PC Power project

Thread Starter


Joined Mar 26, 2009
I need to build a 12 VDC input computer power supply with the following output:

+12V / 12A
-12V / 1A
+5V / 20A
-5V / .5A
+3.3V / 10A

I realize that the 12 - 14 VDC input from a car battery will need to be inverted into a square or triangle wave to feed a main power bus to which individual transformers are connected. Once the transformers are wound and connected to the buss with an output of 1/4 to 1/2 a volt higher than what is needed of each voltage. Once the individual buses are powered then they will need to be rectified and filtered. I'm guessing that once they are filtered they should still be a little higher than the needed output as each one will then be fed into a voltage regulator to fine tune the output voltage. I'm probably forgetting something, so I wanted to post this to the forum. I'm still an adolescent at best when it comes to building power supplies. The need for this is in a backup situation. It seems silly to take 120VAC step down the voltage to 12 to 14 VDC then drive a battery charger for a UPS, only to reverse the process when the mains voltage drops. This would be used to drive a small and compact PC for emergencies in conjunction with my ham radio. I've already got everything in my ham world running from batteries and would like to build a small PC to take along for digital traffic over the ham bands in the case of emergencies.

Thanks for any and all input,

Eric Martin, N5EMX
Faulkner County Amateur Radio Club


Last edited:


Joined Apr 14, 2009
Just a thought...why not just get a small inverter to connect to your batteries, and connect that to a conventional ATX power supply? The switch-mode power supplies for PCs are pretty efficient anyway. That would save lots of trouble.

Otherwise, I think your idea with an inverter, transformers (or a single transformer with multiple taps) and rectifiers/regulators would be fine. You would also need to include some good low-pass filters, to prevent switching noise from making it into the computer.

A 3rd option is to build your own fancy switching supply. That would be the most difficult option. A search of this site or google should turn up some SMPS designs.


Joined Jul 17, 2007
An automotive battery would be a poor choice for powering your supply, as they're not designed for deep discharge. You would have a very short lifespan from such a battery in your application.

For longest battery life, use deep-cycle type batteries, of high enough AH rating that you won't need to discharge them more than 30% for any expected contingencies.