Ovp/uvp circuit

[ki4dkx]

I am building a 13.8v 35a ps and have most of the circuit drawn. The part I am missing is an under voltage circuit. I am using a 15v zener to trip an scr for the over voltage. I would like to trip the same scr with an under voltage circuit. Note; need to keep the parts count to a minimum as room on my already drawn board is limited. Thanks for the help.

 

[JimB]

Why do you feel the need to trip on undervoltage?

I assume that this PSU will be used to power a radio transceiver.
The radio will not come to any harm in an under voltage situation, not like it would with overvoltage.
Also, if you think about it, if you have an undervoltage trip, it will not be possible to start the PSU unless there is a start-up delay to allow the output voltage to rise above the trip point when the PSU is initially switched on.

An undervoltage alarm rather than a trip may be useful, although I am not really convinced.

An overcurrent trip would be usefull to protect the PSU in case of a short circuit across the output.

JimB

 

[ki4dkx]

Hello Jim
Your assumption is correct, I have a fuse in line for over current but something a little faster might just be in order. I am using a ballast resistor on the output transistors. Is there a quicker way to get the current trip I need? I have done some building early on but not really to this amount of output. Thanks much for the help.

 

[JimB]

I have a fuse in line for over current but something a little faster might just be in order

It certainly would.
Often as not the transistor blows faster than the fuse.

I am using a ballast resistor on the output transistors

What is a ballast resistor in this context?

I dont have any instant suggestions for an overcurrent circuit.
When I last built a similar PSU a few years ago, the overcurrent protection circuit was quite complicated in some ways, because of the odd circuit arrangement with the regulator transistors (MOSFETS actually).
The basic regulator circuit was not my original design, but the overcurrent trip was.
I cannot remember if the circuit which I was "copying" had an overcurrent trip, if it did, I decided that I wanted something different and a lot more complicated.

Can you post a schematic of your PSU circut, then we can see what sort of overcurrent circuit is appropriate for you?

JimB

 

[MikeMl]

I would just buy a $8 LED voltmeter, and mount it inside the power supply.

 

[JimB]

A good idea Mike.
But what does that have to do with overcurrent protection?

JimB

 

[MikeMl]

A good idea Mike.
But what does that have to do with overcurrent protection?

JimB

The OP wanted low-voltage indication. I gave him a way of reading out the actual voltage...

 

[ki4dkx]

ki4dkx power supply

Ok here is the diagram with a small description of the happenings. Also thanks for the help in posting this. Joel

 

[fernando_g]

It is fairly simple what you want to do, because you already have ballast resistors.

If those resistors drop about 0.7 volts at overload conditions, I would do the following:

-Take a general purpose PNP transistor, like the 2N3906. Call it Q11
-Connect Q11 emmiter to the junction of R21 thru R24 that comes from the positive source.
-Connect Q11 base to the junction of R21 and Q6's collector. It could also be R22/Q7, etc. Pick one pair.
-Connect Q11 collector to the junction of R30, C20 and SCR4's gate.

Whenever the current causes the voltage drop to increase above Q11's base turnon voltage, it will source current to the SCR gate, turning it on.

I built the circuit exactly as described over 35 years ago.
The only caveat: The current limit is not very precise. You have to give yourself some on the ballast resistors. Say, that they do not drop more than 0.5 volts during normal operation, to prevent nuisance tripping.

MikeMi's idea of adding an inexpensive voltmeter is a good one. Or you could use a bargraph driver like the LM3914. Back then I used an analog voltmeter.

Last piece of advice...even though you are using ballast resistors, try to beta-match your transistors.

 

[JimB]

Hmmm...
There are a LOT of problems with that circuit.

1 The supply voltage is 30 to 40v.
Let us say that it is 36volts.
When the PSU is supplying 20amps at 12 volts (240watts) to the load, the transistors Q6 to Q9 will be dissipating 480watts. Twice as much power as is supplied to the load.
You will need a BIG heatsink.

2 The outout voltage will vary with the load.
The output of the LM317 is nicely stabilised, but just using that output to drive the bases of the pass transistors (via Q5) removes the output voltage from the control loop of the LM317.

3 Resistors R21 to R24 should be in the emitter connection of each transistor Q6 to Q9 if you are to get correct current sharing between each transistor.
The set-up shown will not achieve that.

4 The overvolt protection thyristor SCR4 will turn off the supply to the LM317, and so turn off the base drive to the pass transistors.
But what if one of the pass transistors has a collector-emitter short circuit?
Turning off the LM317 will not be effective, and the output will have the full 30 to 40 volts of the input supply, the output cannot be controlled.

There are probably one or two more things, but for me these four points would tell me not to use this circuit.

JimB

 

[ki4dkx]

Ok let's say that I need to change things, other than the position of the ballast resistors what would make this better.

 

[ronv]

One of the best things you could do is to lower the input voltage as stated so the power in your transistors is not so high. But I bet you have a transformer already so then I would use a switching regulator.

 

[ki4dkx]

Ok JimB you have given me some good advice. If there is a better way please let me know. It is obvious that I am rusty at this. Anything to make it right. The last thing I want to do is burn up a rig.

 

[ki4dkx]

Something I should say is the txfmr I am using is a 12v 20/40a (current output not sure of) it came from a computer backup supply. The filter caps are the smallest I have. The total rating is between 120 to 250kuf (over kill I'm sure) at 30v. The rms of the txfmr is about 16v.

 

[JimB]

If I were building a PSU, I would consider using an LM723.
This is a very old regulator IC, ( I first used one around 1975! ), but it is quite versatile and its current rating can be extended by using external pass transistors.

Over the years there have been comments that the 723 can do some funny things if RF gets into the circuit.
I think that this is true of any device, especially if the builder of the PSU does not take care with decoupling the cables going in/out of the PSU case.

Have a look at the following links.
They both discuss PSUs using the 723.

**broken link removed**


JimB

 

[MikeMl]

Check out the schematics of the Astron linear power supplies. I have been using the 35M version for years to power my Ham station. They use the 723 chip, and are a good example of how to apply it.