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atx power supply problem

Posted: Tue Dec 28, 2010 4:50 pm
by danbob
I make bench power supplies out of atx power supplies for my friends. Basically you ground the green wire and put 1 to 2 amp fixed load on the 5 volt side and you end up with a nice 12-volt supply for your work bench. I have recently run in to a couple atx supplies that wont work like this. They start for a millisecond and then turn off. They both came out of working computers. Any help on what the problem might be would be appreciated. Dan

Re: atx power supply problem

Posted: Wed Dec 29, 2010 4:03 am
by reloadron
The method you are using should still work just fine. The green wire is the PS_ON command and is pin 16 on a standard 24 pin ATX PSU connector. Unless you have a non standard connector or non ATX standard PSU that should work. You are placing a logic low on the PS_ON line and it should turn on the unit.

The ones I have done this to I used a 10 Ohm 10 Watt resistor on the 3.3 volt line as a constant load but a load on the 5 volt rail should work just fine also.

Some power supplies actually use a 3.3 volt sense line. Look at pin 13 of the connector. If it is orange it is a standard 3.3 volt rail output. However, if it is a light gauge brown wire then it is a 3.3 volt sense wire. Now if that is the case try tying it to any orange line (3.3 volt out). Several PSUs use that sense line. The unit will not come up unless that line is connected to 3.3 volts.

3.2.2. Remote Sensing
The +3.3 VDC output should have provisions for remote sensing to compensate for
excessive cable drops. The default sense should be connected to pin 13 of the main power
connector. The power supply should draw no more than 10 mA through the remote sense
line to keep DC offset voltages to a minimum.

Most power supplies don't use the sense as covered in the design guid but many do so you may want to look at that.