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'PWM synch to ac line'
1995\08\03@225407 by Andrew Warren

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Martin Kirk (spam_OUTmlkTakeThisOuTspamASU.EDU) wrote:

>how about some clever (meaning cheap/simple) ideas for PIC
>sense zero crossing from the transformer secondary.

Martin:

If you're REALLY brave, check out AN521, "Interfacing to AC Power
Lines", in the Embedded Control Handbook.

-Andy

--
Andrew Warren - .....fastfwdKILLspamspam@spam@ix.netcom.com
Fast Forward Engineering, Vista, California

1995\08\04@003213 by Mike Keitz

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>Hi all,
>
>        I have an interesting challenge.  I am using a 16C57 to drive a
>nitinol wire actuator with PWM.  The drive goes to multiple actuator and
>as it turns out I need a whopping current to drive them.  I am lifting
>something heavy.  Each takes 6 amps average.  Everything was going along
>happy (in denial) until I started to purchase the power supply to provide
>the current.
>
>        Now I am wondering if any of you have experience with sourcing
>PWM current from an un-regulated supply.  I am thinking that maybe I can
>simply step down the ac line, full wave rectify it, and synch my PIC PWM
>timing to the zero crossing.  I use IRF520 MOSFETs to switch the current.
>This would insure that my driving waveform would be uniform for each cycle.
>I do not believe that having a non-DC source current is any problem.  After
>all, I am only heating wire.
>
>        Also how about some clever (meaning cheap/simple) ideas for PIC
>sense zero crossing from the transformer secondary.
>
Use a full-wave rectifier to develop a bus having positive half-cycles of AC
(i.e. 120 Hz pulses).  The transformer and diodes have to be rated for the
sum of the current through all the wires that will be heated at the same
time.  Connect one end of all the wires to this bus, and the other end to
the anode of a SCR that controls that wire.  The cathodes of the SCRs go to
ground, and the gates are driven by the PIC through resistors (as if they
were NPN transistor bases).  About any SCR will work, for example the S2006L
(about $1.20 each in Digi-Key catalog) will switch 6A RMS and turn on with
15 mA of gate current.  If you're driving multiple wires from each channel,
70 Amp SCRs are available (about $7.00), but you'll need to add a driver
transistor (emitter follower from PIC pin, with collector on 5V bus) to
supply the 50 mA gate current required.

The SCRs will turn on when positive gate voltage is applied, and turn off
during the zero-crossings of the bus voltage.  If you want on/off control
with 1/120 second resolution, synchronizing is unnecessary.  Just leave the
gate drive on while the wire is to heat up, and turn it off when not.  If
proportional control is required, synchronize the PIC with a zero-crossing
voltage derived from the bus (by comparing when the bus voltage rises above
1V or so), and generate a delayed pulse for the gate.  The more delay of
course, the less power will be applied.  But the relation between phase
delay to power is not linear (think of integrating the sine squared
function), easily corrected with a table if required.

As far as conditioning the bus voltage for the PIC, it could be simply
applied to RTCC through a suitably large resistor to limit the current to
<500 uA when the protection diode clamps the pin voltage to 5V.  Loading the
bus with a resistor to ensure that it does return to 0V at each half-cycle
would be a good idea.  The turn-on point would be about 3V.  If earlier
detection is required, a more sophisticated voltage comparator, for example
driving the base of an NPN transistor and using the collector with a pull-up
to 5V, would switch on while the bus voltage is more than 1V or so.

-Mike

1995\08\04@102659 by Paul Greenwood

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> >        Now I am wondering if any of you have experience with sourcing
> >PWM current from an un-regulated supply.  I am thinking that maybe I can
> >simply step down the ac line, full wave rectify it, and synch my PIC PWM
> >timing to the zero crossing.  I use IRF520 MOSFETs to switch the current.
> >This would insure that my driving waveform would be uniform for each cycle.
> >I do not believe that having a non-DC source current is any problem.  After
> >all, I am only heating wire.

I'm wondering - why not just use a triac and NOT convert to DC?  Yes, you need
to sense the zero crossing.  It's EASY!  Just add one resistor.  The app.
notes show you how.

--

           -- Paul Greenwood --  (pablospamKILLspamaustin.ibm.com)

Newton's Fourth Law:  Every action has an equal and opposite satisfaction.

1995\08\05@172229 by First Last

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>>how about some clever (meaning cheap/simple) ideas for PIC
>>sense zero crossing from the transformer secondary.

AN521, "Interfacing to AC PowerLines"
works like a charm.  I have 2 projects where I did just that and they
have been working fine for over a year.

The only real reason to sync to zero cross is if you are using SCR or
Triac as the current switch.
Then you let the zero cross turn off the device.
If you are using MOSFETs, then you can turn them off anytime.
So, set the PWM frequency much higher than 60 Hz and don't worry about
sync.
This does not take into account switching transient noise if that is a
concern.

Hope this helps,  Gary Skinner,  ESI

1995\08\05@182403 by mlk

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       Thanks all for the suggestions.  I am curious about Gary Skinner's
suggestion.  How much higher than 60Hz is "much higher"?  I do a single PWM
control loop that drives 8 separate actuators.  They all function with the
same master cycle.  All turn on at the same time and I just toggle the bit
for each off when the time for that one runs out.  The 20MHz PIC is doing
about 230Hz at best the last I checked.  Is that fast enough to avoid some
sort of beat frequency effect?

Thanks,

Martin Kirk
Arizona State University
.....mlkKILLspamspam.....asu.edu
(602) 582-5718

On Sat, 5 Aug 1995, First Last wrote:

{Quote hidden}

1995\08\06@193046 by First Last

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Martin writes:
ML>        Thanks all for the suggestions.  I am curious about Gary Skinner's
ML>suggestion.  How much higher than 60Hz is "much higher"?  I do a single PWM
ML>control loop that drives 8 separate actuators.  They all function with the
ML>same master cycle.  All turn on at the same time and I just toggle the bit
ML>for each off when the time for that one runs out.  The 20MHz PIC is doing
ML>about 230Hz at best the last I checked.  Is that fast enough to avoid some
ML>sort of beat frequency effect?

I don't really know exactly what much higher means.
The PWM is chopping up the sinewave into pulses.  If it chopped up each
cycle into at least 3 or 4
pieces, then there should not be any "beat frequency" type of problems.
I suppose you could do a lot of math and verify some results, but I
would probably build a
simple device and measure the RMS output as a function of PWM on/off
ratio.
I expect it will be pretty linear with 230Hz period.

Please let us all know how the experiment works out.

Thanks,  Gary Skinner  ESI

1995\08\07@160429 by CRSO.pic

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-> The only real reason to sync to zero cross is if you are using SCR or
-> Triac as the current switch.

Sorry to butt in here Gary...

The main reason to sync to the zero crossing is to minimize in-rush
current and its resulting EMI. Connecting the load to the mains while at
peak voltage results in a high dI/dT surge that many loads don't
appreciate. As well, EMI emanations will abound up into the MHZ region.
An inductive load just complicates this even further.

This is one of the reasons one can't (or shoudn't) use a dimmer switch
with an inductive load.

Regards, Dana Frank Raymond - Foxtrot Systems Ltd.
Internet: EraseMEdana.raymondspam_OUTspamTakeThisOuTcanrem.com. Compuserve: 73362,3052

1995\08\12@193210 by Eric Smith

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On Fri, 4 Aug 1995 00:30:08 +0600, Mike Keitz <mkeitzspamspam_OUTBEV.NET> wrote:
> As far as conditioning the bus voltage for the PIC, it could be simply
> applied to RTCC through a suitably large resistor to limit the current to
> <500 uA when the protection diode clamps the pin voltage to 5V.  Loading the

Note that the RTCC pin does not have an internal protection diode (to the
postive supply rail) as most of the other pins have.  You will need to add
the diode externally, as shown in Microchip application note AN521.

Eric

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