[aprssig] computer power supply question

[Rick WIdmer]

On 12/31/2010 2:44 PM, K8YS wrote:
> Hey guys...
> I need a cheap 12vDC supply to power a computer monitor. I have a 120W PC
> power supply that was removed from an HP piece of garbage.

I consider the $10 to $12 typical buy it now price, with free shipping, 
on eBay pretty cheap.  Also take a look at the local second hand stores 
like Deseret Industries, or the Salvation Army thrift store.  Most have 
a box of random power supplies for a couple of bucks each.


> Does anyone know what pair/pairs need to be shorted to turn the power supply
> ON?

Google knows...  Search for "atx power connector"


The third result down has a good answer for your question:

http://www.duxcw.com/faq/ps/ps4.htm

The power switch connects between 14 the control input and 15 a common wire.

> "Shorted" may be wrong, what pair do I need to place a load to have the
> supply sense that it needs to turn ON?

I am not sure but a switching power supply usually actively regulates 
its highest current output, and depends on transformer ratios to create 
the other voltages.  Back in the AT days it was a sure bet that would be 
the 5 volt, but now it may be the 3.3 volt line.

To find out, put a load similar to the monitor on the +12 line and turn 
on the power supply for a few seconds.  Using the monitor for this load 
is a good way to let the smoke out.

Either the 5 or 3.3 output should be right on, and the 12 should be way 
low.  If only one of the 5 and 3.3 lines is right on, that is the one 
you need to add load on.  If not try them both... Increase the load on 
the one that brings up the 12 volt output, until it is actually putting 
out 12 volts.

The web page cited above mentions that you should not run the power 
supply for very long with no load.


The problem is you are wasting more than half the power coming out of 
the power supply heading the load resistor.  Between the hassle of 
figuring things out, getting the load right and wasting over half the 
power you put into it, a $10 wall wart looks pretty cheap.


Rick  KD7VKZ