Hi,
as a newb with electronics, I've decided to make my own bench power supply from an old computer psu. When I did some testing I discovered that the undervoltage protection shuts the psu off when applying a heavier load (a few amps on 12V rail) even though it's rated for more than 20 amps on 12V.

So I've decided to disable the protection, since I've read that it's a common practice. I've looked at both the schematics of the psu and datasheet of the IC, but I couldn't figure out which pin I have to disconnect/ground, so I ask for your help. If anyone could take a look at the schematics and share your idea, I would be really grateful

I've attached the schematics and the datasheet.
Thanks!

 

Try putting some load on the 5V before loading up the 12V.
It is probably the overvoltage on the 5V line causing your problem.

 

Your schematic is one of an old, very old ATX form factor 200 Watt PSU. That schematic has been floating around the internet for a few decades or close to it. You are sure you have that 200 Watt PSU? If not what PSU do you have? and if an older PSU does it have the main output 20 or 24 pin connector? Most of the newer PSUs regulated off either the 5 volt or 3.3 volt outputs so as mentioned by Albert you want a load out there. Typically a 10 Ohm 10 Watt resistor was used. There is likely a dozen how to articles about using an ATX form factor PSU as a bench type power supply available with Google. Most will work but this generally comes down to the specific PSU you have. Some load the 5 volt buss while others the 3.3 volt buss. A Google will bring up the pin outs for the 20 and 24 pin connectors.

Ron

 

Your schematic is one of an old, very old ATX form factor 200 Watt PSU. That schematic has been floating around the internet for a few decades or close to it. You are sure you have that 200 Watt PSU? If not what PSU do you have? and if an older PSU does it have the main output 20 or 24 pin connector? Most of the newer PSUs regulated off either the 5 volt or 3.3 volt outputs so as mentioned by Albert you want a load out there. Typically a 10 Ohm 10 Watt resistor was used. There is likely a dozen how to articles about using an ATX form factor PSU as a bench type power supply available with Google. Most will work but this generally comes down to the specific PSU you have. Some load the 5 volt buss while others the 3.3 volt buss. A Google will bring up the pin outs for the 20 and 24 pin connectors.

Ron

Thank you for your answer, Ron. The schematics are not for the psu that I have, but it seems to use the voltage control ic the same way. I have a Trust 420W big fan low noise PSU with a 24 pin main connector, but I can't find the correct schematics. I've tried using a 10 ohm 10W resistor on both 5 and 3.3V busses but the psu still shuts down the same way, so disabling the protection circuit seems like the only solution.

 

In that case the PSU may be faulty and in that case disabling the protection may be a very bad idea.

 

Something you can try and this is a guess. Pin #8 is a PWR_OK line and a grey wire. This normally goes High after the PSU starts, If I recall it gets a logic high from the CPU on the motherboard. You can try making the line high and see if the PSU will run under a load. It should have worked with what you tried. As to disabling the other sense lines you got me as I never had to do it.

Beyond that I agree with Albert in that this PSU should not be shutting down with light loads.

Ron

 

PWR_OK is an output from the supply to say that the voltages are within specification. Do not feed anything into this pin.

 

My bad and thank you Albert. I had it backwards.

Ron

 

In that case the PSU may be faulty and in that case disabling the protection may be a very bad idea.

If this is so, then I'll rather not mess with this psu, and get one that I'm sure is working Thanks for the heads up!