Hello, i was planning to use my old pc power supply, to make lab power supply. When i plugged it to the main voltage, everything works normal, normal output voltages and such. The problem came when i tried to connect the load on the 12v rail, it keeps shutting down, due to the overcurrent protection (it may be undervoltage actually), although the current wasn't more than 2 amps, btw it says 15A output current for 12v rail. So i tried to bypass shutdown protection on ka7500 chip, by pulling down voltage on pin 4. That worked, but now when i connected same the load as before, there is some high frequency hissing sound, and 12v rail voltage drops to about 11.2v (this is what may be causing undervoltage protection, and shutting down the smps) although it draws less than 2 amps. I have to mention that while working on that, one of the primary driver tramsistors blew up, so i replaced both of them and the same results, nothing is even warm. Can someone help me to solve this problem, cause I don't want to spend money on another pc power supply.

 

Try putting a load of a few (Say 5) amps on the 5 volt output. The regulation feedback is normally done from the 5 volt rail. The main load on a PC powe rsupply is on the 5 volt rail.
Les.

 

Try putting a load of a few (Say 5) amps on the 5 volt output. The regulation feedback is normally done from the 5 volt rail. The main load on a PC powe rsupply is on the 5 volt rail.
Les.

Exactly the same, seems like the output voltage starts to drop even if I connect 100mA of load, and i cant even get 5A from 5v rail, if i try to connect it to 1 ohm of a load, the current is only about 3 amps, because voltage drops to 3v, and there is very loud hissing high frequency sound

 

It's not worth trying to repair PC power supplies as they are quite cheap to buy new. It would be better to buy a power supply designed for the voltage and current you require or make one yourself.
Les.

 

@MEAx
Days working to modify an ATX PSU to a bench PSU and I am exactly in the same situation as you are.
Voltage varies nicely from 0 to 30 volts but as soon as connecting a <100mA load the PSU starts to hiss.
Not to mention I can draw up to 10 A with a 1 ohm ceramic resistor and the PSU blower to dust due to mistakenly connecting a 0.1 ohm instead of 1 ohm with 3 volt settings.
Now I am trying to figure out the hissing problem (solving the roots) and a current limiter before attempting another one.

 

Hello, i was planning to use my old pc power supply, to make lab power supply.
...
Can someone help me to solve this problem, cause I don't want to spend money on another pc power supply.

if you need lab PSU get lab PSU. you will not regret it. i don't know how much you are paying for PC supply but low cost lab PSU that does 0-30V 0-10A is under $50.

the reason PSU is hissing is because it is defective and overloaded. as a result frequency drops and you hear it as hissing.