ATX Power Supply Mod

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This is a step by step instruction for modifying an ATX power supply to work as a 5V and/or 12V power supply.

Not all power supplies are the same but I will try to cover the differences in them.

[edit] Wiring

ATX Power Supply

First it is important to understand how the different color wires work and what there function is to the power supply. Bellow is a list of there basic functions. The first group is power lines listed in order of decreasing amps available, and the second group is signals.

Black GND
Red +5 VDC
Orange +3.3 VDC
Yellow +12 VDC
Purple +5 VDC VSB
Blue -12 VDC
White -5 VDC

Brown 3.3VDC sense
Green PS-ON

for pinouts of the connector try the following link

Power Lines (±5, ±12, 3.3, 5VSB)

The two main work horses of the Power supply (PS) are the 5V and the 3.3V these have the highest amp ratings, usually 20-50 Amps. The 12V is usually lower in power 10-20 Amps. The 5VSB line is 5V line with 2 Amps, this line is always on if the PS is plugged in this allows the Mother board to run things like WOL and "power on keyboard stroke" functions. The -12V line has about 1 amp of power this almost not used on computers today but is still required on the PS. Lastly the -5V line has .3 amps; this line was used to power the ISA slot but on all ATX PS after the 2.0 revision this line has been discontinued.

Signal Lines (PWR-OK, 3.3VDC sense, PS-ON)

There are three signal lines, each has it's own function and I am unaware of the voltage levels but I believe it is TTL for 2 of them. The PWR-OK line is a output line that tells the motherboard that the power lines on the PS is safe and ready to use. a little testing will tell you if it is an inverted output. The 3.3VDC sense is almost a useless wire, but if your PS has one you will need it. This line is basically connected to the 3.3VDC power line at pin 11 on the connector. this lets the PS sense that it is outputting the correct 3.3VDC level. Finally there is the PS-ON line. This line turns on the power supply it is an inverted input and when the line is pulled to GND (or wired) the power supply will put power on all the power lines.

[edit] Modification

I am not liable for the following modification doing this may void your warranty and may circumvent safety.

This is what everyone wants to know how to do, but be careful and check your wiring or you will ruin your PS. To get the power supply to run on it's own you will have to do a little rewiring. The quick way is to solder the PS-ON wire to the GND wire and your done. If you want to clean up the wires and remove as many wires as possible do the following. First decide what power lines you want to use. Then remove all the wires except the PS-ON, GND, 3.3VDC sense (if available). Take the 3.3VDC sense line and solder it to a 3.3VDC line. I like to take out one of the 3.3VDC wires and then solder the 3.3VDC sense into the now open 3.3VDC hole. Finally the PS-ON wire can be soldered to the GND line, or what I feel is better for the PS and for safety is to put a solder a ground and the PS-ON to a STSP switch and mount it on the case, taking care not to obstruct airflow and avoid shorting anything.

If you have questions on this ask John Szilagyi

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