Science desk · A beginner’s how-to · Power supplies
A power supply, the PSU, turns the power from the wall into the steady low voltages a desktop computer runs on. This page shows how to read those voltages with a multimeter, on the colored plugs only, and what a reading like that can and cannot tell you.
Household power is alternating current (AC): the push reverses many times a second. A computer wants direct current (DC), which flows one way. A rail is one of those steady DC voltages. Volts (V) are how hard that push is. A desktop supply offers +12 V, +5 V, and +3.3 V, a small −12 V rail in the opposite direction, and a standby +5 V that stays up while the computer looks off, so the power button has something to wake.
The plugs follow ATX, the common desktop power-supply standard. The wide one is the 24-pin main connector. A 4-pin or 8-pin plug feeds the processor. A graphics card may take a 6-pin or 8-pin plug. Drives take a flat SATA power plug — SATA is the drive-connection standard — or an older 4-pin Molex plug. Molex here means that four-pin plug, not a brand of supply.
The motherboard does not switch the wall cord. It pulls one green wire, PS_ON#, down to ground. The # means the wire turns the supply on when it is low, near 0 V, not when it is high. Until then, only the purple standby wire should be alive. A paperclip does the motherboard’s job on the bench: green joined to a black ground wire, and the main rails wake with nothing else attached.
This is a no-load voltage test, or a light-load one if you add a fan. No-load means almost nothing is drawing power. You learn whether each rail comes up near its rated voltage, not whether it stays there when the computer asks for real current. The design guide’s windows apply at the plug under the loads that guide lists. Inside the window on an empty plug is a good sign, not a certificate. Far outside has already failed this easier test. The guides also say a supply with nothing plugged into it must not become a hazard, and they allow it to shut itself off. One that will not stay on until you add a fan is not automatically scrap. One that sits at 9 V where 12 V belongs, is.
Ripple is the small fast wiggle on an otherwise steady voltage. The guide caps it at 50 millivolts peak to peak on +5 V, +3.3 V, and standby, and 120 millivolts on +12 V and −12 V, measured with an oscilloscope — a screen that draws voltage against time — from 10 hertz to 20 megahertz. A meter on DC volts averages that wiggle away. A dedicated PSU tester shows the same DC voltages, sometimes with a small load, and it still cannot see full power or ripple. If the meter looks fine and the PC does not, swap in a known-good supply.
A digital multimeter, the kind that shows digits, and its two probes. A jumper: a paperclip bent into a U, or a proper ATX jumper. The paperclip’s bare metal must touch only the two holes you chose. If it can lean into a third hole, tape the middle or use the proper jumper. That one is the better tool.
Optional: a small load. Some supplies shut off, or look wrong, with nothing attached. An old case fan or an old drive is enough for those. It is still a small load, not proof the supply can run a computer. The supply itself sits on a clear dry bench. Unplug the wall cord before you take it out of the case, and leave its lid screwed on.
Turn the dial to DC volts. The mark is a V beside a straight line, sometimes drawn V⎓ or V—. A wavy line, V~, is AC. That is the wrong position for these plugs.
If you must pick a range, use 20 V. It covers 12 V. The 2 V range is too small for 5 V and 12 V. The 200 V range works but shows fewer digits. A meter that picks its own range only needs the DC volts position.
Black probe in the jack marked COM, common, the meter’s zero. Red probe in the jack marked V or VΩ. Not the hole marked 10 A or mA. Those measure current. On a voltage rail the meter becomes a short, the reading is nonsense, and the meter’s fuse often opens.
Look at the end the wires enter. That is the wire side. Hold the plug so the clip, the latch you squeeze to unplug it, runs along the bottom. In Intel’s pin-side figure the latch is on the pin 13–24 side; this drawing turns that figure around so the wires face you, and the clip stays on that same row. Pin 1 is the top-left pin, away from the clip. The green wire, pin 16, is on the clip side, fourth from the left. A number molded in the plastic wins if a housing is stamped the other way. The colors below are the usual ones.
| Pin | Signal | Usual color | Pin | Signal | Usual color |
|---|---|---|---|---|---|
| 1 | +3.3 V | Orange | 13 | +3.3 V, and the sense wire | Orange, often plus thin brown |
| 2 | +3.3 V | Orange | 14 | −12 V | Blue |
| 3 | COM, ground | Black | 15 | COM, ground | Black |
| 4 | +5 V | Red | 16 | PS_ON# | Green |
| 5 | COM, ground | Black | 17 | COM, ground | Black |
| 6 | +5 V | Red | 18 | COM, ground | Black |
| 7 | COM, ground | Black | 19 | COM, ground | Black |
| 8 | PWR_OK | Gray | 20 | Reserved, not connected | Empty. White only on old −5 V supplies |
| 9 | +5 V standby | Purple | 21 | +5 V | Red |
| 10 | +12 V | Yellow | 22 | +5 V | Red |
| 11 | +12 V | Yellow | 23 | +5 V | Red |
| 12 | +3.3 V | Orange | 24 | COM, ground | Black |
±5% of 3.30 V is 3.135 to 3.465. Both design-guide tables round that published window to 3.14–3.47. A few tenths off is the supply, not the rounding.
| Rail | Wire | Nominal | Window | Min | Max |
|---|---|---|---|---|---|
| +12 V | Yellow | 12.00 V | Version 3: +5% / −7%. Version 2.2: ±5% | 11.20 V (3) · 11.40 V (2.2) | 12.60 V |
| +5 V | Red | 5.00 V | ±5% | 4.75 V | 5.25 V |
| +3.3 V | Orange | 3.30 V | ±5%, published as 3.14–3.47 | 3.14 V | 3.47 V |
| +5 V standby | Purple | 5.00 V | ±5%. Present with the rocker on, jumper or not | 4.75 V | 5.25 V |
| −12 V | Blue | −12.00 V | ±10%. Optional; many boards never use it | −13.20 V | −10.80 V |
| Signal | Wire | Plugged in, not jumped | Jumped, supply on |
|---|---|---|---|
| PS_ON# | Green, pin 16 | High. At least 2.0 V, at most 5.25 V | Low. 0 to 0.8 V |
| PWR_OK | Gray, pin 8 | Low. Near 0 V, under 0.4 V | High once the rails are steady. 2.4 V to 5 V, usually near 5 V |
Do not jump these. With the main plug already jumped and the supply on, black probe on black, red on the color you are checking. Yellow is +12 V, red is +5 V, orange is +3.3 V, and black to black is about 0 V. The housings only fit one way. The drawings below are color keys for the design-guide pin numbers. They do not show which edge the clip is on. If a molded pin number disagrees with a drawing, believe the plastic, and believe the wire color either way.
These carry only +12 V and ground. On the 4-pin, pins 1–2 are ground and pins 3–4 are +12 V. On the 8-pin, pins 1–4 are ground and pins 5–8 are +12 V. A split 4+4 lead can fill an older 4-pin socket; measure yellow to black on each half. You should see about 12 V, not 5 V or 3.3 V.
Also +12 V and ground, plus a sense pin the card reads to learn what the cable may deliver. The supply ties that pin to ground, so a meter cannot tell it from any other black wire. On the 6-pin, pins 1–3 are +12 V, pins 4 and 6 are ground, and pin 5 is sense. On the 8-pin, pins 1–3 are +12 V, pins 4 and 6 are the sense pins, and pins 5, 7, and 8 are ground. A 6+2 plug is those eight pins with two on a side piece. Yellow to black should be the +12 V window.
The flat 15-pin plug can carry +3.3 V, +5 V, and +12 V. Pin 1 is at the left of this drawing. Pins 1 and 2 used to be +3.3 V and are retired. Pin 3 used to be +3.3 V and, on newer drives, is Power Disable: if it sits near 3.3 V, a drive with that feature stays off. Intel notes +3.3 V left the SATA 3.2 spec and is only for older gear, so missing orange wires are normal. If orange is present it should read the +3.3 V window. Red, pins 7–9, is +5 V. Yellow, pins 13–15, is +12 V. Pin 11 is a spin-up or activity signal, not a rail.
Four pins in order: pin 1 yellow +12 V, pin 2 black ground, pin 3 black ground, pin 4 red +5 V. There is no +3.3 V on this plug. Yellow to black is the +12 V window. Red to black is the +5 V window.
Type the volts you measured. Blank boxes are skipped. Nothing is sent anywhere; the marks are computed in the browser. The +12 V line uses the current guide’s 11.20–12.60 V window, and it also tells you if the number fell below the older 11.40 V floor.
A rail clearly outside its window is not in spec on this test. Replace the supply. Do not open it to look at the capacitors. One rail at 0 V, with the others healthy, is a dead rail: replace it. If the supply turns on and every rail is 0 V, check the dull mistakes first: dial on AC, red probe in the amp hole, jumper in the wrong holes, or the black probe not on a black wire. If those are right, the supply is not delivering power.
If nothing happens, including a dead purple wire, the outlet, the cord, the rocker, or the supply is at fault. The internal fuse is not yours to change. A supply that stays off until a fan is plugged in can still be within the guide, which allows a shutdown when every connector is empty. Add the fan and read again. That is still not a pass under load.
Voltages in the window, and the computer still restarts or dies under load: this test never asked for real current, and it could not see ripple. Swap in a known-good supply. If the swap fixes the machine, replace the old one. Also replace for a rail out of range, a rail stuck at 0 V, no standby, or a burning smell. If you are unsure, replace it. Do not repair the inside. The cover stays on.
Use continuity or ohms only with the cord unplugged and the rocker off, after the supply has sat. Two black wires should beep. A beep from a colored wire to black is not a verdict: the circuits inside still connect those pins. Do not use ohms while the supply is plugged in.
This is a bench check for a person who can use a meter, not a repair manual and not a promise that a supply in the window is healthy under load. When a label on the supply disagrees with a general window, the label is about that model; the windows here are the design guides’ limits. If you are unsure, replace the supply. Leave the cover on.