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Old 22nd February 2007, 07:35 AM   #1
luka is offline luka  Slovenia
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Default How to make current limit for PC supply?

Hi

How to make current limit for PC supply, but not that kind that shuts down the supply. Something like small current sensing resistor, optocoupler that is connectet to error amplifier or somewhere else on driving chip...

When over current would occurred, opto would start to open and narrowed the gate pulses...

How to do that?
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Old 22nd February 2007, 03:18 PM   #2
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Luka,

I would say that the output from your opto should drive a signal-conditioning op amp, and feed its output into the deadtime pin, pin#4 of the TL494. I assume you're using the '494, right?

Steve
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Old 22nd February 2007, 04:34 PM   #3
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Hi

It is a good chance that it is.So this would increase dead time? Would i need any special opto or any would do?Also would I need anything else like transistor,...?
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Old 26th February 2007, 06:40 AM   #4
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Try any opto with a good CTR (current transfer-ratio) of, say, ~100%. The CNY17-2 & CNY17-3 are a couple of good examples. You could use a transistor to drive the opto, but an op-amp would probably be better. Also, use an op-amp on the primary side coming off the opto as your signal-conditioning amp. This one will drive the deadtime pin.

As drawn current increases to the selected trip point, the circuit will insert additional deadtime until the "overload" condition is removed.
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Old 26th February 2007, 06:53 AM   #5
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Hi

Would I need 1.1xV drop on resistor if I would want to open opto, wich would be parallel to resistor(simplest)
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Old 26th February 2007, 07:04 AM   #6
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You could, but your operation would be crude, at best. try a 0.01W 2-5W resistor in series with your (+) output and take the voltage developed across this resistor to drive your op-amp in differential fashion. Have that op-amp drive the LED-half of the opto.

Using op-amps will give you tighter control and allow some adjustability.
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Old 26th February 2007, 07:15 AM   #7
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Hi

The problem is that I would like to limit current to 20-25 amps.
Somehow like that

Click the image to open in full size.

Does opro really opens at 1.xxV or does it open earlyer enough for dead time?
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Old 26th February 2007, 07:44 AM   #8
luka is offline luka  Slovenia
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Hi

If this would work I would be really happy

Click the image to open in full size.
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Old 27th February 2007, 12:25 PM   #9
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Luka,

I would play around with the values of R1 & R3 of your last post. R3 controls the trip point, and R3 controls the LED current. BTW, What is the PNP with its collector & base tied together? Is this some sort of diode?

Steve
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Old 27th February 2007, 02:45 PM   #10
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Hi

Never mind uper one, what I tried to do was when there would be 0.6-0.7V on resistor, transistor would open and draw current from E to B and C.

Other one is better one. I don't think I can change R1 or R3, since at this point as it is, R1 would need to be low value as possible, but so is R3 too to react to lowest voltage cross resistor. Am I thinking this right?

I really don't want to use op-amp here if not necessary.
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