To check a power supply, you take it out of the computer, make it start without the motherboard and measure its voltages. There are two ways. Without a tester, a jumper (a paperclip bent into a U) connects the green wire, PS_ON#, to a black ground wire on the 24-pin connector, as the motherboard does at power-on; with the power supply running, the multimeter measures its voltages one by one. With a tester, you just plug the power supply into it: it starts it and measures all the voltages at once. In this lab you will do both.
Components in this lab
- MSI MAG A550BN power supply: 550 W ATX, non-modular, 140 × 150 × 86 mm with a 120 mm fan. Connectors: one 24-pin (motherboard), one EPS 4+4 (processor), two PCIe 6+2 (graphics), five SATA, two 4-pin peripheral and one floppy.
- PS_ON# jumper: a paperclip bent into a U and wrapped in electrical tape except for its ends, a piece of solid wire stripped only at the ends, or an ATX jumper starter.
- Digital multimeter (Ferve F-1830) with probes: black in COM and red in VΩmA.
- Power supply tester with display: with the 24-pin connector and the EPS it measures +12V1, +12V2, +5, +3.3, −12 V, +5VSB and the timing of the PG signal.
- Anti-static mat and wrist strap: worn and connected to the mat, the strap drains static electricity from your body before you touch the connectors.
Safety and preparation
- Never open the power supply: its capacitors hold a charge even when it is unplugged.
- Test it out of the computer with no components connected: if it is faulty, it damages nothing.
- Switch off and unplug the power supply before fitting or removing the jumper or connecting or disconnecting the tester.
- Plug in the power supply with the switch at O and work on the anti-static mat, wearing the anti-static wrist strap connected to it, on a dry table with no loose metal objects.
The voltages of an ATX power supply
+12 V powers the processor, the graphics card and the drive motors, and it is the output that carries the most load. +5 V and +3.3 V power the motherboard, the drives and the USB ports. −12 V is an old, optional output that is hardly used. +5VSB is the standby voltage: it is there even if the power supply hasn’t started.
More theory: power supply parts, voltages and how to read its label (Eniun, in Spanish).
Wire colors
In the example wiring, yellow is +12 V, red +5 V, orange +3.3 V, blue −12 V, purple +5VSB and black ground (COM). Green is PS_ON# (pin 16) and grey is PWR_OK (pin 8). Pin 20 is unconnected. Some harnesses use all-black wires. Before measuring or fitting a jumper, verify the pins against the connector pinout.
Allowed ranges
No power supply gives the exact value. Intel’s ATX12V power supply design guide allows ±5% on +12 V (11.40 to 12.60 V), +5 V (4.75 to 5.25 V), +3.3 V (3.14 to 3.47 V) and +5VSB (4.75 to 5.25 V), and ±10% on −12 V (−10.80 to −13.20 V).
PS_ON# and +5VSB
When you press the power button, the motherboard connects PS_ON# (green wire) to ground and the power supply turns on +12, +5, +3.3 and −12 V. Without a motherboard, the jumper does the same. +5VSB doesn’t depend on PS_ON#: the power supply gives it whenever it is plugged in and its switch is at I, even if it hasn’t started. It powers the motherboard circuits that stay on standby, such as the one that detects the power button or wake-on-LAN.
The PG (Power Good) signal
PG or PWR_OK signals that the main power outputs are stable. The tester shows its delay in milliseconds. This example uses the legacy 100 to 500 ms range in Intel’s guide. For another power supply, check the requirements of its ATX revision.
Two ways to test it
The jumper starts the power supply and the multimeter measures each output separately. The tester starts the power supply, displays the voltages from the connectors under test and measures the PG delay.
The paperclip test (PS_ON jumper)
With the power supply switched off and unplugged, the jumper connects PS_ON# (pin 16) to COM (for example, pin 15). Check the connector pinout first. This power supply’s fan spins at startup. On models with a semi-passive mode the fan may stay still. Use a meter to check the voltages.
Measure the power supply with a multimeter
With the power supply started by the jumper, set the multimeter to V⎓ 20, the black probe on a black wire (ground) and the red probe on the one you want to measure: yellow, +12 V; red, +5 V; orange, +3.3 V. The 4-pin peripheral connector is the easiest place for +12 V. +5VSB (pin 9, purple) is measured without the jumper, with the power supply plugged in and at I. Each reading is compared with its range.
Test the power supply with a tester
With the power supply switched off and unplugged, connect the 24-pin ATX cable to 24P and the EPS 4+4 cable to 8P. When you switch it on, the tester asserts PS_ON# and displays the voltages from the connectors under test and the PG delay. On this screen, 11.9 in the −12V field means −11.9 V. Consult the tester manual to check other cables. When finished, switch off and unplug the power supply before removing the connectors.
Faults and what to do
A voltage out of range, a missing PG signal or a power supply that doesn’t start (it only gives +5VSB) are reasons to replace it. Before declaring it faulty, check the power cord, the outlet and the switch. A power supply isn’t repaired in the workshop: it is replaced with another one with suitable wattage and connectors.
Lab sequence
Start the power supply with the jumper, measure +12 V, measure +5VSB, turn off and put away, and test it with the tester.
The tester and the multimeter follow photos of real models; the power supply, the MSI datasheet. Voltages, colors and times come from Intel’s ATX12V power supply design guide. The 3D representation is educational and approximate.
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