Articles :: Power Supplies 101: A comprehensive guide ::

Jon Gerow · 06-26-2006 · Category: Guides

ATX power supplies DO NOT turn on at the flip of a switch

An ATX power supply is always "live", even when it seems turned off, as long as it's plugged in and the rear power switch (if applicable) is switched on.

The +5V standby is always coursing through your motherboard whether your PC is on or not. This is how the CMOS keeps it's settings with or without it's CMOS battery and wake on LAN, wake on ring and keyboard power on is accomplished.

The power supply is instructed to energize the other power leads throughout the system when the "Power On" lead (typically the green wire on the main connector) is grounded. Without grounding the green wire on the power supply, you will not be able to test ANY of the power leads on the power supply except for the +5V standby.

To test the power supply, you can do this: Unplug the main power connector from the motherboard. Connect ("bridge") the green and black wires on the main power connector together with a paper clip or a piece of wire. This will "energize" the power supply. Now, using a multimeter, the +12V should read 12V, the +5V should read 5V and the +3.3V should read 3.3V (within +/- 5% per Intel specifications.)

I typically use a paper clip, but here I use a piece of solder. Just connect the green and black and you're live!

Now I know what some of you are going to say, "You want me to shove a metal wire into a live power supply??!" Again, the leads are dead until the Power On lead is shorted to ground. The Power On lead is a signal, so it's not going to shock you. Even if you were to stick your paper clip into the only live lead on the power supply: the +5VSB, it's not going to hurt you. You're talking about a 10 to 15W lead and as soon as that lead goes to ground the power supply is going to trip and shut itself down.


  1. Introduction
  2. The PC power supply:
  3. The PC power supply label:
  4. Defining the connectors of an ATX power supply
  5. ATX power supplies DO NOT turn on at the flip of a switch
  6. Testing your power supply's voltage: Software vs. Multimeter
  7. Power Supply Efficiency
  8. The Derating Curve
  9. Power Factor Correction
  10. The resistance of modular connectors, adapters and splitters
  11. Conclusion

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