PC Hardware

Measuring Electricity

A multimeter is an instrument that is used to measure several aspects of electricity. All multimeters are designed to provide at least four major measurements:

The Multimeter

  • AC voltage
  • DC voltage
  • Continuity
  • Resistance

A multimeter consists of two probes, an analog or digital meter, and a multiposition switch to select the type of test you wish to perform.

Testing AC Power

On any new building installation, failure to properly test AC outlets can result in damaged or destroyed equipment, as well as possible injury and electrocution. In the event a wiring error was made that causes the voltage to be outside of the specifications (either two high or too low), problems are sure to arise. Don't take for granted that the building power supply provides the correct voltage, or that all of the other inputs are wired correctly.

When testing an AC power source, check these three things:

  • Is the hot wire sending the correct voltage, and is it wired to the correct pin?
  • Is the neutral wire connected to ground and to the correct pin?
  • Is the ground wire connected to ground and to the correct pin?

Testing AC Outlets with a Multimeter

The first step when testing an AC outlet is to set up the multimeter. Then you need to know how to read the meter. You can also use special equipment if the multimeter does not provide enough information.

Setting Up the Meter

Basic multimeter usage with AC circuits is quite straightforward:

  1. Attach the black test lead to the negative (-) marked hole. In some low-cost meters, leads are permanently attached to the meter.
  2. Attach the red test lead to the volts (+) hole. Be careful-if this lead is placed in the wrong hole (ohm or amp) it can cause permanent damage to the meter.
  3. Set the selector switch to AC volts (this choice is often denoted by red lettering). If there are multiple selections, use the highest setting possible (if voltage is unknown), or select one level higher than the estimated voltage. For standard household outlets, 200 VAC is a good selection. Some digital meters use "Auto-range" and don't need any selection except AC volts.

Reading the Meter

After the meter is set up, you are ready to test a wall outlet. There are three tests to perform. With AC voltage, it does not matter which lead is placed in which connector:

  • Hot to Neutral: Place one lead in hot (smaller of the two vertical slots) and the other in neutral (larger of the vertical slots). The reading should be between 110 and 120 volts AC.
  • Hot to Ground: Place one lead in hot (smaller of the two vertical slots) and the other in ground. The reading should be between 110 and 120 volts AC.
  • Neutral to Ground: Place one lead in neutral (larger of the two vertical slots) and the other in ground. The reading should be 0 volts.

Using AC Testers

An alternate method for testing electrical outlets is to purchase an AC tester. These small devices are made especially for testing outlets and can be purchased at any home improvement or electronics outlet store. By simply inserting the tester into an outlet, all voltages for all combinations can be tested at the same time. Many testers provide several LEDs that tell whether or not each function passes the test. This device is not as accurate as a multimeter but it is more convenient. It will provide a pass/fail indication rather than an accurate voltage reading.

Testing AC Ripple

The function of the power supply is to convert AC to DC voltage. When working properly, a pure DC signal will be produced. However, sometimes, as the power supply ages, its ability to produce pure DC falters. A power supply uses electrolytic capacitors (discussed later in this lesson) to filter or smooth the voltage after it has been converted from AC to DC. These capacitors are second only to fuses as the part of a power supply most likely to fail. When an electrolytic capacitor begins to fail, it allows more and more AC voltage to pass through. This small amount of AC voltage is superimposed on top of the DC voltage and called noise or ripple. To test for ripple, set a meter to read AC. Then connect a .1mfd (microfarad) capacitor to the red lead. With the power turned on, measure the DC voltage. Any ripple present will be displayed as AC voltage.

Testing Resistance

Resistance is an opposition to the flow of current through a conductor. Resistance is measured in ohms. The symbol for an ohm is W. Resistance is measured by placing one lead of the meter on each side of the circuit or component to be measured. Taking resistance measurements for a component while it is still soldered in its circuit can lead to inaccurate readings because any other component connected to the circuit can affect the total resistance measured. Unlike voltage checks, you should test resistance with the power off. If a meter is set up to read resistance, you will damage it if you connect it to an electrical outlet.

Be careful when measuring resistance. If the meter is set too high or the resistance is too high for the meter, you will get an inaccurate reading. Also, before taking a measurement, be sure that any charge stored in a capacitor is properly discharged. Refer to the applicable product manual for details.

Testing Continuity

Continuity is a term used to indicate whether or not a connection exists between one point in a circuit and another. It is used to determine the presence of breaks in wires and electrical circuits.

If no continuity setting is available, use the resistance setting (see the next section). If the multimeter measures infinite resistance, there is no continuity. This indicates a break in the line. If the multimeter shows little or no resistance, there is continuity and the circuit is complete.

Testing DC Voltage

Testing for DC voltage is the same as testing for AC voltage, but with one important difference: DC voltage is sensitive to polarity. As mentioned earlier, DC voltage has a positive pole (+) and a negative pole (-). When measuring DC voltage, it is important to place the positive (red) lead on the positive side and the negative (black) lead on the negative side of the circuit. If the leads are positioned backwards, the polarity of the reading will be the opposite of what it should be.

When using an analog meter (one with a dial and needle), connecting the leads backward will cause the needle to move in the opposite direction, possibly damaging the meter.