A digital multimeter can be an indispensable tool for testing, diagnosing, and troubleshooting electrical circuits, components and devices. The first digital multimeters were introduced in the late-1970s, and has proven much more accurate and reliable than the old needle-based analog meters. It’s used primarily to measure voltage (volts), current (amps), and resistance (ohms). But that’s just the beginning of what this surprisingly useful tool can do.

What Is A Multimeter?

A multimeter is a measurement tool absolutely necessary in electronics. It combines three essential features: a voltmeter, ohmeter, and ammeter, and in some cases continuity.

• is the switch on?
• is this wire conducting the electricity or is it broken?
• how much current is flowing through this led?
• how much power do you have left on your batteries?

These and other questions can be answered with the help of a multimeter.

Understanding Your Multimeter

Your basic multimeter will have a dial that you’ll use to indicate what type of measurement you want to take. Voltage indicators are labeled as DCV for direct current voltage and ACV for alternating current voltage. You’ll use the former for testing batteries and the latter for testing outlets, fixtures, appliances and electronics.

When measuring electrical resistance, look for the omega symbol, which looks like a horseshoe. And when testing continuity, choose the diode symbol, which looks like an arrow pointing to the right. Resistance tells you how easily electricity can flow through a circuit, and continuity verifies whether a circuit is complete or broken.

Most multimeters will also let you measure direct current amperage (look for the DCA label) but not all models measure alternating current amperage (ACA). There may be additional settings for functions like measuring temperature, direct current gain, frequency or special functions for testing common batteries by size.

While you get your readings from the multimeter itself, you can only test electrical sources with the use of compatible probes. You should see at least three ports for probes, usually labeled “COM”, “mA” and “10ADC”.

COM stands for common and is where you plug in the black probe. The red probe will go either in the mA port if you’re measuring a voltage or current of less than 200 milliamps (mA) or in the 10ADC port if you’re measuring voltage or current greater than 200 milliamps. When in doubt, use the 10ADC port.

How Do You Use It?

Step One: Decide what to test for

Both analog and digital multimeters require you to decide which to test for first: ohms, voltage or current.

Step Two: Select a range

Next, select the range you’ll be testing. For example, if you were to test AC voltage in a 120-volt wall outlet on an analog multimeter, but only set the function switch to 30 AC volts, you’d get a faulty reading. Instead, select a setting greater than 120 AC volts.

Step Three: Contact a circuit

Multimeters come with two colored test leads that connect to ports in the meter. The leads have electrically insulated handles with metal tips, called “probes”. The black test lead is always plugged into the black port on the meter, labeled “COM”. The red lead is plugged into one of the other ports, depending on which type of test is being performed. When the test probes contact a circuit, the findings are displayed on the LCD readout or analog meter scale.

For resistance (ohms) and continuity tests, batteries inside the VOM send a weak current through the circuit being tested to get the reading.  Before running a resistance test, avoid possible injury and damage to the multimeter by disconnecting power to appliances and shutting off circuits.

A List of Common Terms

• 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.

Trust The Experts

Making the right investments in your home can lead to saving thousands of dollars in the long run. Before the thermometer starts to drop this season, review your energy consumption habits and areas in the home that can be improved. For a home electrical inspection contact D.O.C. Electrical Services today.