Using a computer power supply as a bench supply

Thread Starter


Joined Oct 4, 2011
I have an ATX power supply from and old PC and want to convert it to a 12V 5V and 3V bench supply to run a charger to power up lipos etc. I have connected all the 12V 5V and 3V and -Ve cables together and connected the green cable to ground.

When I first just connected the green wire to ground with all the computer plugs still connected to the leads but unplugged from the computer the unit worked fine. The fan was running and all seemed OK. Now that I have removed the plugs ends and have all the cables grouped together with the white and purple are left not connected to anything, all that happens is the fan spins up for a fraction of second and stops. There is no 12V 5V and 3V at the terminals.

What am I doing or have done wrong?

Many thanks Steve


Joined Oct 2, 2009
Some SMPS require a minimum load before they start/continue operating.
Try installing a resistor load on the 5V supply line.


Joined Jan 28, 2011
While you're at it, look up one or two of the many articles on the web on how to perform such a conversion, then compare that with what you have in front of you.


Joined Apr 28, 2012
They are rather bad to use for such purposes, no good short circuit protection.

Electronic transformer is better, and not expensive.


Joined Mar 24, 2008
On the other hand, computer power supply power supplies are much cheaper, even free, in many cases, much cheaper than other alternatives. It is a common project, if you were to do a search on them you would find many writeups.

I buy into the concept that current limiting is needed. But it is not absolutely necessary.


Joined Dec 26, 2009
Have you tried applying the +5V to the power OK terminal (pin 8 on an ATX Power Supply - I believe this is normally a gray wire).
The power on pin (pin 14 on an ATX power Supply) will turn on the power supply, but it will shut down if +5v isn't supplied to the Power OK pin (pin 8 on ATX units). And yes - you can use the +5v from the same power supply to keep it running, just in case you were wondering.
Incidentally, here is a link to converting a PC Power supply to a bench power supply..