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ATX power supply unit testing

Testing an ATX PSU

When powered on with nothing connected, no load connected, to it an ATX might not switch on

  1. Power the power supply on
  2. Connect the green wire to a black wire. Now the power supply might switch on. In any case continue
  3. Measure the purple +5VSB wire. This is the standby supply. It should show a voltage of about +5V
    • If this voltage is available continue. If not
      • put a load of 470 Ohm between the purple wire and a black wire. Now the powersupply might turn on
      • consult a technician to check any fuses
      • you might need to replace the power supply
  4. Measure a yellow, red and orange wire according to the table below. If you have a powersupply with all black wires you will have to count to find the right wire. Here you can find the ATX connector pinout. If the voltages are there the power supply is ok. If not, which can be the case if the powersupply needs a load to turn on, continue with the next test
  5. Preform a load test
    • Connect the green wire of the ATX connector to a black wire
    • Load the ATX power supply by connecting to the 20 / 24 pole ATX connector
      • For the +12V, connect a 12 Ohm 20W resistor between a yellow and a black wire
      • For the +5V, connect a 4,7 Ohm 10W resistor between a red and a black wire
      • For the +3,3V, connect a 3,3 Ohm 10W resistor between a orange and a black wire
    • Warning: The resistors will get hot
    • Switch the power supply on
    • Check the voltages according to the table below

The wires color coding

NameWire colorExpected voltageRemark
ONGreen When connected to ground (GND) the power supply should turn on
PWR_OKGray+5VIf the power is good. Otherwise 0V
+5VSBPurple+5VStandby. Might need a (tiny) load for the powersupply to turn on
GroundBlack0VThis is the reference to which all other voltages are measured
-5VWhite-5VObsolete on newer supplies

ATX on Wikipedia
Minimum load
Using a computer PSU
PC Power Supply
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atx_powersupply_testing.txt · Last modified: 25-02-2021 18:18 by wim