Getting Ready for Work

Getting Ready for Work

One of the skills you should have to be truly effective as a PC repair professional is the ability to use a multimeter, whether digital or analog, to troubleshoot PC electrical problems. On the exam, you can expect to see at least one question that deals with the use of a multimeter to measure the voltage of the power coming from the power supply or into the motherboard or a hard disk drive. So this section is worth reviewing, if for no other reason than it may help you get that question right.

Meeting the multimeter

 Time Shaver  The A+ Core Hardware exam expects you to know how to use, when to use, and what to measure with a multimeter. Expect questions in the trouble-shooting area that ask which settings to use when measuring volts, amps, and ohms. If you don’t have much experience with a multimeter, get one and either read the booklet that comes with it or ask a more experienced PC service technician to explain it you.

A multimeter measures the properties of AC and DC electrical power circuits. The range of capabilities available on multimeters is wide; even the simplest (and cheapest) have features you will rarely (or never) use.

A multimeter has two probes: a red (positive) probe and a black (negative) probe. When you test a device, place the red probe on the hot point, or high point, of the current and place the black probe on the ground, or low point. The voltage of the circuit is calculated as the difference in the readings of these two points.

The two major categories of multimeters are analog and digital. Here’s a brief description of these two types of multimeters:

  • Analog multimeter: An analog multimeter performs the same measurements as a digital multimeter, but uses a scaled dial and a needle to register the value measured.

  • Digital multimeter: Digital refers to the fact that the output numbers that represent the results of a measurement are displayed on an LCD (liquid crystal diode) screen. In other words, the results are displayed as digits.

 Shocking Information  Never, I repeat, never connect a multimeter to the main (building) electrical supply line, which carries at least 20 kilowatts of power. This is bad for your multimeter and it won’t do you much good, either.

Using a multimeter

 Remember  You can make four measurements on a PC using a multimeter:

  • Amps: Measure the strength of an electrical current

  • Farads: Measure the capacitance of an electrical device

  • Ohms: Measure the resistance in an electrical medium

  • Voltage: Measure the electrical potential of a circuit

Figure lists the type of problems that can be identified in some systems and the property that is measured.

Figure: Troubleshooting with a Multimeter



Potential Faults

Power supply connectors


Defective power supply, cables, or connectors

Cable connections


Broken or defective cable

Cable shielding


Broken or defective cable sheath or insulation

Electrical FRMs

Volts or Ohms

Improper electrical operation to specifications

Probing for power

 Remember  When preparing a multimeter for use, you must set three things:

  • The type of current you’re measuring: AC or DC (also known as VDC — voltage direct current).

  • What you’re measuring: Set the appropriate indicator or dial to voltage (volts), current (amps), or resistance or continuity (ohms).

  • The range of values expected: If you’re measuring voltage from the power supply, the voltage range is 3V to 12V, and for the AC wall plug’s output, the range is around 105V to 125V. You can find Autorange multimeters that sense the incoming power and set the range automatically.

     Shocking Information  Remember to put on your ESD wrist strap and connect it to either a static ground mat or the PC case.

Measuring volts

Voltage is the easiest and most common measurement that is made with a multimeter on a PC. Here are the general steps to follow when measuring voltage:

  1. Set the dial to measure DC voltage.

  2. Choose the voltage range. Usually, this will be 20V or lower, based on what you are measuring.

  3. Hold the black (negative) probe to a grounding point on the circuit.

  4. Touch the red (positive) probe to a hot point on the circuit.

Measuring resistance

The amount of resistance, or continuity, a circuit has to allow an efficient flow of electricity is measured in ohms. The general steps to measure resistance are

  • Be sure the circuit (power connector, trace, or cable) has no power running through it. If the circuit to be tested is inside the PC, turn off the PC’s power supply. You could damage your multimeter if you test a circuit that has power running through it.

  • Set the multimeter to test ohms and set an appropriate range.

  • Touch the multimeter’s probes to two metal points on the circuit. If you get a zero value, set the range higher or lower.

The multimeter should give a reading, buzz, or beep to indicate continuity in the cable. If you do not get buzzed or beeped, then either no continuity exists in the cable, or you have not made a good connection. Try a few more times — if you still get no reading, probably there is no continuity and you have discovered a bad cable. Before you chuck out the cable, try testing all of the remaining pins, using the cable’s pinout for which pins are connected to each other.

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