Home-Made 12V DC Power Supply

[Gerowen]

So like many of you, I have several old junk computers that are inop for one reason or another.  I'm salvaging still working parts from them before I take them out to my rifle range and fill them full of holes and sell them for scrap.

 

I had the idea of taking an old ATX power supply, wiring in an on/off switch to replace the computer's power button, and removing all of the wires except one ground and one 12V positive wire.  This gives me an extra 14 amp power supply to test CB/HAM radios that I'm working on, as well as other devices you would normally use in your car.  Tonight I verified it is functional.  Tomorrow I'm going to dig out the dremel and remove the case, and cut out spots for the switch and some red/black studs so that all of the wires will remain inside the case, and I'll have two color coded studs to attach electrical wires to instead of splicing/twisting each new device and fatiguing the wires built into the power supply.

 

I tested 3 different power supplies, and although all of them "said" 12V DC on the stickers, they all came in a little on the low side, but it's within safe operating range so what the heck.  The longest part of the process was unsoldering all of the extra wires I didn't need from the PCB and removing them.  Between making sure I was soldering the correct spot on the back side of the board and waiting for my soldering iron to melt those huge globs of solder at points where 3 and 4 wires ran together, it took about 30 minutes just to do that.  I thought about cutting them off and leaving them inside, but then I figured that eventually through movement one of the hot wires inside would end up touching the outside shell of the power supply or something else important and frying the whole thing, so I removed them completely from the PCB and left only the ones I needed.

 

Anyway, this is the first time I've done this, and thought I'd share.  The CB radio in the picture is just an extra I had laying around to test it with, and is running on the power supply in this picture.

 

[Ian S.]

Impressive!

[Gerowen]

I am going to do some research and see if there's a way to crank it up to between 12 and 14 volts.  11.8 will work, but it's a tad low, and CB radios will "work" on 12, but they are designed for optimal performance at 13.8V, so I'd like to crank it up a bit if I can.

[Obi-Wan Kenobi]

Very cool man! Thank you for sharing. I wanted to build one too, just never got around to it. Good post, let us know how it goes! (y)

[Gerowen]

After being un-happy with the 11.8 volts I was getting, I managed to find an adjustable pot inside the power supply and managed to get it up to between 12.8 and 12.9 volts DC.  When I turn on the CB radio it drops a little, and when I key down it dips to about 12.5, but no more than what my other factory made power supplies do.  Just a little bit of glue to hold the switch down in place and it's g2g.

 

This is actually a separate power supply.  In this one I decided to leave all of the wires in place and just cut them off short and wrap them in electrical tape.  That allows the possibility of re-wiring it for higher or lower voltages in the future.

 

[vcfan]

if you open the power supply,make sure you know how to properly discharge the caps before working on anything. you can get shocked with hundreds of volts.there are usually shunt resistors to drain the caps in a bit after unplugging,but do you seriously want to put your life at risk and in the hands of a potentially shoddy manufacturer?

[Gerowen]

if you open the power supply,make sure you know how to properly discharge the caps before working on anything. you can get shocked with hundreds of volts.there are usually shunt resistors to drain the caps in a bit after unplugging,but do you seriously want to put your life at risk and in the hands of a potentially shoddy manufacturer?

Yeah, already torched one power supply by accident.  I had half the case off to drill holes in it for the wire studs and switch, and I couldn't remember exactly where I wanted it, so I held it over top of the power supply which was turned off, but plugged in, with the intent of seeing where the open spaces were, and accidentally grounded out one of the voltage rails, so it's toast.  Generally speaking though I always unplug everything and give it a chance to wind down before doing anything.

[n_K]

If it's low be default then it's probably because you've not attached the length of the 3.3v wire to the 3.3v sense wire (it should be as long as your longest 12v lead from the PSU).

No modern PSU has an adjustable pot for controlling the output voltage, it's all done automatically via feedback from the 3.3v sense only.

[Gerowen]

If it's low be default then it's probably because you've not attached the length of the 3.3v wire to the 3.3v sense wire (it should be as long as your longest 12v lead from the PSU).

No modern PSU has an adjustable pot for controlling the output voltage, it's all done automatically via feedback from the 3.3v sense only.

I have another one here that's only outputting about 10.5 volts on a 12 volt wire.  I have found what I "think" is the 3.3 volt sense wire and connected a 3.3 volt wire to it, but no change.  I can't seem to find any information on this sense wire though so I'm not sure what color wire I should be looking for.  Any ideas?

[n_K]

If you still have the PSU output connectors attached, you should be able to find it right away.

On the 20 pin cable, there should be a 3.3v rail with 2 wires instead of 1, one being orange (the rest should all be just 1 wire) the other can be any colour. The non-orange cable should be the 3.3v SENSE cable.