It may seem unlikely, but over a quarter of all PC problems are in some way related to trouble with the power supply. Power-supply problems can also cause lockups, unexpected reboots, intermittent boot problems, Blue Screen of Death crashes, hard drive errors, or just plain won’t boot. I’ll explain how to test the PC’s power supply using a multimeter on your power connections and motherboard.
The power supply’s purpose is to convert the alternating current (AC) supplied by an electrical outlet into direct current (DC) that the PC can use. Typically, the power supply converts the AC into a 12-volt, 5-volt, or 3.3-volt direct current. The 12-volt direct current is used to power devices with motors, such as hard drives and CD-ROM drives. The 5-volt and 3.3-volt outputs are used to power various electronics on the system board.
Almost every PC power supply in use today is an ATX power supply.
1. To begin the diagnostic process, verify that the PC is unplugged from the wall and is receiving no power.
2. Disconnect the power supply cables from all of the components inside the case. Follow each cable from the power supply to the component to make sure that everything is properly unplugged.
3. Short out pins 15 and 16 on the motherboard power connector with a small piece of wire. You can also use a paper clip to help test your power supply and trick it into thinking that it has been switched on. To do this, straighten a paperclip and then bend it into a “U” shape. This paperclip will act as the pins that are inserted into the power supply that give it the “Power ON” signal.
4. Find the 20/24 Pin connector that normally attaches to your computer’s motherboard. It is typically the largest connector for the power supply.
5. Find the green pin and a black pin (pins 15 & 16). You will be inserting the ends of the paperclip into the green pin (there should be only one) and a neighboring black pin. Before you do this, double check to make sure that the power supply is completely disconnected from any power outlet, that it is switched off, and that it is not connected to any computer components.
6. Once you have placed the paperclip into each of the pins, Plug the power supply back into the outlet, and flip the switch in the back.
7. Once the power supply is receiving power, you should be able to hear and/or see a fan moving. If the power supply does not turn on at all, double check your pins (after unplugging) and try again. If it still does not turn on, then it is most likely dead.
Important: Just because the fan is running does not mean that your power supply is supplying power to your devices properly. You'll need to continue testing to confirm that.
8. Turn on your multimeter and turn the dial to the VDC (Volts DC) setting.
Note: If the multimeter you're using does not have an auto-ranging feature, set the range to 20V.
9. I recommend testing every pin on the 24 pin connector that carries a voltage. This will confirm that each line is supplying the proper voltage and that each pin is properly terminated. Connect the negative probe on the multimeter (black) to any ground wired pin and connect the positive probe (red) to the first power line you want to test. The 24 pin main power connector has +3.3 VDC, +5 VDC, -5 VDC (optional), +12 VDC, and -12 VDC lines across multiple pins.
10. Are any voltages outside the approved tolerance? If yes, replace the power supply. If all voltages are within tolerance, your power supply is not defective.
Below is a table listing the tolerances for each power supply voltage rail according to Version 2.2 of the ATX Specification.
Voltage Rail Tolerance Minimum Voltage Maximum Voltage
+3.3VDC ± 5% +3.135 VDC +3.465 VDC
+5VDC ± 5% +4.750 VDC +5.250 VDC
+5VSB ± 5% +4.750 VDC +5.250 VDC
-5VDC (if used) ± 10% -4.500 VDC -5.500 VDC
+12VDC ± 5% +11.400 VDC +12.600 VDC
-12VDC ± 10% -10.800 VDC - 13.200 VDC
11. If your power supply passes your tests, it's highly recommended you continue testing to confirm that the power supply can operate properly under a load.
12. Turn off the switch on the back of the power supply and unplug it from the wall.
13. Reconnect all of your internal devices to power. Also, don't forget to remove the short you created in Step 5 before plugging back in the 24 pin motherboard power connector.
14. Plug in your power supply, flip the switch on the back if you have one, and then turn on your computer as you normally do with the power switch on the front of the PC.
15. Test the voltages for other power connectors like the 4 pin peripheral power connector, the 15 pin SATA power connector, and the 4 pin floppy power connector.
16. Just as with the 24 pin motherboard power connector, if any voltages fall too far outside the listed Power Supply Voltage Tolerances you should replace the power supply.
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