How to Test a PSU with a Multimeter

Preperation

Read this guide over, top to bottom, at least once before attempting. Working with electricity can be very dangerous, you could harm the components, your multimeter, your house’s wiring, or even yourself!

Note: The power supply does not need to be physically removed from the case.

Warning: Dell computer power supplies may have the wrong colors. You can refer to the ATX pinout at the bottom to cross reference

Procedure

  1. Switch the Power Supply Unit (PSU) to the OFF position or unplug the power cord if it does not have a switch.

  2. Remove the side panel of the computer case and unplug the ATX power harness from the motherboard.

  3. Unplug all other power connectors including CPU and any graphics cards (if present).

  4. Using a piece of wire, a paper clip or bobby pin form it into a U-shape to construct a jumper.

  5. Insert one end of the of the jumper into the green wire socket of the 24 or 20 pin ATX power connector.

  6. Insert the other end of the jumper into any black wire socket, to ground it out and trigger it to start once powered later. **Important: On the multimeter, ensure the red probe is attached to the mAV Ω connection and that the black probe is connected to COM (-). On an auto-ranging multimeter, select the VDC position. Otherwise, turn the dial to 20 VDC setting. > If you are ever unsure of the output voltage you should always use the highest choice first, ex: 200V DC. Once confirmed low disconnect and select the 20V DC setting.

 

Taking Measurements

Note: It is important that your power outlet is properly grounded. Always use caution when working with live power!

  1. Switch the PSU to the ON position. It should become energized because you made a connection between the green wire and (any) black wire on the ATX connector. Some fans are thermally activated, meaning they only come on when they are hot (to save power), so just because the fan isn’t spinning does not mean it isn’t on. If the fan is not spinning, you should consider adding some load to the PSU by connecting a molex powered fan.

  2. Insert the black probe of the multimeter into any molex connector and set it down where it won’t be disturbed. This frees you both hands for taking measurements. Every black wire is grouped to the same point, which is the same point as ground and the case (hence why grounding is a little important).

Hold the ATX connector in your non-dominant hand. Using your dominant hand and the red probe, you can check each socket as follows:

PWR_OK

Power OK (PWR_OK) is a voltage high signal generated by the power supply to tell the computer that the DC voltages are within the ranges required for proper operation.

  1. Insert the red probe of the multimeter into the gray wire socket of the ATX connector.

  2. Observe the reading on the meter’s display then compare it to the acceptable voltage range listed below:
OutputTolaranceMinimumNominalMaximumUnit
+5 V±5%4.75+5.005.25V DC

If there is no 5V DC voltage present, verify to ensure the black probe is engaging with the socket properly. If properly connected still yields no result, move the black to any black wire socket on the ATX connector.

> Note: In cheap PSUs, the 5V side could be connected to this inside the PSU, negating this to save parts and allow the motherboard to turn on. Hopefully not.

12V

  1. Insert the red probe into any yellow wire socket of the ATX connector.

  2. Observe the reading on the display then compare it to the value ranges listed below:
OutputTolaranceMinimumNominalMaximumUnit
+12 V±5%11.40+12.0012.60V DC

Note: If your video card requires power connectors, the yellow wires should be checked for each set looking for the tolerance listed above. Although they supply voltage from the same source, each power connector may have its own overcurrent protection.

5V

  1. Insert the red probe into any red wire socket of the ATX connector.

  2. Observe the reading on the display then compare it to the acceptable value ranges listed below:
OutputTolaranceMinimumNominalMaximumUnit
+5 V±5%4.75+5.005.25V DC

-5V

If you are using an old ISA slot check this, otherwise continue to the next voltage check.

  1. On the ATX connector, insert the red probe into the white wire socket. It should be -5V DC ±10%. To avoid confusion, no table is listed.

    3.3V

  2. Insert the red probe into any orange wire socket of the ATX connector.

  3. Observe the reading on the display then compare it to the acceptable value ranges listed below:
OutputTolaranceMinimumNominalMaximumUnit
+3.3 V±5%3.14+3.303.47V DC

-12V

  1. Insert the red probe into the blue wire socket of the ATX connector. If you have an analog meter (not digital to indicate polarity +/-), you will have to reverse the leads: red probe to black, black probe to blue.

  2. Observe the reading on the display then compare it to the acceptable value ranges listed below:
OutputTolaranceMinimumNominalMaximumUnit
-12 V±10%-10.80-12.00-13.20V DC

5VSB (5V Standby)

  1. Insert the red probe into the purple wire socket of the ATX connector to check for standby power.
OutputTolaranceMinimumNominalMaximumUnit
+5 V±5%4.75+5.005.25V DC
  1. Remove the jumper wire from the ATX connector to turn the PSU off.

  2. Once again, check the standby power.

 

Results

Compare your measured values to these to ensure your PSU is within tolerance:

OutputTolaranceMinimumNominalMaximumUnit
+12 V±5%11.40+12.0012.60V DC
+5 V±5%4.75+5.005.25V DC
+3.3 V±5%3.14+3.303.47V DC
-12 V±10%-10.80-12.00-13.20V DC
+12 V±5%4.75+5.005.25V DC

For a complete verification, you may wish to check all the wires. Below is a table of the ATX voltages which are the same for both 24 and 20 pin ATX standard connectors.

With the black lead still inserted in a spare Molex connector, hold the ATX connector in your non-dominant hand. Using your dominant hand, you can probe each socket as follows:

Expect to see 0V on any COM/Ground.

Left Side Pinout (without tab)

PinNameColorDescription
13.3VOrange+3.3 VDC
23.3VOrange+3.3 VDC
3COMBlackGround
45VRed+5 VDC
5COMBlackGround
65VRed+5 VDC
7COMBlackGround
8PWR_OKGray+5 VDC if OK
95VSBPurple+5 VDC (standby, power off)
1012VYellow+12 VDC
1112VYellow+12 VDC
123.3VOrange+3.3 VDC

Right Side Pinout (with tab)

PinNameColorDescription
133.3VOrange+3.3 VDC*
14-12VBlue-12 VDC
15COMBlackGround
16PS_ONGreenLow/Short = ON, High/Open = Off
17COMBlackGround
18COMBlackGround
19COMBlackGround
20-5VWhite-5 VDC**
21+5VRed+5 VDC
22+5VRed+5 VDC
23+5VRed+5 VDC
24COMBlackGround

* (ATX V2.3 / EPS12V V2.92)

** Optional on newer ATX-2 supplies)

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