From About This Project
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. But only using these instruments to check batteries is like harnessing a draft horse to pull a roller skate.
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.
How To Troubleshoot With a Multimeter
Using VOMs
Both types of multimeters 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. Before running a resistance test, avoid possible injury to yourself and damage to the multimeter by disconnecting power to appliances and shutting off circuits.
Good TechniquesWhen 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.
Digital vs. Analog VOMs
For appliance and electronic repairs, buy a digital, not analog, multimeter. This type is much simpler to read and you can change the functions on it more easily. Digital multimeters have LCD readouts, do continuity testing, and cost from $35 on up. Some digital multimeters also feature auto-ranging and overload protection and other advantages analog multimeters lack.
Analog multimeters have multiple scales on the dial, a moving needle and many manual settings on the function switch. It’s tricky spotting the correct scale to read on the dial, plus you sometimes have to multiply the reading by 10 or 100 to get your final value. Depending on features (make sure it can do continuity testing), prices start at about $15.
For easier, hands-free viewing, choose a multimeter with a stand that will prop it up or hang it on a wall. If a multimeter doesn’t come equipped with either jumper wires or alligator clips (both about $4 each), buy them. Alligator clips are often used to firmly grip wiring or contacts for hands-free safe and accurate readings. Both types of multimeters and these accessories can be purchased at electronics stores, home centers and hardware stores.
Safety Above All
VOMs are tools that an impulsive and foolish person should avoid. When it comes to electricity, a little knowledge is a dangerous thing. When I got my first multimeter, I was so excited to start using it, I gave a quick glance at the owner’s manual and then proceeded to stick the multimeter’s probes into the closest appliances and wall outlets to see what the tester could do. I fried that VOM. I was more studious and careful with the next multimeter. Even now, there are still some repairs I let the electricians and repair technicians handle—with their VOMs.