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'[PICLIST] [EE] Drivin Suzie Q (transistor troubles'
2002\02\18@012041 by Dave King

flavicon
face
I've been trying to get a power transistor to behave for the last week or so.
I've been trying to drive 3x 3v 350 ma led (yes each one is 350ma) with a
specific pattern.
The pic generated signal is of course flawless but when I've tried to get
the power
transistor to function I get either spurious signals or it just sits there.
I've been trying
to use a 2N2222 to drive various power mosfets that have been suggested.
Nothing seems
to get this close to what I need. The biggest problem is that this needs to
be done with
3.3V which seems to be proving the big problem. The 3.3 was chosen to
eliminate a few parts
and keep the heat down, ie some of the parts eliminated were power resistors.

I've tried drivers, converters and tried tweaking the 2222 to the nth
degree without satisfaction.
I've even rtfm'd just to make sure I wasn't pulling a total boner. I'm sure
this is doable it would
be close to a 20% duty cycle motor controller (yes tried that way too).
Could anyone help
me sort this out?

Tnx

Dave

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2002\02\18@140825 by hard Prosser

flavicon
face
Dave,
I'd suggest you include at least an ASCII diagram of the circuit as it
makes it a lot easier to visualise what you are trying to do - or at least
how you are trying to do it.
Alternatively put a gif up on a web site somewhere or send it to me direct
& I'll have a quick look.

If you are using MOSFETS you will need more than 3.3V on the gate to turn
them on properly. If you have only this voltage to play with you will need
to change to a bipolar power transistor or arrange a higher voltage gate
drive supply using a charge pump/aux supply or whatever. It would only need
a mA or so depending on switching rate.

Richard



I've been trying to get a power transistor to behave for the last week or
so.
I've been trying to drive 3x 3v 350 ma led (yes each one is 350ma) with a
specific pattern.
The pic generated signal is of course flawless but when I've tried to get
the power
transistor to function I get either spurious signals or it just sits there.
I've been trying
to use a 2N2222 to drive various power mosfets that have been suggested.
Nothing seems
to get this close to what I need. The biggest problem is that this needs to
be done with
3.3V which seems to be proving the big problem. The 3.3 was chosen to
eliminate a few parts
and keep the heat down, ie some of the parts eliminated were power
resistors.

I've tried drivers, converters and tried tweaking the 2222 to the nth
degree without satisfaction.
I've even rtfm'd just to make sure I wasn't pulling a total boner. I'm sure
this is doable it would
be close to a 20% duty cycle motor controller (yes tried that way too).
Could anyone help
me sort this out?

Tnx

Dave

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2002\02\18@211943 by Dave King

flavicon
face
>Alternatively put a gif up on a web site somewhere or send it to me direct
>& I'll have a quick look.

I've put the information up on the backend of my website
at  http://members.shaw.ca/gnat/led.htm
That has the schematic for the problem section and a image of the scope
signals.

>If you are using MOSFETS you will need more than 3.3V on the gate to turn
>them on properly. If you have only this voltage to play with you will need
>to change to a bipolar power transistor or arrange a higher voltage gate
>drive supply using a charge pump/aux supply or whatever. It would only need
>a mA or so depending on switching rate.
>
>Richard

Ok I'll dig out my books and get on the net and start looking for a
bipolar. It does need
to be able to handle about 1.7A at a 20% duty cycle. I'm almost stuck on
having to deal with
3.3 as my main supply voltage as going up to 5 will do a number on the cost
of things.It could
be done but I'd rather shy away from doing that unless I have to.

I really appreciate any and all help on this. I haven't had to play with
transistors for quite a few years
so I'm out of touch with what works in the 3.3v regime and what won't.

Cheers

Dave

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2002\02\19@065104 by Thomas C. Sefranek

face picon face
Take all the JUNK out, use a PNP power transistor driven by the control
signal.
(Your 2N222 connection is nonsense.)
Emitter to +3.3 V.
Base is the drive signal.
Collector goes to the LEDs.

Dave King wrote:

{Quote hidden}

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2002\02\19@142454 by hard Prosser

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face
Dave
Had a quick look at your circuit & there seem to be a number of issues.
1. There is no positive supply for the FET gate. The collector of the
2N2222 needs to be taken to a positive rail.
2. Even then it won't work as the maximum voltage that could be delivered
would not be enough to turn the IRF540 on as the emitter voltage will be
about 0.6V less than the base voltage which is going to be the PIC power
Supply voltage more or less.
3. The resistor values shown are unrealistic.
4. Driving LEDs in parallel like this can work but you may get significant
brightness variation due to unequal current share.
5. The 100 ohm input resistor is not needed unless the input signal is
going open circuit. In any event 100ohms is too low. 10k would be adequate.
If you are limited to a 3.3V supply then you will need to use a bipolar
power transistor - I would suggest a darlington such as the TIP122.

e.g  - If you can read it (Use a monospaced font eg Courier)

But even with a darlington you may not have enough base current for a
3x385mA (or so) drive. Dependent on the PIC you are using you may be able
to drop the 330ohm resistor to 100 ohms (20mA out maximum ? ) to get more.
LED current can be adjusted by changing the 3.3ohm resistors in each LED
line. They are based on a 3.3V supply, a 1.5V LED drop, and a  0.5V
transistor drop .i.e. 1V across the resistors = 330mA  Actual values may
need adjustment.

If you can't get the LEDs to full brightness, the PIC may not be able to
supply enough drive current (I think some of the older ones are limited in
this regard) so in this event you would need to add another transistor. -
and complicate the circuit somewhat.

If you need to switch the LEDs quickly (e.g this is being used for infra
red transmission purposes, then, again, you may need a better circuit.

Richard P
____________________________________
 3.3V Supply            |     |    |
                        V     V    V  LEDs
                        -     -    -
                        |     |    |
                       3.3   3.3  3.3
                        |_____|____|
                             | C
                        B    |
---------- 330 ------------ TIP122
PIC Drive                     |
                             | E
                            GRND






>Alternatively put a gif up on a web site somewhere or send it to me direct
>& I'll have a quick look.

I've put the information up on the backend of my website
at  http://members.shaw.ca/gnat/led.htm
That has the schematic for the problem section and a image of the scope
signals.

>If you are using MOSFETS you will need more than 3.3V on the gate to turn
>them on properly. If you have only this voltage to play with you will need
>to change to a bipolar power transistor or arrange a higher voltage gate
>drive supply using a charge pump/aux supply or whatever. It would only
need
>a mA or so depending on switching rate.
>
>Richard

Ok I'll dig out my books and get on the net and start looking for a
bipolar. It does need
to be able to handle about 1.7A at a 20% duty cycle. I'm almost stuck on
having to deal with
3.3 as my main supply voltage as going up to 5 will do a number on the cost
of things.It could
be done but I'd rather shy away from doing that unless I have to.

I really appreciate any and all help on this. I haven't had to play with
transistors for quite a few years
so I'm out of touch with what works in the 3.3v regime and what won't.

Cheers

Dave

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2002\02\19@171902 by Dave King

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face
Hi Richard (and anyone else that looked at my mess)

What I had put up was a non working schematic, I've tried so many
many variations that I just put up the section that I though needed to
be sorted out. I was hoping it might allow someone the idea of what
I was trying to do. Ie using the available current from an output pin on a
pic to
drive a 2n2222 that in turn switches a larger "power" transistor which
hopefully
switches off and on with my desired pattern..

I think I'll correct it to the point where it might make sense and get rid
of the
schematic portion I have on there. That might eliminate a lot of the confusion.

If you can look at it again (give me an hour to correct and upload) it
should make
more sense.

Thanks!

Dave

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2002\02\19@185446 by hard Prosser

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face
Dave,
Looks much better. Still a few problems however.
With 3.3V drive from the PIC, the emitter of Q2 can only reach 2.7V, the
emitter of Q23 about  2.1V. The 220ohm resisor will limit the maximum total
LED current to (3.3 - 1.5) /220 = 8mA max or about 3mA per LED. But you may
get also current limiting due to the 47k base resistor on Q23

Your best bet with this circuit is short out the 220ohm resistor and the
47k resistor. LED current will be about (2.1-1.5)/47 = 13mA per LED or 40mA
total. For more current, reduce the 47ohm resistors. (Down to maybe 1.5 ohm
for 380mA  or 1.2 ohm for 500mA - it will depend on LED froward voltage
drop - I have allowed 1.5V for this
).
If you are really likely to run down to 2.3V for Vcc, then for constant LED
brilliance,  some sort of constant current supply for the LEDs would be in
order. Not a real easy task at  such a low voltage.

(Or if you have a spare PIC pin you could squarewave, then you could use a
boost circuit to produce a higher voltage & make life a lot easier - or
maybe just introduce a complication somewhere else!)

Richard P




                   Dave King
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                   CA>                   cc:
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Hi Richard (and anyone else that looked at my mess)

What I had put up was a non working schematic, I've tried so many
many variations that I just put up the section that I though needed to
be sorted out. I was hoping it might allow someone the idea of what
I was trying to do. Ie using the available current from an output pin on a
pic to
drive a 2n2222 that in turn switches a larger "power" transistor which
hopefully
switches off and on with my desired pattern..

I think I'll correct it to the point where it might make sense and get rid
of the
schematic portion I have on there. That might eliminate a lot of the
confusion.

If you can look at it again (give me an hour to correct and upload) it
should make
more sense.

Thanks!

Dave

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2002\02\20@020410 by Dave King

flavicon
face
Hi Everyone

If you've been looking at this trying to sort it out I've uploaded a version
with actual resistor values and such as well as showing a image of the signal
that is at the base of the transistor Q23.

I haven't played with the resistor vales on the emitter side of Q23 yet
(R21, R23, R24)
so you can ignore these components for now. A few of you have mentioned the
problems
coming there so I'm not ignoring it ;-]

From what everyone has said It looks like this will be a no go on 3.3v
unfortunately. I'm going
to redo the entire board to 5vdc later on unless someone mentions the
"miracle" transistor
part number that will make this work. I would still like to have a 3 or
3.3v version but it looks
like i'd have to pretty much scrap the way I'm doing the board and the
external leds.

If anyone would like to make any suggestions I've upped the images again to
http://members.shaw.ca/gnat/led.htm

I think I can see the light here and I really appreciate all the help from
everyone! THANKS!

Cheers

Dave

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2002\02\20@061737 by Russell McMahon

picon face
Dave,

I think your application is almost certainly entirely &v easily doable with
3.3 V but I am somewhat uncertain EXACTLY what you are trying to achieve.
I have looked at your posts and website cct. While I THINK I know what you
want I've seen enough such things to realise there may be something required
which isn't quite spelt out.

If you could spell out as exactly as possible what you want to achieve -
RATHER THAN how you think you might achieve it, then I and others can almost
certainly help.

Is it

- Turn LED display on and off under PIC pin control.
- Operate from 3V3 supply.
- Operate common cathode LEDS
-  Drive LEDS with up to 350 mA total consumption at any one time.
- LEDs operate OK with <= 2.5V drop across them.

   ie are these LED strings or single LEDS?
   What is the voltage across the LEDs when running at full desired
current.

If this is the whole requirement then consider it done.

Note that you should almost certainly NOT be using an emitter follower as
you are here with such low supply voltages.
The emitter follower has a MINIMUM 0.6v drop across it. Unacceptable in this
application.

In your latest circuit replace the MPS3904 with a PNP bipolar transistor
with suitable rating.
Swap collector and emitter connections.
Drive this transistor's base directly from a PIC pin with a 470 ohm
resistor.
LEDS will be on when PIC pin is low.
A second driver transistor can be arranged if needed when we know answers to
the above questions.

The above PNP needs to be suitably high current, high Beta (current gain)
and low saturation.
A Zetex ZTX749 would be very suitable but there are others.
A BC327 may be marginally suitable but is a good starting point.

A FET may be possible but would need to be a logic FET with an extremely low
threshold (operating) voltage and you don't really need one here.

If there are other requirements please specify fully.

If you copy your response to RemoveMEruslTakeThisOuTspamparadise.net.nz I will probably see it
sooner.



           Russell McMahon

.


     Russell McMahon
_____________________________


> If you've been looking at this trying to sort it out I've uploaded a
version
> with actual resistor values and such as well as showing a image of the
signal
> that is at the base of the transistor Q23.
>
> I haven't played with the resistor vales on the emitter side of Q23 yet
> (R21, R23, R24)
> so you can ignore these components for now. A few of you have mentioned
the
{Quote hidden}

to
{Quote hidden}

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2002\02\21@024856 by Russell McMahon

picon face
> >-  Drive LEDS with up to 350 mA total consumption at any one time.
>
> Drive 3 leds at 550ma each. Normal is 350ma but these can be
> over driven at 550 when pulsed.
>
> >- LEDs operate OK with <= 2.5V drop across them.
>
> 2.95 (range is 2.3 to 3.3)
>
> >     ie are these LED strings or single LEDS?
>
> These are 3 single leds (big ones...)
>
> >     What is the voltage across the LEDs when running at full desired
> >current.
>
> 3.0v  (but these will operate down to 2.31)

If you make the mods I suggested before you will get a working but not ideal
solution which can then be refined.

You are going to have to look carefully at current versus voltage across the
LEDs, what level you MUST have for this voltage, what the variations are
between LEDs and with temperature. The problem is that the voltage available
is VERY close to what you need. The normal practice is to have current
limiting resistors which are large enough to make the supply used 'look"
somewhat like a current source. You do not have that luxury here. Knowing
the exact LED type and characteristics and their spread and variation in
operation is essential. That said, something that works "OK" will be easy
enough to do (probably).

What is the LED part number(s)?
Where can an on web datasheet be found.

An active current source may be desirable. Whether this requirement is more
annoying than reworking the board to 5 volts is something you will need to
determine. Simple current sources using a transistor require drops of at
least 0.6 volts plus whatever is dropped across the controlling resistor.
Losing 0.6v is not an option here. Current sources using 1 op-amp and a
transistor will work well here. The opamp is cheap and compact but may be
annoying to add depending what stage the rest of the design is at.

A very useful set of questions is - "

- What current will the display draw if connected directly to 3.3v,
- Is the brightness and current draw acceptable,
- Is this outside the display's specs"
- What is the maximum rated OPERATING voltage for the display.



       Russell McMahon
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2002\02\21@183345 by Alan Shinn

picon face
Dave,
Fairchild has a nice little high side driver that looks perfect for your application.
Spec'ed to run at 3.3V, drops .3V at 1A with 3.3V signal on the input
(it'll work with 3V), - needs one resistor for turn off.
Integrated with large P channel FET on the high side with a low power N
chan Fet for level shifting and with a couple of protection diodes here
and there.
$.22 ea @ 1K pcs.
pn  FDC6323L Integrated Load Switch
http://www.fairchildsemi.com

Hook Vin to 3.3V supply, drive On/Off with 3.3V, 1MegOhm from Vin to
high gate for turn off, and the load goes between Vout and ground.

Seems good to me but note that I have not tried them. I bet you could
parallel them for less drop. Oh, also note that they are SuperSOT-6
package  (read as small surface mount device)
For general interest, these high side drivers come in different ratings.
$.32 gets you .1V at 2.5A with 2.5V drive



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2002\02\22@025907 by Dave King

flavicon
face
Hi Alan

This just seems too good to be true ;-] Its a cruel hoax by Fairchild who will
discontinue the part the day before I order ;-]

I'll put you on my beer distribution list if this works out ;-]

>$.22 ea @ 1K pcs.
>pn  FDC6323L Integrated Load Switch

If it works this is much cheaper than the combination of other parts ie 2222
and power trans.

My only question would be if its possible to hand solder this. I didn't see a
through hole model and I haven't tried to solder a sot 6 before with an iron.

Cheers

Dave

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2002\02\22@032633 by Stuart Meier

flavicon
picon face
Lots of discussion on hand-soldering this scale (and much smaller) in
the archives.

I do it all the time, and would NEVER drill a hole if I can avoid it.

My approach for SMT (my designs are mostly SMT)
1 flux the board with spray rework flux
2 pretin all traces and pads as thinly as possible with solder
3 put a small dab of solder on pin1
4 with IC held by tweezers in left hand, and iron in right, melt the dab
and s l i d e the ic into alignment
5 solder farthest pin (eg pin 5 on a soic8) to lock alignment
6 solder remaining pins one side
7 solder remaining pins other side

Doing dozens one after the other, takes me around 20 seconds for 8pin
IC. (find it is easiest to rotate 180 degrees after steps 4 and 6, so I
do ALL ICs for step 4, rotate, do all steps 5 etc etc)

Stuart Meier

{Original Message removed}

2002\02\22@153429 by Dave King

flavicon
face
At 08:24 AM 2/22/02 +0000, you wrote:
>Lots of discussion on hand-soldering this scale (and much smaller) in
>the archives.
>
>I do it all the time, and would NEVER drill a hole if I can avoid it.
>
>Stuart Meier

Thanks Stuart

I'll dig through the achieves and sort it out. Was just curious to know
how big a pia this would be.
I've saved and printed out every mention of the toaster oven and will
try that one of these days too. I've found it absolutely amazing how much
has changed (and I've forgotten) not having played with this stuff for
10+ years...

What I'm finding real pathetic is I actually get excited when I see someone
mention a low power transistor to use....  ;-]

Cheers

Dave

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2002\02\23@221924 by Russell McMahon

picon face
> This just seems too good to be true ;-] Its a cruel hoax by Fairchild who
will
> discontinue the part the day before I order ;-]
>
> I'll put you on my beer distribution list if this works out ;-]
>
> >$.22 ea @ 1K pcs.
> >pn  FDC6323L Integrated Load Switch
>
> If it works this is much cheaper than the combination of other parts ie
2222
> and power trans.
>
> My only question would be if its possible to hand solder this. I didn't
see a
> through hole model and I haven't tried to solder a sot 6 before with an
iron.


Don't be seduced by the dark side of the force, Luke, er Dave.
This part indeed sounds wonderful BUT do you need it.
Did you follow what I said about using a single pnp transistor and 1
resistor (possibly 2 resistors).
There is no need for an extra "power" transistor if you choose the part
well.
It is possible that the IC will be cheaper than a single transistor but
maybe not.
For small volumes or hand soldering the transistor is also likely to cause
you less mechanical pain.

Required:
1 x PNP TO92 transistor.
1 x 470r or so resistor
Try BC327, ZTX749 or anything you can find with similar specs.
The Zetex ZTXxxx Elines generally have better specs than almost anything
else but are not as cheap.
A BC327, which may work OK, costs about $US0.05 in any sort of volume (or
less).

Transistor emitter to 3.3v+
Transistor collector to load.
Transistor base to 470r.
Other end of 470r to PIC pin.

Done.

If you want this to turn OFF when the PIC is powered down then add a 10k
from base to 3.3v+
If you want it to turn on (and start the PIC) if the PIC is asleep (which
you probably don't) connect the 10k from base to ground.

Resistor values will need a little thinking about.
In above case with 3v3 supply the PIC provides SAY 0.5v low to turn on.
460r sees (3v3-0.5) = 2.8v.
Resistor = 2.8/470r = 6mA
PIC can easily supply this.
Transistor needs to supply 350 mA so beta required = 350/6 = 60 or more.
The BC327 will have rather more than this and most applicable Zetex
transistors will have much more than this.

Vsat should be a few tenths of a volt at worst.

I'm not sure what the attraction of using the dark side solution is but by
all means try it.

Note that both solutions do NOT address the current limit problem which I
and others mentioned.
(Unless the driver has selectable current limiting).
That's why I asked the questions about how the LEDs would go on 3.3v or
close to it.
You need to be able to answer the questions I asked to be able to actually
design this rather than just hoping it will work OK.

Tr the one transistor solution -  you may be pleasantly surprised (and maybe
not :-) ).


       Russell McMahon

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