Using a ATX power supply for other things

[spacegoogie]

I have an ATX power supply that I use to power up and test other things. My question is, if I wanted to power something that was 24v or even 17v can I combine a Yellow (12V) and a red (5v) to get a 17v line of power or even two yellows for a 24v of power on one line or will combining wires like this short it out?
Thanks guys!

[SavannahLion]

To answer your question.

No, PSU's don't work that way. When you join a 12V + 5V you actually get 7 Volts. The difference between 12 and 5. Clarification on this point: 12v on the positive, 5v on the negative (eg for a fan) gets you 7v. You don't use the black ground for this. If you join 12v + 5v on the anode and use the ground (black) on the cathode, I believe you'll definitely get an imbalance on the rails and burn the PSU out. You won't get 17v.

When you join 12V to 12V, you won't get 24V, you may increase the amperage at best, most likely nothing will happen and at worst, you'll get an imbalance on the rails and blow the PSU out (not likely).

There is a really good write up on how PSU's work and how to modify them into project power supply boxes. I'll see if I can find it.

[spacegoogie]

To answer your question.

No, PSU's don't work that way. When you join a 12V + 5V you actually get 7 Volts. The difference between 12 and 5. Clarification on this point: 12v on the positive, 5v on the negative (eg for a fan) gets you 7v. You don't use the black ground for this. If you join 12v + 5v on the anode and use the ground (black) on the cathode, I believe you'll definitely get an imbalance on the rails and burn the PSU out. You won't get 17v.

When you join 12V to 12V, you won't get 24V, you may increase the amperage at best, most likely nothing will happen and at worst, you'll get an imbalance on the rails and blow the PSU out (not likely).

There is a really good write up on how PSU's work and how to modify them into project power supply boxes. I'll see if I can find it.

Just the answer I was looking for. Thanks much. 

[WhereEaglesDare]

you can combine the 12 volt bus with the -12 volt bus to get 24 volts.  I use it to run a bank of 24 VDC fans.  Make sure that you dont let either the 12 volt or the -12 volt to touch a ground bus you may have.  Ive ran my fans, 6 inches and I run 6 of them in parallel, for days at a time with no issue.

Just remember when dealing with voltage, especially in DC, Voltage is a difference in potential so if you put 12 and 5 together you get 7 but if you put 12 and -12 together you get 24.

[SavannahLion]

Aha! It took a while to find it. I guess I never bookmarked it.
http://web2.murraystate.edu/andy.batts/ps/powersupply.htm

Please try not to set you or your house on fire.

you can combine the 12 volt bus with the -12 volt bus to get 24 volts.  I use it to run a bank of 24 VDC fans.  Make sure that you dont let either the 12 volt or the -12 volt to touch a ground bus you may have.  Ive ran my fans, 6 inches and I run 6 of them in parallel, for days at a time with no issue.

Just remember when dealing with voltage, especially in DC, Voltage is a difference in potential so if you put 12 and 5 together you get 7 but if you put 12 and -12 together you get 24.

That's true. I forgot PSU's have -12v. I was thinking -12v was removed in modern PSU's, but I think it's actually the -5v. I'm not in the mood to Google-fu and find it.

[WhereEaglesDare]

Aha! It took a while to find it. I guess I never bookmarked it.
http://web2.murraystate.edu/andy.batts/ps/powersupply.htm

Please try not to set you or your house on fire.

you can combine the 12 volt bus with the -12 volt bus to get 24 volts.  I use it to run a bank of 24 VDC fans.  Make sure that you dont let either the 12 volt or the -12 volt to touch a ground bus you may have.  Ive ran my fans, 6 inches and I run 6 of them in parallel, for days at a time with no issue.

Just remember when dealing with voltage, especially in DC, Voltage is a difference in potential so if you put 12 and 5 together you get 7 but if you put 12 and -12 together you get 24.

That's true. I forgot PSU's have -12v. I was thinking -12v was removed in modern PSU's, but I think it's actually the -5v. I'm not in the mood to Google-fu and find it.

I think you are right about the -5 bus, I remember it use to be there now that you mention it, but it isnt any longer I dont think.