ATX Psu 12v rail undervoltage

[ITSYSTEMSALBANIA]

Hi all

I have an old delta electronics atx psu in the 12v rail output 10v no load when i connect a load voltage go to 9v-9.5v i have opened the psu the capacitors looks fine.Please help me to trubleshoot this psu.

[budm]

Re: ATX Psu 12v rail undervoltage

Bad caps can look normal, you need to check the capacitance and the ESR of the caps.
BTW, other Voltage rails are OK?

[ITSYSTEMSALBANIA]

Re: ATX Psu 12v rail undervoltage

Hi

Thank you for reply

All other rails are fine only 12v rail is undervoltage i have changed the output cap of 12v rail and nothing has changed the cap was 16v 2200 micro farad and i dont have an esr meter.

[stj]

Re: ATX Psu 12v rail undervoltage

foto's ?

[Agent24]

Re: ATX Psu 12v rail undervoltage

Maybe the 12v rectifier is bad

[ben7]

Re: ATX Psu 12v rail undervoltage

It might regulate the 5V output, so if there is low/no load on that, then yes the 12V will be low. Try loading the other outputs as well.

[stj]

Re: ATX Psu 12v rail undervoltage

i would say replace any small caps near opto-isolators incase the feedback is the problem.
it could mistake ripple for actual constant voltage.

[ITSYSTEMSALBANIA]

Re: ATX Psu 12v rail undervoltage

It might regulate the 5V output, so if there is low/no load on that, then yes the 12V will be low. Try loading the other outputs as well.

Hi

Thank you very much this was the problem i connected a dvd rom and measured the 12v rail the voltage was 11.6v i connected a 5watt resistor between ground and +5v and the 12v volt rail is 11.6v.

[Agent24]

Re: ATX Psu 12v rail undervoltage

11.6v is still a bit low...

I had a really old Enermax PSU which had this behaviour too, low voltage rails without load, which I had never seen before on an ATX PSU - all other ones I've seen keep the voltage in spec even without a load.

It worked OK for about a week or two, then the PWM controller (UC3842 or whatever) exploded.

[Th3_uN1Qu3]

Re: ATX Psu 12v rail undervoltage

If it's a really old PSU that kind of behavior is normal for some designs. Prior to the Pentium 4 and later model Athlon XPs, most motherboards had the CPU VRM powered by the 5v rail so there was always load on 5v, and 12v was mostly used just for hard disks, CD-ROM drives and fans.