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# High DC voltage on a salvaged computer ATX power supply

(@coalrxs)
New Member
Joined: 10 months ago
Posts: 3
2019-11-17 2:23 am

After watching the DroneBot video on using an ATX power supply I salvaged one from an old Dell computer.  While it powered on after I connected the green (power on) to ground, my voltage reading for the 5VDC line indicated 10 volts and for the 12VDC line it registered 25.5 to 25.7 volts.  I did not test a 3.3VDC circuit.  I did connect a 5 Ohm resister across a 12VDC-Ground connection.  It is set for 115 VAC.

In general I spent most of my career working with rocks and avoided large amounts of moving electrons.

This topic was modified 10 months ago by coalrxs

(@codecage)
Joined: 1 year ago
Posts: 721
2019-11-17 11:31 am

@coalrxs

Welcome to the forum!

But I think I would look for another ATX supply!  Since the 5V and 12V lines were double what they should have been, the 3,3V line may have been doubled as well.  Maybe that is why the original PC no longer works? ?

SteveG

(@zeferby)
Reputable Member
Joined: 12 months ago
Posts: 380
2019-11-17 11:41 am

I'm wondering if you measured voltage between the +12v and -12v / +5v and -5v ?

Beware all voltages are relative to GND and the -12v is an independant output from the +12v one (and has very low current capacity conpared to the +12v).

So you have a multi-amperes capable 12v supply between +12v and GND, and a X hundred milliamps capable -12v supply between -12v and GND, and they are totally separate.

Same goes for +5v : the -5v is an independant output with much less juice than the +5v.

...or else...what @codecage said above...

Eric

codecage liked
(@codecage)
Joined: 1 year ago
Posts: 721
2019-11-17 1:15 pm

Good point about where he might have been measuring the voltages.  Measuring between the + and - of each of the voltages would have produced those "doubled" voltage levels.  Never even dawned on me that is something that could have easily been done.

SteveG

(@coalrxs)
New Member
Joined: 10 months ago
Posts: 3
2019-11-17 1:32 pm

@zeferby

I measured between one of the voltage wires (red or yellow) to black in each case.  However, I will check the voltages again.  I suspect that codecage's advice may be the best.  Considering the price of them new that may be the better way to go although I have another old Dell sitting on the shelf that is a candidate.

Thanks folks

Greg