Good to have you back, Olin, and thanks for responding.  So let's assume
> > I'm going to use a PWM rate of a couple hundred Hz.  Would something
> > simple, like going from 0/256 to 256/256 PWM duty cycle over a period of a
> > second or so, be sufficient, or am I missing something?


If you are fiddling with PWM and you are using a 16F87x chip, maybe you
could use the ZapLab interface. Then, while testing, you can quickly
adjust the PIC PWM using slider controls from the PC. The software is
compatable with ROMzap and loads like any bootloader software.

The link is at the bottom of this page.

http://www.bubblesoftonline.com/demo/ROMzap.html


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2002\02\18@233244 by Spehro Pefhany

picon face
At 06:45 PM 2/18/02 -0600, you wrote:

>As a bonus, it looks like I could use one more pin and
>turn on just one MOSFET to drive the lights with half-wave rectified DC.

Your transformer will not be happy with you doing this.

>I took a quick look at Digikey - 2SK2614 looks like a winner.  50V/20A,
>less than .08 Ohm Rds(on), 4V gate drive and a tad over a buck each.  I
>can drive the gates with RA4 and a pull-up.

Ok, the one IRLZ44N I suggested is about $1 in 100's and is logic level, 41A
0.022-0.025-0.035 ohms depending on 10/5/4V drive level. About $1.70 in
singles. The IRF1010N (72A /55V( is 3.24 in singles and is 0.011 ohms
with 10V drive (not logic level).

Total I^2R or forward conduction losses with 4V/10V drive

Current:                                10A             20A     Cost (qty
1/1000 Digikey)
Triac Q2040J 40A/200V (isolated
case)   10W             20W             7.16/3.97
2SK2614 0.08 ohm at 4V/0.045
10V        16W/9W          64W/36W         2.04/1.05
IRLZ44
0.035/0.022                      7W/4.4W         28W/17.6W       3.46/2.02
IRF1010N 0.011 ohms
(10V)               --/2W           --/8.8W         6.48/3.62

With the possible exception of two IRF1010N's with 10V drive running a <10A
load,
heat sinks required for all these options.

Best regards,

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2002\02\19@051133 by Vasile Surducan

flavicon
face
On Mon, 18 Feb 2002, Spehro Pefhany wrote:

{Quote hidden}

 eh, eh you have too rested minds guys !
 This will never work, take a look better ...

 So, people said: relays, triacs, bridge and tranzistor ( the best
solution here ) other "ingenious ideea" which does not take care of
polarisation and the fact that not every time a mos-fet can be used as
a controlled switch.
 What about a transformer magnetic amplifier ? Does any of the youngest
to project such things ?

 Dale, take care about the iminent distruction of the bulbs filament at
high current off ( bulb totaly cold ) - on switch

best regards, Vasile

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2002\02\20@162158 by Dale Botkin

flavicon
face
On Tue, 19 Feb 2002, Vasile Surducan wrote:

{Quote hidden}

Vasile,

Can you tell me why you think it won't work?  It looks to me like each
MOSFET's body diode will conduct "in reverse" during the half-cycle when
Vds < 0, and the opposing MOSFET will conduct if the gate is driven
high...  I do see one thing less than ideal, though, which appears to be a
loss of the Vf of the body diode.  For an IRLZ44 this looks like 2.5V max,
or around 1.7V for the 2SK2614.

Looks like at my 7A or so load this would work out to...  let me see...
1.7V * 7A = 11.9W dissiption, is that right?  But only for half the cycle,
figure roughly 2.7W or so for the other half cycle (P=I^2R where I=7A and
R=.055 typ.) meaning an average of about 7.3W per package for the 2SK2614,
9.4W for the IRLZ44.  That's 14+ to nearly 20W total...  Sure wouldn't
have to worry about condensation in the box!

The relay is looking better now, but I don't see where the dual-MOSFET
idea wouldn't work.  But of course IANAEE...  I may be all wet on part
or all of this, someone please tell me if I am.

Dale

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2002\02\20@172113 by Dwayne Reid

flavicon
face
At 03:20 PM 2/20/02 -0600, Dale Botkin wrote:
{Quote hidden}

Dale - this will work just fine.  I've used it before.

Remember that a conducting MOSFET looks like a resistor, NOT a saturated
bipolar transistor.  As such, it conducts BOTH directions when enhanced
(on).  In other words, the Vf of the body diode is swamped by the ON
resistance of the FET.  Because Dave drives both gates at the same time,
both FETs are turned on.

Remember Bob Pease's cute little reverse battery polarity protect
circuit.  I believe he even got a patent on it!  There are only 2
components: a gate resistor and the MOSFET.  The MOSFET is wired backwards:
source and drain swapped with regard to normal polarity.    As such, the
body diode conducts when the battery is installed correctly.  He then uses
the gate to fully enhance the FET (also only when the battery is installed
correctly) and the Vf of the body diode is completely swamped by the
on-resistance of the FET.  If you were protecting the positive lead of a
battery, you would use a P channel MOSFET, with D tied to battery +, S to
load, G to gnd via a 10K resistor.  It works!

dwayne


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2002\02\20@172950 by Dale Botkin

flavicon
face
On Wed, 20 Feb 2002, Dwayne Reid wrote:

> Remember that a conducting MOSFET looks like a resistor, NOT a saturated
> bipolar transistor.  As such, it conducts BOTH directions when enhanced
> (on).  In other words, the Vf of the body diode is swamped by the ON
> resistance of the FET.  Because Dave drives both gates at the same time,
> both FETs are turned on.

See, I didn't know that.  I *thought* it might be the case, but couldn't
find any confirmation of it anywhere.  I couldn't find any reference in
the data sheets as to whether Rds(on) was the same as Rsd(on)...  but I
suppose the R part should have been a give-away, in hindsight.

Thanks for the correction.  Now that I've learned something I guess I can
go home and call it a day!  8-)

Dale

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2002\02\20@173204 by Olin Lathrop

face picon face
> I do see one thing less than ideal, though, which appears to be a
> loss of the Vf of the body diode.  For an IRLZ44 this looks like 2.5V max,
> or around 1.7V for the 2SK2614.

You could parallel the FET with an external power diode that would have
lower drop and also spread the dissipation to another package.


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(978) 742-9014, EraseMEolinspamspamspamBeGoneembedinc.com, http://www.embedinc.com

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2002\02\20@173626 by Dave Dilatush

picon face
Dale Botkin wrote...

>On Tue, 19 Feb 2002, Vasile Surducan wrote:
>
>>   eh, eh you have too rested minds guys !
>>   This will never work, take a look better ...
>
>Vasile,
>
>Can you tell me why you think it won't work?  It looks to me like each
>MOSFET's body diode will conduct "in reverse" during the half-cycle when
>Vds < 0, and the opposing MOSFET will conduct if the gate is driven
>high...  I do see one thing less than ideal, though, which appears to be a
>loss of the Vf of the body diode.  For an IRLZ44 this looks like 2.5V max,
>or around 1.7V for the 2SK2614.

The circuit DOES work, and the MOSFET body diodes do NOT conduct.  
I have two IRF512's connected to a 12V transformer and a light bulb
running on the workbench right now; the circuit gives full-wave ON/OFF
control (I can turn it on and off just by touching the gates with my
fingertip... neat!), with no current other than leakage flowing when OFF
and no voltage drop when on, other than that due to Rds(on).

When the MOSFET gates are biased positive with respect to their sources,
the MOSFETs conduct regardless of applied drain-source polarity.
They're field-effect devices, remember: the field under the gate
modulates the channel resistance, and neither the gate nor the channel
gives a fat rat's patootie which direction the channel current is
flowing.  Think about it: if it were otherwise, how would MOS analog
switches ever work?

As to the FET whose body diode would have been conducting because of the
"backward" current flow had the FET channel not been turned on, consider
it to be acting as a FAD*.

>Sure wouldn't
>have to worry about condensation in the box!

Well, you'll have to; your MOSFETs aren't going to be much help in
keeping it warm in there for you.  :)

>The relay is looking better now, but I don't see where the dual-MOSFET
>idea wouldn't work.  But of course IANAEE...  I may be all wet on part
>or all of this, someone please tell me if I am.

IAAEE; I do circuits for food.  This one works as advertised.

Cheers,

Dave

* FAD = Field-Assisted Diode  :)

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2002\02\20@182506 by Sean H. Breheny

face picon face
I haven't followed this from the beginning, so perhaps I am missing
something, but you have to remember that you won't be able to turn the
MOSFET off if the drain is more negative (or positive for PMOSFET) than the
body (usually connected to the source). In other words, it is true that it
conducts in both directions when fully on, but it will still conduct in one
direction when off. This is due to the inherent diode (body diode) created
by the PN junction between drain and body (substrate), and the fact that
the source is usually connected to the body.

If you have a MOSFET where the source is not connected to the body (it has
4 terminals instead of 3), then the asymmetry is gone: as long as the body
is kept more negative (or positive for a PMOSFET) than both source and
drain, then source and drain are interchangeable.

AFAIR, MOSFET analog switches work by keeping the body separate from the
source and keeping it more negative than both source and drain (this is
easy, tie body to ground and then specify that source and drain must be
greater than 0 volts with respect to ground).

Sean



At 04:27 PM 2/20/02 -0600, you wrote:
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2002\02\20@184716 by Dwayne Reid

flavicon
face
At 06:22 PM 2/20/02 -0500, Sean H. Breheny wrote:
>I haven't followed this from the beginning, so perhaps I am missing
>something, but you have to remember that you won't be able to turn the
>MOSFET off if the drain is more negative (or positive for PMOSFET) than the
>body (usually connected to the source). In other words, it is true that it
>conducts in both directions when fully on, but it will still conduct in one
>direction when off. This is due to the inherent diode (body diode) created
>by the PN junction between drain and body (substrate), and the fact that
>the source is usually connected to the body.

What you are missing is that there are 2 series-connected MOSFETs wired in
series with the load such that the Source leads are tied together, as are
the gate leads.  The switch terminals are the 2 Drain leads.  1 FET handles
1 half-cycle, the other FET handles the remaining half-cycle.

So, yes, one FET does not turn off on a given half-cycle.  But the other
one does.

dwayne


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2002\02\20@204043 by Sean H. Breheny

face picon face
Hi Dwayne,

Ah, I see, that makes sense now,

Thanks,

Sean

At 04:44 PM 2/20/02 -0700, you wrote:
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2002\02\20@204718 by Dale Botkin

flavicon
face
On Wed, 20 Feb 2002, Dave Dilatush wrote:

> The circuit DOES work, and the MOSFET body diodes do NOT conduct.

Cool.  I understand now.  Neat idea, I think I'll kick this project up a
couple of notches on the list!  Time to order more parts...  8-)

> I have two IRF512's connected to a 12V transformer and a light bulb
> running on the workbench right now; the circuit gives full-wave ON/OFF
> control (I can turn it on and off just by touching the gates with my
> fingertip... neat!), with no current other than leakage flowing when OFF
> and no voltage drop when on, other than that due to Rds(on).

So am I correct in thinking that turning on just one of the gates will run
the lamp at about half intensity, while the other MOSFET gets warm?

> flowing.  Think about it: if it were otherwise, how would MOS analog
> switches ever work?

Beats me, IANAEE, remember?  8-)  But now I understand how they do!

> * FAD = Field-Assisted Diode  :)

I just try to avoid NEDs -- Noise Emitting Diodes (SNAP!!)

Dale

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2002\02\20@213437 by Dave Dilatush

picon face
Dale Botkin wrote...

>On Wed, 20 Feb 2002, Dave Dilatush wrote:
>
>> The circuit DOES work, and the MOSFET body diodes do NOT conduct.
>
>Cool.  I understand now.  Neat idea, I think I'll kick this project up a
>couple of notches on the list!  Time to order more parts...  8-)

The IRLZ44N's that Speff suggested would be good candidates.  The lower
the Rds(on) the better.

>> I have two IRF512's connected to a 12V transformer and a light bulb
>> running on the workbench right now; the circuit gives full-wave ON/OFF
>> control (I can turn it on and off just by touching the gates with my
>> fingertip... neat!), with no current other than leakage flowing when OFF
>> and no voltage drop when on, other than that due to Rds(on).
>
>So am I correct in thinking that turning on just one of the gates will run
>the lamp at about half intensity, while the other MOSFET gets warm?

Yup.  Just tried it.  Though "warm" in the case of the setup I have
running is kinda theoretical, as I'm only driving a 250 mA bulb and the
FET is in a TO-220 package.  But if I put my scope across drain and
source of the "off" MOSFET, I can see the drop across its body diode
when the other MOSFET is conducting.  So it appears to be doing what you
said.

The more I fool around with this circuit, the stranger it gets.  What we
have here is a "Logic Controlled Diode" with two control inputs:

 0 0 = doesn't pass any current
 0 1 = passes current to the right
 1 0 = passes current to the left
 1 1 = passes current both ways

Wonder what applications there might be for such a thing?  Hmmm...

>> * FAD = Field-Assisted Diode  :)
>
>I just try to avoid NEDs -- Noise Emitting Diodes (SNAP!!)

There are also DEDs- which is what you get when you put exactly the
right amount of current into an LED to change the doping levels and
crystal structure so that it emits darkness instead of light.

Technically, at the LED/DED Transition Point the device is said to be
"on its last GaAsP"...

Cheers,

Dave

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2002\02\20@230913 by Dale Botkin

flavicon
face
On Thu, 21 Feb 2002, Dave Dilatush wrote:

> The IRLZ44N's that Speff suggested would be good candidates.  The lower
> the Rds(on) the better.

Yah...  I'm going with the 2SK2614's.  Lower Rds(on) and cheaper, and
still plenty of V and A to handle my particular load.

> The more I fool around with this circuit, the stranger it gets.  What we
> have here is a "Logic Controlled Diode" with two control inputs:
>
>   0 0 = doesn't pass any current
>   0 1 = passes current to the right
>   1 0 = passes current to the left
>   1 1 = passes current both ways
>
> Wonder what applications there might be for such a thing?  Hmmm...

Oh, yeah.  The wheels are turning!  Hmmm indeed.

Dale
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2002\02\21@023127 by Vasile Surducan

flavicon
face
On Wed, 20 Feb 2002, Dwayne Reid wrote:

> Dale - this will work just fine.  I've used it before.
>
> Remember that a conducting MOSFET looks like a resistor, NOT a saturated
> bipolar transistor.  As such, it conducts BOTH directions when enhanced
> (on).  In other words, the Vf of the body diode is swamped by the ON
> resistance of the FET.  Because Dave drives both gates at the same time,
> both FETs are turned on.
>

 Well, what you probably forgot Dwaine, ( even if this circuit is
probably working ) is the ideea that conducting MOSFET looks like a
resistor,ONLY for small Vdrain-source voltage. I have not see any diode on
this drawing from drain to source, so I believed it not works.
 However, two transistors for just driving a bulb is too much !
The problem was already solved more than 10 years ago ( in the same way
like magnetron filament current limiting inside any domestic oven ) by
using a simple high loose magnetic core suply transformer. Than a simple
scr or relay is solving the problem. No pic ( pwm, zero cross detection
and so on) is required for this.
Transforming a standard 110 or 220/12V ac transformer into a high loose
one or computing a new one is an easy task ( not working on toroidal
core... )

best regards, Vasile

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2002\02\21@090757 by Olin Lathrop

face picon face
> In other words, it is true that it
> conducts in both directions when fully on, but it will still conduct in
one
> direction when off. This is due to the inherent diode (body diode) created
> by the PN junction between drain and body (substrate), and the fact that
> the source is usually connected to the body.

True, but Dave's circuit takes this into account.


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2002\02\21@150645 by Dwayne Reid

flavicon
face
At 09:32 AM 2/21/02 +0200, Vasile Surducan wrote:

>   However, two transistors for just driving a bulb is too much !
>The problem was already solved more than 10 years ago ( in the same way
>like magnetron filament current limiting inside any domestic oven ) by
>using a simple high loose magnetic core suply transformer.

You are right.  But consider this: Dale already has a suitable
transformer.  We buy IRCZ24 FETs in thousands - they are less than a dollar
each.  I expect IRZ44 FETs to be not much more.

Bottom line: the FET idea is a simple, inexpensive way for Dale to
electronically control his lights.

dwayne



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2002\02\21@152241 by Dale Botkin

flavicon
face
Dwayne Reid <dwaynerEraseMEspamPLANET.EON.NET> said:

> Bottom line: the FET idea is a simple, inexpensive way for Dale to
> electronically control his lights.

And it uses very little board space, very little power (not that it's a big
issue), can easily handle the current of the string of lamps, has no moving
parts, and will cost me less (from my supplier) than a good quality relay to
do the same job - with better control.  Not to mention the geek factor...  8-)

Dale

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2002\02\21@172328 by Peter L. Peres

picon face
> > >            0--------------------------0
> > >
> > >         FROM 12V                  TO LAMP
> > >         XFORMER                    STRING
> > >
> > >            0------+           +-------0
> > >                   |           |
> > >                   |           |
> > >                   |           |
> > >                   |           |
> > >                   D           D
> > >               Q1    G---+---G    Q2
> > >              Nch  S     |     S  Nch
> > >                   |     |     |
...

That circuit works just fine. Also the body diode is shorted by the FET
and the voltage drop is to be calculated wrt. the Rdson of the FET (FETs
conduct backwards just fine when turned on - not just through the diode).
So the circuit will dissipate 2*I^2*Rdson when on. At 10A and 45E-3R this
is 2E2*45E-3 = 9W and it will loose about 0.9V on the open devices. This
is comparable with a triac however (1.2-1.5V loss). Primary switching with
a SSR that will only be rated 2A at mains looks better than ever (about $8
in 1's last time I checked and will need no heatsink) ...

You could also use 10 VFETs in parallel (5+5) and reduce Rdson to 9E-3R
which will cut dissipation to about 2W (total) so the devices could run
w/o any heatsink (each would dissipate 0.125W). The voltage drop would
become 0.2V. You would need a push-pull driver to turn them on properly
however. I often use a 3-transistor push-pull dc amplifier (class B) set
to gain = 3. This serves as level converter from CMOS logic (PIC) and
drives many (many) FET gates in parallel without problems. The low gain
makes it suitable for PWM and switching up to 100kHz in despite of using
cheap transistors (1A,20MHz finals and a 0.5A,80MHz driver). A small trick
is used to avoid the forbidden state (output never sits at 1/2 Vcc).

Peter

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2002\02\21@233514 by michael brown

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face
Dale Botkin wrote:
> Dwayne Reid <RemoveMEdwaynerEraseMEspamspam_OUTPLANET.EON.NET> said:
>
> > Bottom line: the FET idea is a simple, inexpensive way for Dale to
> > electronically control his lights.
>
> And it uses very little board space, very little power (not that it's a
big
> issue), can easily handle the current of the string of lamps, has no
moving
> parts, and will cost me less (from my supplier) than a good quality relay
to
> do the same job - with better control.  Not to mention the geek factor...
8-)

Absolutely, geek factor is very important.  Still though, I find the
"scalability" and finesse of the FET's to be far more apealing than the
"clack-on clack-off" aproach.  ;-) This is the 21st century and scalability
is everything.

michael brown

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2002\02\22@032205 by Vasile Surducan

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On Thu, 21 Feb 2002, michael brown wrote:

> Absolutely, geek factor is very important.  Still though, I find the
> "scalability" and finesse of the FET's to be far more apealing than the
> "clack-on clack-off" aproach.  ;-) This is the 21st century and scalability
> is everything.
>
 I'm sorry, what is "geek factor" ? My dictionary said nothing about...

best Vasile

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2002\02\22@065947 by Dave Dilatush

picon face
Dale Botkin wrote...

>And it uses very little board space, very little power (not that it's a big
>issue), can easily handle the current of the string of lamps, has no moving
>parts, and will cost me less (from my supplier) than a good quality relay to
>do the same job - with better control.  Not to mention the geek factor...  8-)

One added thought, here:

If you want soft-start for those yard lights, one easy way to obtain it
would be to put an RC low pass filter between the MOSFET gates and the
PIC pin doing the control; something with a time constant of a few
hundred milliseconds or so.

As the gate voltage is ramped up past Vth, the MOSFETs start conducting
but initially act as current limiters, giving a true soft-start that
absolutely prevents the cold filaments from drawing excessive current.

During the soft-start interval the MOSFETs are going to be dissipating
considerable power, so it would probably be best not to make the
interval any longer than necessary so they don't overheat.

Of course, soft-start controlled by a ramped PWM would have a higher
"geek factor" than just a simple RC, wouldn't it...

Dave

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2002\02\22@072943 by steve

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> If you want soft-start for those yard lights, one easy way to obtain
> it would be to put an RC low pass filter between the MOSFET gates and
> the PIC pin doing the control; something with a time constant of a few
> hundred milliseconds or so.

I did this on a promotional thing that had lots of flashing lights. It
worked really well. With around 50 bulbs and startup current being
10 times the nominal value, it saved heaps in power supply cost.

Steve.

======================================================
Steve Baldwin                Electronic Product Design
TLA Microsystems Ltd         Microcontroller Specialists
PO Box 15-680, New Lynn      http://www.tla.co.nz
Auckland, New Zealand        ph  +64 9 820-2221
email: @spam@stevebRemoveMEspamEraseMEtla.co.nz      fax +64 9 820-1929
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2002\02\22@104347 by Dale Botkin

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face
On Fri, 22 Feb 2002, Dave Dilatush wrote:

> If you want soft-start for those yard lights, one easy way to obtain it
> would be to put an RC low pass filter between the MOSFET gates and the
> PIC pin doing the control; something with a time constant of a few
> hundred milliseconds or so.

Not a bad idea, I may do that.

> Of course, soft-start controlled by a ramped PWM would have a higher
> "geek factor" than just a simple RC, wouldn't it...

Well, yeah, but the PWM would really need to be synced to the AC waveform,
which would be a PITA.  My original idea was to just turn on one MOSFET at
first, then the second one a second or so later.  I like your idea better,
though.

And for Vasile: "Geek factor" is related to the amount of cool technology
you can use for any particular application.  A light switch has no geek
factor compared to a relay, and a relay controlled by a PIC is even
better.  Relays have little or no geek factor compared to MOSFETs,
especially when the FETs are used in a new and unusual way.  The fact that
you could just use a ligth switch is irrelevant.  Same reason we carry a
Palm 505 with a mini Linux kernel instead of a notepad...  8-)

Dale

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2002\02\22@152803 by Olin Lathrop

face picon face
> Well, yeah, but the PWM would really need to be synced to the AC waveform,
> which would be a PITA.

Not if the PWM frequency was much higher than the line frequency.  If so,
you could just ignore line sync and output asynchronous PWM which will
average nicely each 1/2 line cycle.  1.2KHz would be 10 PWM periods in each
1/2 line cycle, which should be good enough.  The FET transition time should
still be a negligeable percentage at that frequency if driven right.  Note
that the lowest hardware PWM frequency a 20MHz 16xxx PIC can produce is
1.22KHz.  Frankly I like this much better than low pass filtering the gate
to turn on the FETs slowly.  I guess it can be OK if done just right, but it
I don't like the power those FETs are dissipating even for a short time.


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2002\02\22@153615 by Martin Peach

flavicon
face
> > Well, yeah, but the PWM would really need to be synced to the AC
waveform,
> > which would be a PITA.
>
> Not if the PWM frequency was much higher than the line frequency.  If so,
> you could just ignore line sync and output asynchronous PWM which will
> average nicely each 1/2 line cycle.  1.2KHz would be 10 PWM periods in
each
> 1/2 line cycle, which should be good enough.  The FET transition time
should
> still be a negligeable percentage at that frequency if driven right.  Note
> that the lowest hardware PWM frequency a 20MHz 16xxx PIC can produce is
> 1.22KHz.  Frankly I like this much better than low pass filtering the gate
> to turn on the FETs slowly.  I guess it can be OK if done just right, but
it
> I don't like the power those FETs are dissipating even for a short time.

You might not like the whining coming from the lights either though....
PWM at 40kHz would be easier on the ears, and would possibly repel mice
too...
/\/\/\/*=Martin

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2002\02\22@191853 by Dale Botkin

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On Fri, 22 Feb 2002, Olin Lathrop wrote:

> > Well, yeah, but the PWM would really need to be synced to the AC waveform,
> > which would be a PITA.
>
> Not if the PWM frequency was much higher than the line frequency.  If so,
> you could just ignore line sync and output asynchronous PWM which will
> average nicely each 1/2 line cycle.  1.2KHz would be 10 PWM periods in each
> 1/2 line cycle, which should be good enough.  The FET transition time should
> still be a negligeable percentage at that frequency if driven right.  Note
> that the lowest hardware PWM frequency a 20MHz 16xxx PIC can produce is
> 1.22KHz.  Frankly I like this much better than low pass filtering the gate
> to turn on the FETs slowly.  I guess it can be OK if done just right, but it
> I don't like the power those FETs are dissipating even for a short time.

I can understand your position...  I was more concerned with what I
suspect would be horrible wideband RF hash, but of course it would only
last a second or so once a day.  Guess even I could live with that.

Dale

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2002\02\22@201604 by Dave Dilatush

picon face
Dale Botkin wrote...

>On Fri, 22 Feb 2002, Olin Lathrop wrote:
>
>> Not if the PWM frequency was much higher than the line frequency.  If so,
>> you could just ignore line sync and output asynchronous PWM which will
>> average nicely each 1/2 line cycle.  1.2KHz would be 10 PWM periods in each
>> 1/2 line cycle, which should be good enough.  The FET transition time should
>> still be a negligeable percentage at that frequency if driven right.  Note
>> that the lowest hardware PWM frequency a 20MHz 16xxx PIC can produce is
>> 1.22KHz.  Frankly I like this much better than low pass filtering the gate
>> to turn on the FETs slowly.  I guess it can be OK if done just right, but it
>> I don't like the power those FETs are dissipating even for a short time.
>
>I can understand your position...  I was more concerned with what I
>suspect would be horrible wideband RF hash, but of course it would only
>last a second or so once a day.  Guess even I could live with that.

I have a distinct distaste for having potential problems with my designs
pointed out in public, because it makes me say, "Dave, you doofus, you
should have done your homework before suggesting that!!"

In this case I didn't do my homework beforehand, and Olin's concern
about the instantaneous power dissipation in the MOSFETs is a very
realistic one.  It's an issue that should be addressed before proceeding
with any sort of "ramped gate drive" soft-start scheme such as the one I
proposed.

So now I'll go and do my homework and figure out how much stress these
MOSFETs might be subjected to by this soft-start arrangement.

I downloaded the datasheet for the 2SK2614's you mentioned, and note
they're rated for a typical Rds(on) of 32 milliohms; are these what
you're planning to use?

Also, what's the current rating of the bulbs in your yard lights?  I
recall you saying the total current drain for all the bulbs together is
7 amps, but what about each individual bulb?  
This is important because the filament size is going to determine how
fast the bulb heats up and gets past the "inrush current" period during
turn-on.  It's the inrush current that's of concern here, and how fast
it decays as the bulbs heat up and reach incandescence.  A single bulb
which draws 7 amperes steady-state will have a thick, massive filament
that will probably take several scores of milliseconds to come up to
temperature, whereas smaller bulbs, with light, thin filaments, will
reach operating temperature much more quickly.

Frankly, I don't think we have a problem, here; but I'll do some
calculation and some modeling to make sure we don't once I get some data
from you.

Dave

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2002\02\22@211259 by Dale Botkin

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On Sat, 23 Feb 2002, Dave Dilatush wrote:

> I have a distinct distaste for having potential problems with my designs
> pointed out in public, because it makes me say, "Dave, you doofus, you
> should have done your homework before suggesting that!!"

I know how you feel...  I get that with almost every post.  8(

> I downloaded the datasheet for the 2SK2614's you mentioned, and note
> they're rated for a typical Rds(on) of 32 milliohms; are these what
> you're planning to use?

Yup.

> Also, what's the current rating of the bulbs in your yard lights?  I
> recall you saying the total current drain for all the bulbs together is
> 7 amps, but what about each individual bulb?

They're 8W bulbs, little wussy thangs.

> Frankly, I don't think we have a problem, here; but I'll do some
> calculation and some modeling to make sure we don't once I get some data
> from you.

The transformer supplying power to this whole thing is rated at 121W
according to the label, if that makes a difference to you.  I have no
other information about it, unfortunately.  This is actually an
interesting question, I'm going to dig around myself and see if I can
figure this one out.

Dale

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2002\02\23@090818 by Olin Lathrop

face picon face
> You might not like the whining coming from the lights either though....
> PWM at 40kHz would be easier on the ears, and would possibly repel mice
> too...

You'd have to try it to be sure, but the bulbs don't "whine" much at 120Hz,
so I wouldn't expect much at higher frequencies either.


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(978) 742-9014, @spam@olinspam_OUTspam.....embedinc.com, http://www.embedinc.com

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2002\02\23@090838 by Olin Lathrop

face picon face
>>
In this case I didn't do my homework beforehand, and Olin's concern
about the instantaneous power dissipation in the MOSFETs is a very
realistic one.  It's an issue that should be addressed before proceeding
with any sort of "ramped gate drive" soft-start scheme such as the one I
proposed.
<<

Dave, I didn't say (and certainly didn't mean) that the soft start design
was bad, only that I personally didn't like it.  What bothers me most is how
the FETs will be taking pulses of serious power.  Even if it can be proven
that everything stays within current, power, and temperature ratings, I
don't like the sudden thermal stress.  In the long run, this has got to have
*some* effect.  Maybe it will only lower the expected life from an
irrelevantly large number to a slightly smaller irrelevantly large number.
There is also the issue of what if the bulbs get changed or more get added
in the future.  Again, this isn't based on defensible numbers, just
instinct.  Others could legitimately argue I'm being silly.

I certainly haven't done homework on this either, so here are some rough
calculations.  I'm making this up as I'm typing.  I remember something about
7A steady state.  Let's say the initial current is 70A at full voltage.  If
the FETs were 1/2 on with cold bulbs (it won't happen like this, I'm just
trying to pick something to see where it leads), that would be 35A x 60V =
2100 watts.  If this happened for 20mS, that would be 42W averaged over 1
second.  Hmm.  I think this is saying that this very crude model doesn't
provide conclusive evidence on way or the other, but that a much more
realistic model needs to be properly worked thru to prove this is OK before
implementing it.  That fact that it is close enough to require more detailed
calculations makes me nervous about this method again.


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2002\02\23@114826 by Chris Loiacono

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here are some rough
calculations.  I'm making this up as I'm typing.  I remember something about
7A steady state.  Let's say the initial current is 70A at full voltage.

I have been taught, and have used 7:1 as an inrush current ratio for
tungsten for the past 20 years without any problems.
If
the FETs were 1/2 on with cold bulbs (it won't happen like this, I'm just
trying to pick something to see where it leads), that would be 35A x 60V =
2100 watts.  If this happened for 20mS, that would be 42W averaged over 1
second.

So, I get abou 30 watts/sec avg.

Hmm.  I think this is saying that this very crude model doesn't
provide conclusive evidence on way or the other, but that a much more
realistic model needs to be properly worked thru to prove this is OK before
implementing it.  That fact that it is close enough to require more detailed
calculations makes me nervous about this method again.

As long as the ramping starts below 15% and the ramp is long enough, this
scenario should never happen. With large and more costly loads, I like
longer ramps, say as much as 5 sec full-scale. This gives the tungsten time
to heat a bit more and the heat sink time to actually do something since the
resistance increases significantly along the way up the ramp. I haven't
followed this thought from start to finish, nor have I done the analysis,
but I do a fair bunch of AC power controllers for tungsten, and when I use a
5 second soft start with a 7:1 inrush, I have gotten into the practice of
using devices at/near their rated power level without inrush failure.
Besides, with a 1 or 2 second soft-start, most non-geeks wouldn't even
notice it, and the 'wow-factor' would be mostly lost.

I have found that the most common cause for failure occurs when the output
to the controlling  devices is left to do spurious things during power-up or
reset, etc..

Chris


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2002\02\23@154959 by Dave Dilatush

picon face
Dale Botkin wrote...

>They're 8W bulbs, little wussy thangs.

"Wussy" is right.  I'm sure the bugs will have no difficulty finding
them, but are you going to be able to see OK with those little things?  
The data I have so far indicates that this inrush current issue is
perhaps being overstated a bit.

My 1968 edition of the Motorola Power Circuits Handbook (written back
when TRIACs were apparently still "rocket science") has a good
discussion on solid-state relay design and in one subchapter about
soft-start circuits for lamps, goes into some detail on inrush current.

They measured current versus time for a 500W 120V tungsten lamp with a
triac triggered right at the peak of the line voltage waveform, and
found the inrush current decayed by 60% within two line cycles of
turn-on.  In another test, with 120V DC applied, the inrush decayed from
its initial 60A down to 30A in about 5 milliseconds.

Now those results are for 4 amp filaments, about 6 times more massive
than the ones in your yard lamps which are rated at just under 700 mA.
So I would expect the current to decay quite a bit faster in your case.


And if that's so, inrush may well be a only a minor issue here.

>The transformer supplying power to this whole thing is rated at 121W
>according to the label, if that makes a difference to you.

I don't think it does, other than presenting a small additional
impedance in series with the lamps and the MOSFET switch.  If it has any
effect, it will be in the direction of alleviating--albeit slightly--the
inrush current problem if one exists.

>This is actually an interesting question, I'm going to dig around
>myself and see if I can figure this one out.

I've been an engineer for nearly a quarter-century and was a technician
for more than a decade before getting my degree; and I've been an
electronics hobbyist for almost 45 years.  So you would think by now I'd
be well past the point where I could get absolutely fascinated by dumb
stuff like "What do light bulbs **REALLY** do?"

But I'm not.  And that scares me...

Dave

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2002\02\23@160404 by Dave Dilatush

picon face
Olin Lathrop wrote...

>Dave, I didn't say (and certainly didn't mean) that the soft start design
>was bad, only that I personally didn't like it.

I didn't take any offense, Olin; I'm just irked that this is one of
those instances where I've "winged" an answer to a question and possibly
missed a significant pitfall.  My gut sense is usually pretty good, but
it isn't infallible.

>...Others could legitimately argue I'm being silly...

You?  Silly?  Never-- even if you did once plug a carbon microphone into
a 120V outlet to show people what REAL sparks were like...

>...If the FETs were 1/2 on with cold bulbs (it won't happen like this, I'm just
>trying to pick something to see where it leads), that would be 35A x 60V =
>2100 watts.  If this happened for 20mS, that would be 42W averaged over 1
>second.

Ummm... they're 12 volt bulbs, not 120 volt bulbs.  The current's right,
but the voltage and the power dissipation are off by an order of
magnitude.

I've done some simulation of this MOSFET switch with soft-start, based
on a 50 millisecond turn-on period (from beginning of current flow to
the point where current is limited by Rds(on)), and modeling the lamp as
a conductance which decays exponentially from the "cold" value to
one-seventh that amount with a time constant of 50 milliseconds,
starting at the instant where current flow begins.  
This model is biased very strongly toward the pessimistic side, since
the Motorola data (see my other post, in response to Dale, for details)
suggest filaments this size heat up much, much more quickly--at least
ten times as fast as the model I constructed.

I don't have a SPICE model for a 2SK2614, so I used a Fairchild BUZ11 in
my simulation.  It has very similar characteristics and it also has a
much more complete datasheet, including a chart showing Forward Bias
Safe Operating Area for different pulse widths.  According to this
chart, the BUZ11 can dissipate 120W in a 50 millisecond pulse over a
wide range of voltage/current combinations.

The simulations I did showed the MOSFET dissipating a little over 18
watts average over the three line cycles during which current limiting
was taking place; and this is with a current inrush whose duration is
something like an order of magnitude longer than might reasonably be
expected.  
With no inrush (that is, the lamp starts out at its "hot" resistance
value), the MOSFET dissipation during the slow-start interval drops to 7
watts for 50 milliseconds.

Even if the 2SK2614 SOA curves aren't exactly like those of the BUZ11,
it appears the MOSFETs in this soft-start scheme will be experiencing
turn-on stresses roughly an order of magnitude below what they're
capable of withstanding-- and even lower if the soft-start interval is
shortened from 50 milliseconds down to 10 or 20.

A two-to-one safety margin would be cutting it close; but when the
margin goes up to ten-to-one, I start getting fairly confident of having
room enough for error.

Dave

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2002\02\23@181235 by Olin Lathrop

face picon face
> here are some rough
> calculations.  I'm making this up as I'm typing.  I remember something
about
> 7A steady state.  Let's say the initial current is 70A at full voltage.
>
> I have been taught, and have used 7:1 as an inrush current ratio for
> tungsten for the past 20 years without any problems.
> ...

Chris, please do whomever you are replying to and the rest of the list the
courtesy of differntiating your responses from the original post.  I use ">
" at the start of each line to denote the original post, as most people do,
but anything that makes it obvious is sufficient.  I particularly object to
you reproducing my signature line in your post as if I had written the whole
thing.  (Of course that should have been trimmed altogether, but that's
probably too much to ask at this stage).


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2002\02\23@205206 by Edson Brusque

face
flavicon
face
hello Dave et all,

>No one ever said you can't use MOSFETs on AC; you just have to be a
>little clever, is all.  Usually, that means you end up having to use two
>of them in series-- one for each half-cycle-- because of the intrinsic
>body diode that shunts the drain and source.
>The circuit below uses no voltage-dropping, power-wasting TRIAC, no
>likewise bridge rectifier, no relay^H^H^H^H^H "electromechanical logic",
<snip>

   this circuit is really very interesting. I've tried it with low current
(about 200mA) at 30VAC and worked like a charm. I'll try to use it with
something that requires more current to see how it works.

   Now, how about using an IGBT? I'm thinking of using this circuit to make
reverse dimming control (a topic that was discussed last year) in 100-240V
and I'm afraid I will burn some MOSFETs. Maybe IGBT would be a better
solution as they're made for high voltages, but the only way I can think is
to put the IGBTs on the MOSFETs place on your circuit and use a pair of
diodes. But then we'll have a large voltage drop.

   Have someone tried anti-paralleling IGBTs?

   Best regards,

   Brusque

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2002\02\23@221936 by Chris Loiacono

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Dave:

OK, help me understand...
Does this mean that a longer soft-start period will cause a higher inrush?

Chris
(ps: that's why I love this list - I actually learn things here...)

The simulations I did showed the MOSFET dissipating a little over 18
watts average over the three line cycles during which current limiting
was taking place; and this is with a current inrush whose duration is
something like an order of magnitude longer than might reasonably be
expected.

With no inrush (that is, the lamp starts out at its "hot" resistance
value), the MOSFET dissipation during the slow-start interval drops to 7
watts for 50 milliseconds.

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2002\02\23@223817 by Chris Loiacono

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I remember an ap note - perhaps from I.R. or Eupec on using IGBT's
similarly.
I will try to dig it up. This is really getting interesting...

Chris

ps: see Olin, I'm learning! >>>

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2002\02\23@225354 by Dave Dilatush

picon face
Chris Loiacono wrote...

>OK, help me understand...
>Does this mean that a longer soft-start period will cause a higher inrush?

No, it just means that if the soft-start period is drawn out longer than
it needs to be for the sake of the lamp filament, it will cause
unnecessary stress on the MOSFET.

Think of what happens in the extremes:

At one extreme, we have no soft-start period at all; the MOSFET is
abruptly turned on and, if this happens to occur when the voltage is at
its peak, the lamp experiences huge inrush currents-- 7 to 10 times the
current that would normally flow through the filament.

At the other extreme, let's say we turn the MOSFET on partially, so that
it is operating as a current limiter--and then just let it twist in the
breeze indefinitely, turned partially on.  The lamp experiences no
inrush of current at all; it can't, because the MOSFET is governing how
much current passes through it.  But the MOSFET is dissipating a lot of
power, and within a minute or so it overheats and goes "pop!".

Those are the extremes, and we are attempting to find a happy medium
between them-- a soft-start period long enough that the lamp experiences
a gradual rise in current so its filament isn't shocked, yet short
enough that the MOSFET doesn't dissipate more energy than it can handle.

That's what I was trying to figure out in my simulation: how short did
the soft-start period have to be, to avoid overstressing the MOSFET.

Dave

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2002\02\23@233518 by Dave Dilatush

picon face
Edson Brusque wrote...

>    Now, how about using an IGBT? I'm thinking of using this circuit to make
>reverse dimming control (a topic that was discussed last year) in 100-240V
>and I'm afraid I will burn some MOSFETs. Maybe IGBT would be a better
>solution as they're made for high voltages, but the only way I can think is
>to put the IGBTs on the MOSFETs place on your circuit and use a pair of
>diodes. But then we'll have a large voltage drop.

What's reverse dimming?  Is that where conduction begins on the zero
crossing and then shuts off partway through the cycle?

I've never used IGBTs and I only know a little about them.  But from
what I've read they should function in this circuit, provided you put a
diode in parallel with each one so current can go around it during its
"OFF" half of the cycle.

This circuit, using MOSFETs, would certainly not be very good for high
voltage applications because of the relatively high Rds(on) of FETs with
high breakdown voltages.  Also, I would expect real problems--maybe
insurmountable ones--with the huge current surges that occur during lamp
burnout in high voltage circuits.

Maybe IGBTs would alleviate these problems; I don't know.

Dave

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2002\02\24@080800 by Edson Brusque

face
flavicon
face
Hello Chris, Dave et all,

Chris:
>I remember an ap note - perhaps from I.R. or Eupec on using IGBT's
>similarly.
>I will try to dig it up. This is really getting interesting...

   I've downloaded about 15 app notes today from I.R., but can't find this
information. Please, tell me when you've found it.

Dave:
>What's reverse dimming?  Is that where conduction begins on the zero
>crossing and then shuts off partway through the cycle?

   yes, you got it.

>I've never used IGBTs and I only know a little about them.  But from
>what I've read they should function in this circuit, provided you put a
>diode in parallel with each one so current can go around it during its
>"OFF" half of the cycle.

   I've tried with a pair of IRG4BC30UD (D is from Diode) and this worked,
but I haven't measured the voltage drop.

>This circuit, using MOSFETs, would certainly not be very good for high
>voltage applications because of the relatively high Rds(on) of FETs with
>high breakdown voltages.  Also, I would expect real problems--maybe
>insurmountable ones--with the huge current surges that occur during lamp
>burnout in high voltage circuits.

   Yes, bipolar transistors work ok for high voltage, MOSFETs are perfect
for large currents, if you want both you must use IGBTs.

>Maybe IGBTs would alleviate these problems; I don't know.

   Yes, this is the idea.

   Best regards,

   Brusque

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2002\02\24@122039 by George Tyler1

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face
I made a reverse dimmer in about 1987 using an irfp450 and a 555. The
curcuit had bult in current limit so you could short the output and it just
shut off until you removed it. Later I used it without the dimmer to limit
the inrush current in lamps for traffic lights, to improve the life.
   I was a very simple curcuit, just semse the drop across the fet with a
small bipolar transistor wich then shunted the gate to ground.

{Original Message removed}

2002\02\24@130723 by George Tyler1

flavicon
face
You set a limit to the current that the fet will conduct - then switch it
off. As the fet is on or off the dissipation is low.
{Original Message removed}

2002\02\24@183723 by Edson Brusque

face
flavicon
face
Hello George,

> I made a reverse dimmer in about 1987 using an irfp450 and a 555. The
> curcuit had bult in current limit so you could short the output and it
just
> shut off until you removed it. Later I used it without the dimmer to limit
> the inrush current in lamps for traffic lights, to improve the life.
>     I was a very simple curcuit, just semse the drop across the fet with a
> small bipolar transistor wich then shunted the gate to ground.
>You set a limit to the current that the fet will conduct - then switch it
>off. As the fet is on or off the dissipation is low.

   have you actually seen the circuit proposed by dave?

   The high current protection you are proposing (saddly) will not work
work in this circuit but only on DC ones.

   I've been thinking of a pair of very small resistances between the
MOSFETs sources and the ground, but this will reduce the circuit efficience
and a lot of power will have to be dissipated on the resistors.

   Best regards,

   Brusque

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2002\02\24@213633 by Spehro Pefhany

picon face
At 07:18 PM 2/24/02 +0200, you wrote:
>I made a reverse dimmer in about 1987 using an irfp450 and a 555.

Did you publish this circuit anywhere? I've read that the reverse phase
control is patented by a theatrical dimmer company.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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2002\02\24@234319 by George Tyler1

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face
Hi Edson and others,
       I have not seen his circuit, but my one would run on AC,although I
think I have not actually used it like this. (fet across dc output of a
bridge rectifier, ac side of bridge in series with mains.)
{Original Message removed}

2002\02\24@234451 by George Tyler1

flavicon
face
Interesting.... I wonder when they patented that? The main reason I went to
reverse is for  a  switchmode halogen lamp "transformer". This had a
capacitive input, so a normal dimmer generated high peak currents.

{Original Message removed}

2002\02\25@070830 by Alan B. Pearce

face picon face
>As long as the ramping starts below 15% and the ramp is long enough, this
>scenario should never happen. With large and more costly loads, I like
>longer ramps, say as much as 5 sec full-scale. This gives the tungsten time
>to heat a bit more and the heat sink time to actually do something since
the
>resistance increases significantly along the way up the ramp. I haven't
>followed this thought from start to finish, nor have I done the analysis,
>but I do a fair bunch of AC power controllers for tungsten, and when I use
a
>5 second soft start with a 7:1 inrush, I have gotten into the practice of
>using devices at/near their rated power level without inrush failure.
>Besides, with a 1 or 2 second soft-start, most non-geeks wouldn't even
>notice it, and the 'wow-factor' would be mostly lost.

I would think that you probably do not want a linear ramp up anyway. Most of
the preheat to stop inrush current will happen in the first 15-20% of a long
ramp, so shortening the time for the last portion where the current is
highest may improve the average dissipation in the FETS. You will need to do
a bit of calculating in a spreadsheet or similar to produce a graph to prove
or disprove this, but it is my gut feeling.

The geek factor may be enhanced by having the bulbs sit there with an orange
glow for a couple of seconds, and then zoom up rapidly to white anyway.

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2002\02\25@072330 by Alan B. Pearce

face picon face
>Now those results are for 4 amp filaments, about 6 times more massive
>than the ones in your yard lamps which are rated at just under 700 mA.
>So I would expect the current to decay quite a bit faster in your case.


>And if that's so, inrush may well be a only a minor issue here.

And if the transformer is in the house, with a reasonable run of wire down
the yard, then that will limit the absolute peak as well, even with multiple
bulbs.

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2002\02\25@074417 by Vasile Surducan

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On Fri, 22 Feb 2002, Dale Botkin wrote:

> On Fri, 22 Feb 2002, Dave Dilatush wrote:
>
> > If you want soft-start for those yard lights, one easy way to obtain it
> > would be to put an RC low pass filter between the MOSFET gates and the
> > PIC pin doing the control; something with a time constant of a few
> > hundred milliseconds or so.
>
> Not a bad idea, I may do that.
>

 There are many bulb producers, for example chinese ( veeery cheap )
bulbs needs more than a few hundred milisecons for safetly heating. And we
go back, what is cheaper for a production, a transformer, two mos-fets,
one pic, some software for pwm, some filtering, and so and so, or a good
designed transformer ( saturated core at power-up ) and a relay/scr in
primary coil ? Everything you will say the truth is just one...

best regards, Vasile

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2002\02\25@101908 by Dale Botkin

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On Mon, 25 Feb 2002, Vasile Surducan wrote:

>   There are many bulb producers, for example chinese ( veeery cheap )
> bulbs needs more than a few hundred milisecons for safetly heating. And we
> go back, what is cheaper for a production, a transformer, two mos-fets,
> one pic, some software for pwm, some filtering, and so and so, or a good
> designed transformer ( saturated core at power-up ) and a relay/scr in
> primary coil ? Everything you will say the truth is just one...

If I were going to produce and market something like this a a commercial
product, I'd probably go cheap and simple.  Since it's a one-off just for
my yard lights, and something for me to have fun with, I'll play with some
new stuff.  The transformer is a given, it's already there, and the parts
are all in my junque box as it is - other than the MOSFETS, which ar
cheape than a relay anyway.  Besides, I already *know* how to drive a
relay, no challenge in that! 8-)

Dale

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2002\02\25@125241 by Edson Brusque

face
flavicon
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Hello PICple,

   while we're still on the subject, someone knows where to find appnotes
regarding MOSFETs in AC circuits? The big majority of app notes only tell
about MOSFETs in DC circuits.

   Thanks,

   Brusque

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2002\02\26@042850 by Vasile Surducan

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On Mon, 25 Feb 2002, Dale Botkin wrote:

> On Mon, 25 Feb 2002, Vasile Surducan wrote:
>
> >   There are many bulb producers, for example chinese ( veeery cheap )
> > bulbs needs more than a few hundred milisecons for safetly heating. And we
> > go back, what is cheaper for a production, a transformer, two mos-fets,
> > one pic, some software for pwm, some filtering, and so and so, or a good
> > designed transformer ( saturated core at power-up ) and a relay/scr in
> > primary coil ? Everything you will say the truth is just one...
>
> If I were going to produce and market something like this a a commercial
> product, I'd probably go cheap and simple.  Since it's a one-off just for
> my yard lights, and something for me to have fun with, I'll play with some
> new stuff.  The transformer is a given, it's already there, and the parts
> are all in my junque box as it is - other than the MOSFETS, which ar
> cheape than a relay anyway.  Besides, I already *know* how to drive a
> relay, no challenge in that! 8-)
>
 Agree, for this particular project. However designing a startup
saturating transformer is a challenge !

best, Vasile

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