When confronted with a PC that won't power on, a multimeter can be the perfect tool for troubleshooting. Find out how to safely and effectively use a multimeter to test a bad cable, faulty switch, or broken power supply.
When a PC won't power on, there are many possible causes, such as a bad cable, faulty switch, or blown power supply. A multimeter can help you find the cause of your particular problem quickly and easily. With this tool, you can perform continuity tests on cables and switches and test the voltage on the power supply.
Follow vendor instructions carefully
Keep in mind that working with electricity can be very dangerous. My instructions are intended to as general guidelines. Every multimeter is different, and if the instruction manual for your specific multimeter differs from any of my pointers, you should follow the manufacturer’s recommendations. Using a multimeter incorrectly can cause damage to your PC or multimeter and can lead to injury or death.
As I explained in a previous article, a continuity test is a test of circuit integrity to determine if a circuit is complete between two points. When dealing with a PC, the most common continuity tests involve testing cables and testing switches. Remember that the PC should be disconnected from the power source (not just turned off) before performing any type of continuity test.
Testing a cable
To perform a continuity test on a cable, set the multimeter to test the lowest possible number of ohms and work your way up. When the probes are disconnected from the cable, the multimeter will display infinite resistance. On an analog meter, the pointer will remain on the peg at the left end of the scale. There will normally be an infinity symbol at this end of the scale. (On a digital multimeter, infinite resistance is indicated by the numeral 1 without a decimal.)
Attach the probes to matching wires within a cable, and the resistance will drop to near zero if the cable is good. You can see an example of this in Figure A, in which I am testing a power cable.
|You can use a multimeter to test most types of cable.|
Testing a more complex cable, such as a drive ribbon, works on the same principle. You’ll still attach one probe to each end of the cable. The only trick is figuring out which ports on the cable ends share a common wire.
Testing a switch
Another common use for a continuity test is to test a switch. Typically, this is used when testing the PC’s on/off switch. If the PC uses an ATX power supply (which most newer PCs do), the power supply keeps the motherboard powered even when the PC is turned off. The power button is designed so that the circuit is only connected while the button is held in. This is known as a momentary. The simple act of pressing the button trips a logic circuit that begins the power cycling process.
There is no standard type of ATX switch. Some switches use two wires and some use four wires. In a two-wire model, you can connect the multimeter’s probes to the two wires (with the PC unplugged) and press the button in to watch for a drop in resistance. If the resistance drops to near zero, the switch is good. You can see an example of this test shown in Figure B.
|You can use a multimeter to test your PC’s power switch.|
The four-wire switch is divided into the left and right side. Usually wires on the left go together and the wires on the right go together; however, these are sometimes crossed. Be sure to test all combinations to check for unintended current paths. When testing this type of switch, unplug the PC and test the two wires on the left followed by the two wires on the right. Like the two-wire switch, the switch will show infinite resistance until you press the button in. When the button is pushed in, the resistance should drop to near zero.
The most common troubleshooting task in which you use a multimeter is to troubleshoot a power supply. An ATX power supply converts a 120-volt AC power source into 12-volt, 5-volt, and 3.3-volt DC voltages. Different parts of the system board require different voltages. An ATX power supply constantly supplies a minimal amount of power to the system board. Unless the power supply is properly attached to the system board, the fan won’t spin and the power supply will appear to be dead.
Power to the people
For more information on installing and troubleshooting PC power supplies, check out these other TechRepublic articles:
- "Learn to troubleshoot power supply problems"
- "Learn to re-fuse a power supply"
- "Resolving common problems caused by power supplies"
- "Choosing the right PC power supply"
A common test for a power supply is to check the peripheral connectors, which are used to supply power to hard drives and CD-ROM drives. These connectors have eight wires arranged into four pairs. From left to right, the wires are colored yellow, black, black, and red. If you attach the multimeter’s red probe to the red wire and the black probe to the black wire that’s next to the red wire, it should read +5 volts DC.
Perform the same test by placing the multimeter’s red probe on the yellow wire and the black probe on the black wire that’s next to the yellow wire. This test should produce a reading of +12 volts DC. You can see an example of this test shown in Figure C. The industry standard is that all red power leads are +5 volts DC, all yellow leads are +12 volts DC, and all black leads are common, or system grounds.
|You can use a multimeter to test how much current the peripherals are receiving.|
If either measurement is off by one volt or more, you have either a bad power supply or too many peripherals connected. Try disconnecting all peripherals and repeating the test to see if your voltage levels are normal. If the voltage levels return to normal, try using fewer peripherals or getting a higher wattage power supply. If the voltage levels are still too low with everything disconnected from the power supply, it’s time for a new power supply.