How to Use a Multimeter

Using your multimeter

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Photo 1: How to test batteries

Test battery life using a multimeter or volt-ohm meter (VOM). Analog-type multimeters use a needle and printed scales to display results from a variety of electrical tests.
Function setting: Direct current (DC) voltage
Scale setting: 12 volts
Test probes: Match the polarity (“+” to “+” and “-” to “-”) of the test probes and the battery.
Good reading: 6 volts (this battery's original rating)
Bad reading: Batteries with voltage readings 20 percent below their original rating should be replaced.

A multimeter is a black box of electronic circuitry that allows you to troubleshoot just about any type of electrical wiring or device. You simply dial the proper function and scale, touch the two test leads to the wiring or device in question and check the meter reading. Depending on the setting, the multimeter will tell if you have a broken connection, no power, poor connections, faulty parts and more.

With all its numbers, dials and switches, a multimeter (also known as a volt-ohm meter, or VOM) can be pretty intimidating. Yet it's well worth learning, is relatively inexpensive and should be in the toolbox of anyone seeking to do wiring diagnostics and appliance repair. Here, we'll walk you through the basics and show you six quick, down-to-earth tests you can safely use today.

Your VOM can pay for itself quickly by simply analyzing whether the dozens of batteries devoured by toys and electronic devices are still good (Photo 1). But only using these instruments to check batteries is like harnessing a draft horse to pull a roller skate. Explore a sampling of other possible uses, as shown in Photos 2 – 6.

A multimeter gives you a variety of readings,
depending on the setting.

Multimeter Terms

It's hard to visualize electricity, so I've always thought of it like a river. In a river, there's a certain volume of water (similar to electrical amperage, or amps) flowing with a certain potential force (like voltage) that encounters obstructions as it flows (resistance measured in ohms). Hold that big picture in your mind, then add these key concepts about both electricity and VOMs:

Alternating current (AC) voltage: The type of electricity that powers your house.

Direct current (DC) voltage: The type found in auto and household batteries.

Resistance (measured in ohms): The lower the reading, the easier electrical current (measured in amps) flows through circuit material.

An open circuit equals trouble: There is high resistance from a broken connection, a faulty part or a switch that's been turned off. There isn't a complete circuit path and no current will flow.

A closed circuit is good: It means a minimum of resistance is present because a connection or part is working. Note: Check the pathways in the wiring or device being tested for any random loose wiring that's touching the circuit you're testing. Sometimes a broken connection (“short circuit”) can look like a closed circuit. Short circuits can harm you, destroy equipment and start fires.

Continuity testing determines if an open, shorted or closed circuit exists in an appliance, electrical or electronic device and is a common use for multimeters.

On a VOM, infinity signifies an open circuit. On an analog multimeter, infinity shows up as an unwavering needle that won't move off the far left side on the display. On a digital multimeter, infinity reads “0.L.”

On a VOM, “zero” means a closed circuit has been detected. The display needle moves to the far right side of an analog scale; “zero” reads “0.00” on a digital VOM.

Selecting the proper range is very important and refers to setting the function switch on your multimeter to a voltage or amperage value that's higher than the top value you anticipate testing. Digital multimeters have a nifty feature, auto-ranging, that automatically selects the widest possible range once you set the function switch for ohms, current and voltage (AC or DC). Auto-ranging gives you the safest testing capacity each time you change back and forth from, say, measuring resistance to voltage readings.

Using VOMs

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Photo 2: Testing a broken extension cord

Check for a broken wire or bad plug connection in an extension cord by bending the cord along its length.
Function setting: Ohms.
Scale setting: Auto-ranging.
Test probes: Connect the prongs together at one end of the cord with a jumper wire, creating a closed circuit. Insert the testing probes in the other end to complete the circuit, and then turn on the multimeter. Don't forget a second test to check along the safety ground wire too.
Good reading: A “0.00” reading would indicate “zero”—a closed circuit.
Bad reading: This “0.L.” reading indicates “infinity”—an open circuit. This cord has a broken wire or loose connection.

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Photo 3: Check wall switches

Test whether this wall switch is bad. First, shut off the power to the main electrical panel. Then disconnect one terminal wire—so you don't measure the entire circuit a switch is tied into—and turn the switch “on” to test for a short circuit inside the switch.
Function setting: Ohms (continuity).
Scale setting: “X-1.”
Test probes: Hold them steady on the terminals. You don't have to match the polarity.
Good reading: A “zero” reading would indicate a closed circuit, a good switch.
Bad reading: This needle reads “infinity”—an open circuit. The wall switch is bad.

Both types of multimeters (digital and analog VOMs) require you to decide which to test for first: ohms, voltage or current. Next, select the range you'll be testing. For example: On an analog multimeter, if you were to test AC voltage in a 120-volt wall outlet, but only set the function switch to 30 AC volts, you'd overload and damage the multimeter. Instead, select a setting greater than 120 AC volts.

VOMs come with two colored testing probes that connect to jacks in the meter. The probes have electrically insulated handles with metal tips. Generally, the red probe plugs into the “+” meter jack and the black probe plugs into the “-” jack. When the test probes contact a circuit, the findings are displayed on the LCD readout or analog meter scale.

A test for voltage and amps uses the electricity present in a live circuit to power the meter. For resistance (ohms) and continuity tests, batteries inside the VOM send a weak current through the circuit being tested to get the reading. We suggest you stay away from live circuit tests until you've mastered using the VOM on the resistance and continuity tests we show.

Using good techniques

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Photo 4: Check household appliances

Troubleshoot a food blender that isn't running or is running sluggishly by testing the blender's switch block. UNPLUG the blender, set up the VOM and probes, and push each of the blender's speed switches.
Function setting: Ohms (resistance).
Scale setting: Auto-ranging.
Test probes: Attach alligator clips (a must-have accessory) to the testing probes, connect the clips to the power plug, and turn on the VOM.
Good reading: This low reading (28.4 ohms) indicates a good switch. Check the manufacturer's specification for the “good” reading on your blender.
Bad reading: A “0.L.” reading (“infinity”) indicates a switch that's dirty or defective.

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Photo 5: Test electric stovetops

Investigate a possibly faulty electric cooktop heat element.
Function setting: Ohms (resistance).
Scale setting: “X-1.”
Test probes: Attach alligator clips to each lead on the heating element and turn on the VOM.
Good reading: This low reading (22.5 ohms) indicates a good electric range element. Check the manufacturer's specification for the “good” reading on your stove's element.
Bad reading: An “infinity” reading or one exceeding 120 ohms.

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Photo 6: Troubleshoot large appliances

Determine why a dehumidifier won't run by first testing the humidistat switch for continuity. UNPLUG the appliance, remove the front panel and disconnect the mounting bracket that holds the switch. Label and disconnect the two switch wires, set the VOM, and slowly twist the switch several times from on to off.
Function setting: Ohms (continuity).
Scale setting: Auto-ranging.
Test probes: Attach the alligator clips to the switch's flat prongs and turn on the VOM.
Good reading: If the humidistat is functioning, each time the switch “clicks,” the reading should change from “0.L.” for “infinity” (if switch is off) to “0.00” for “zero” (if switch is on).

• When testing DC voltage or amps, match the polarity of the probes to the “+” and “-” terminals of the DC source being measured. Matching polarity isn't necessary for testing either AC voltage or amps, or for continuity or resistance readings.
• For the most accurate readings, hold the probe tip points (not the sides) tightly to a contact. Avoid touching the metal tips with your fingers. Your body could act as a circuit and influence a reading (and get you zapped!).
• Each time you do an ohms test using an analog multimeter, touch the two probes together and use the calibration dial to “zero it out.” For a digital multimeter, touch the probes together and it automatically calibrates itself.
• Ensure accurate readings by periodically cleaning oxides off the metal probes and keeping the tip points sharp with fine-grit sandpaper or emery cloth.