Build Your Skills: Learn to use a multimeter the right way

When and how to use a multimeter to troubleshoot PC power problems

For the electrically uninitiated, using a multimeter to troubleshoot PC power problems can be an intimidating task. But it doesn't need to be. With some basic electrical knowledge and a little practice, you can safely and effectively use this handy gadget to troubleshoot a variety of common PC problems.

Follow vendor instructions carefully
As you read through the article, keep in mind that working with electricity can be very dangerous. My instructions are intended to be 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.

Analyzing a circuit
Any time that you’re checking a circuit with your multimeter, you must know whether the circuit is AC (alternating current) or DC (direct current) and about how much current and voltage you’re potentially handling. You must also decide whether you want to measure volts, amps or continuity.

When working with a PC, you'll almost always use DC volts. Modern ATX power supplies convert the fixed-levels power they receive into +12 or -12, +5 or -5, and +3.3-volt power sources for various system components. When testing a PC with a multimeter, you'll be testing these various power cables (peripheral power, motherboard power, and auxiliary device power—auxiliary device power is +12 volts and runs such auxiliary devices as case fans and come CPU fans) to verify that they are delivering the correct voltage.

Many multimeters have a dial that allows you to select various modes of operation. As you can see in Figure A, there are four different areas of my multimeter’s dial that deal with measuring current. These areas are designated by the codes V DC (volts DC), V AC (volts AC), A DC (amps DC), and A AC (amps AC). You’d select one of these four areas of the dial depending on whether you wanted to measure volts or amps of DC or AC current. When checking circuits inside the computer, you would use either DC volts (V DC) or DC amps (A DC).

Figure A
The multimeter’s dial selects modes of operation.

The next step in the process is to select an appropriate range. Each of my multimeter’s sections has several values that designate range. You wouldn’t want to use a two-volt setting to measure 600 volts. There are a variety of ranges available for both volts and amps of DC and AC.

If you’re unsure of the approximate voltage range, a good technique is to start with the maximum range and work your way down. Suppose that you had a two-volt power source but weren’t sure if you were dealing with two volts or 200 volts. You should initially measure the voltage with the highest possible range (in my case 600 volts). Two volts won’t even show up on a 600-volt scale, so next you’d disconnect the meter, set it to the next lowest range, and try measuring the voltage again. Eventually, as you repeat this procedure, you’ll find the appropriate range.

A word about safety
It’s always important to pay attention to the colors on a multimeter’s display. You might have noticed in the figure that my multimeter has a red probe and a black probe. When measuring AC, it doesn’t really matter which probe goes to which wire because AC alternates positive and negative on each wire. However, when measuring DC, you should always observe polarity by attaching the red probe to the positive and the black probe to the negative. Often the power source will already use red and black wires, making it easy to determine which is positive and which is negative. Most digital multimeters have a reverse polarity display, but most analog meters do not. Reversing the polarity on an analog meter could possibly bend the needle and damage the meter movement. If your power source uses different colors, CHECK THE MANUAL for the correct polarity of each conductor.

You may also have noticed that of my multimeter’s various groups of available functions, only the electrical current measurement functions that I described earlier are shown in red. That’s because measuring current and voltage are the only operations that should be performed with the power on. There are exceptions to this rule, but as a general rule, never take any other type of measurements with the power on. If you’re measuring something besides current or voltage (i.e., resistance or continuity), the device you’re testing should be unplugged.

Even if the power source is disconnected, you should make sure to touch only the plastic on the multimeter’s probes and never the metal. Capacitors store electricity, and accidentally touching the wrong spots on a circuit board with the probes can release this charge. Some capacitors within devices such as TVs and computer monitors can hold a large, very dangerous charge for several years after the device has been unplugged. These latent charges can hold enough electricity to kill a person. So don’t assume that the old monitor that’s been collecting dust in the basement for 10 years doesn’t have any juice in it.

Testing continuity
Sometimes, you just need to find out if power is getting from point A to point B. This is called a continuity test. Last summer, my brother shorted out the dashboard on my boat. I needed to find out if the problem was actually in my dash or if the line that connected the dash to the battery had somehow been damaged. To accomplish this, I did a continuity test.

A continuity test involves using the Ohms section of your multimeter (ohms are the standard unit for measuring electrical resistance). Set the range to the smallest possible resistance level (lowest number of milliohms). Now, place one probe at one end of the wire that you’re testing and place the other probe at the other end. If the resistance level is very low, then the connection is good and current can get through the wire, or the circuit is “closed.” If the resistance level is infinite, then the circuit is “open” or has a break in it. Many meters produce an audible signal when continuity is present, but the signal is only enabled in the lowest range of resistance. If the circuit is continuous, you will hear the tone.

Testing resistance
You can also use the Ohm section of your multimeter to test a resistor or the terminators used on coaxial cable or old token ring networks. First, make sure that there’s no power going through the circuit. Then place one probe on each side of the resistor or terminator that you’re testing and set the appropriate ohm level. Terminators typically have the ohm level printed somewhere on or near them. Terminators that are used in coaxial network cable have two options, either 50 or 75 ohms. Resistors use a series of colored bands to indicate their ohm level and their tolerance (how many ohms they can be off by and still be within specs). Unfortunately, the stripe pattern is a science in and of itself and can take a while to figure out.

Stay tuned
Now that I've given you the basics of multimeter use, you’re probably already thinking of PC problems you can troubleshoot with this new knowledge. My next article will offer a series of real-world PC problems that can be solved using a multimeter. Just remember that any time you work with a multimeter or any electrical devices, safety is always your first priority.

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