Adapting an old Computer Power Supply

SoledWorker · Nov 6, 2003

I've got an old "AT" power supply that I salvaged in the hope of using it as a clean 12V source. This particular unit is from the days when a switch tied into the power supply was used to turn the computer on, unlike the newer ones where the motherboard turns the power supply on.

I did some research on the internet to find out more about these devices and learned that they shouldn't be run unless connected to a motherboard. Does anyone know if it is possible to simulate the load of the motherboard with a simple resistor, or is the motherboard providing some kind of feedback that means the powersupply can't be used for my purposes? (I'm just after a source of 12V for hobby purposes, charging batteries without using the car battery/cig lighter)

Comcokid · Nov 6, 2003

Switchmode power supplies designed for use within a product usually have minimum current requirements on the outputs to operate properly. In some old AT computers, if the computer was setup to operate without floppy drives, a floppy shelf with just a large power resistor was installed to insure the minimun load on the DC output.

In short answer - Yes, resistors on the output(s) to provide for a minimun current load are needed. I do not know how much, or on which outputs.

dpc · Nov 6, 2003

You might go to

http://www.arrl.org

and check the archives of QST magazine. I recall an article maybe a year or so ago regarding use of computer power supply for a 13.8 V power source. I can't remember which issue it was, but I believe they have a search engine that might help you.

dspDad · Nov 6, 2003

Try a 12.5+ volt zener across the 12 volt output. When you supply a real load there won't be any current loss in the zener. Otherwise you'll probably have to pull about an amp from the 12 volt output.

DspDad

cbarn24050 · Nov 6, 2003

Hi, the main problem is the 5v output, as long as you load it to 50% or the rated current the others will be ok.

dhwilliams · Nov 7, 2003

I've used a PC power supply the same way - I just connected 2.5 ohms across the 5V to draw about 2A and this seemed to stabilise the 12V. Because the power dissipation is 10W I used 4 x 10 ohm 10W resistors in parallel and clamped them to a bit of aluminium to get rid of the heat.

killabyte · Nov 11, 2003

if youre usin it for hobby it will work well, ive used those power supplies a lot and i have not had any trouble.

when used to power audio it was too bad cause 6oHz peaks are very strong, high freq peaks are sometimes troublesome.
just draw some resistive power from 5 VDC and other different DC supplies. and ready to go...

if you go up to the limit current, the supply just will turn off the 12 VDC (it will be in short but no trouble) just turn off and on again.

regards

killa

andrewg85 · Dec 17, 2003

I have tried using an AT power supply just to power a cd rom drive connected to a radio as a cd player and that works without any trouble. There was no load on the motherboard connectors (which also includes 3.3 volts). Tested with a multimeter and the 12 volt and 5 volts were correct. This was fed from a 240 volt 50Hz mains. When a power supply is unloaded, the voltage may be a little bit higher due to regulation but comes down to rated voltage when a load is placed on it.

ScottyUK · Dec 18, 2003

Hi Andrewg85,

If the PSU has 3.3V output, that marks it as a (relatively) new model. The 'older' PC's didn't have any rail voltage below 5V; the lower voltages such as 3.3V and 3.0V came about as processors got faster.

SoledWorker · Dec 18, 2003

Thanks all. I was successful in getting this to work, used a pair of 2.5Ohm resistors across the 5V, and situated them near the fan. I clipped the other output wires, except the 12V and common, lead these to a couple of banana plugs and some speaker-type connectors mounted thru the case. The result isn't pretty but works great. I also stumbled across more info at:

http://www.rcbatteryclinic.com/

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