Example ATX computer power supplies.

Test Reuse Surplus PC Power Supplies

by Lewis Loflin

I have several PC power supplies as shown above. For the hacker they are a source of high-current D.C current. The 3V, 5V, and 12V outputs often exceed 10 amps.

Also because these were pulled from non-working computers, how do we know they operating correctly?

Better yet, can I control the main power output with an Arduino or Raspberry Pi? Yes I can even power them with the main power turned off.

Home built ATX power supply adaptor.

Above I built this adaptor cable to control the supply and the terminal blocks provide easy electrical connection.

When plugged into a power outlet the 5-volt standby is always on producing 2 amps or more of current. I have an LED connected across 5SB and GND.

When the switch is closed PON (green) is grounded. The PGOOD (normally gray wire but white here) terminal produces an output of 3-volts. (5-volts in an ATX1 supply.) The associated LED will turn on.

ATX power connector voltages.

My original test used a 24-pin ATX2 connector but it works just as well on the 20-pin ATX1 as shown above.

The differences between the ATX1 and ATX2 is pins 11,12, 23, 24 are missing.

The ATX1 produces -5-volts (white) while the ATX2 does not. Some ATX1s have a brown wire connected to 3.3-volts (orange) while most have this as an internal connection.

Home built ATX power adaptor demo.

An ATX1 requires a 10-Ohm resistor from 5-volts (red) to ground. Without this the 5-volt regulation doesn't work. The ATX2 worked fine without it.

ATX power adaptor schematic.

Finally here is the main schematic. Note the use of a switching transistor allows an external microcontroller to turn on/off the high current supply.

The constant +5VSB supplies permanent power for the control circuits.

This is also good for testing these supplies as one of the five turned out defective.

Happy hacking.

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Basic Electronics Learning and Projects

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