Multimeters are invaluable tools for troubleshooting a wide range of electrical components. The possible applications are many, but they're typically used to test:
- Wall outlets
- Power cords
- Consumer electronics
- Vehicle electrical systems
What Is a Multimeter?
A multimeter is a measurement device used to measure multiple electrical values. Multimeters can measure voltage, current, resistance, and (sometimes) test for continuity.
The multimeter's impressive versatility is owed to its ability to test for the three basic units of electricity, including:
- Voltage: Multimeters can test two forms of voltage: alternate current (AC) and direct current (DC). AC is the voltage used in your home’s electrical wiring, while DC is used primarily in vehicles, residential solar systems, and consumer electronics. Voltage is measured in volts.
- Resistance: Resistance is the opposition to the flow of current through a circuit, and is measured in Ohms.
- Current: Generally speaking, current is the amount of electricity flowing through the circuit, and is measured in amps. The amount of amps flowing through a circuit is the difference between the voltage applied and the resistance present. Milliamps (1/1,000 of an amp) are the most common unit of amps measured with a multimeter.
In addition to testing those three functions, some multimeters test for continuity, or the absence of resistance within a circuit. Not all multimeters have dedicated continuity functions, but those that do emit an audible beep when continuity is present. If this setting isn't present, testing the resistance can be used instead.
Finally, there are two different types of multimeters: analog and digital. The main difference between the two is their display. Analog multimeters display their measurements with a needle that moves over a progressed scale. Digital multimeters have an LCD screen that displays the measurements in four or five digits. Another difference is that analog multimeters don't usually offer a continuity function, though some do.
Parts of a Multimeter
- Display: The window where the electrical measurements are displayed.
- Selection Knob: A round dial that allows you to select the type of electrical unit to measure. You can switch between AC volts (V with a squiggly line on either side or above it), DC volts (DC-), and resistance (Ω), amps (A), and milliamps (mA). Continuity is marked with diode symbol (triangle with a line on the right side) and/or a soundwave symbol on the selection knob.
- Probes: Red and black wires used to physically test the electrical component. Each one is equipped with a pointy metal tip on one end, and a banana plug on the other. The metal tip is used to probe the component being tested, and the banana plug connects to one of the multimeter's ports. While there's no actual difference between the two probes, the black cord is often used to test ground and neutral (or common) terminals, and the red wire is usually used for hot terminals. When testing for resistance and continuity, it makes no difference which probe is used where.
- Ports: Most multimeters are equipped with three ports: COM (or “-”), mAVΩ, and 10A. COM stands for common and is what the black probe is usually plugged into. The mAVΩ port is where the red probe plugs into to measure volts, resistance, and current. The 10A port is a specialty port that’s used when measuring currents greater than 200 mA. Some multimeters have four ports that split the functions of the mAVΩ setting in two, with one VΩ setting for volts and resistance and a µAmA setting for current.
When using a multimeter to test live electricity, it's important to take certain safety precautions to avoid any chance of electrocution as accidents can occur. These include:
- Wearing insulated gloves
- Not using a multimeter with any broken or frayed wires on the probes
- Not performing any electrical testing in wet conditions
- Not touching the tips of the probes together while testing
How to Use a Multimeter
Turn the selection knob to either AC or DC volts, depending on what you're testing. Place the black probe on the negative terminal of the component you're testing, and the red probe onto the positive terminal.
As an example, let’s assume you’re testing a standard, 3-pronged electrical outlet in a US home. Turn the selection knob to AC volts and ensure the probes are plugged into the appropriate port. Place the black probe into the slot on the top left of the outlet (neutral) and the red probe into the upper right slot (hot). If the measurement doesn't read around 120-volts, a wiring issue is likely to blame. Another indication of a wiring issue is if you place the black probe into the u-shaped slot on the bottom of the outlet (ground), the red prong into the neutral slot, and the measurement is greater than 0.
You can also test the outlet's ground by inserting the black prong into the bottom slot, and the red probe in the hot slot. The measurement should be roughly 120-volts, or else the outlet has a bad ground.
If your multimeter has a dedicated continuity setting, turn the selection knob to continuity. Verify the meter and probes are working by touching the tips of the probes together. The meter should beep if it’s working properly.
A common use of continuity testing is checking the functionality of a power cord. Start by touching one of the multimeter probes to one of the prongs on the male side of the power cord. Insert the other probe into the corresponding slot on the female end of the cord. The multimeter will make an audible beep if there is continuity. Repeat this process on the remaining male prong and female slot. If there wasn't a beep on either side, the power cord needs to be replaced.
Next, touch one of the probes to one of the prongs on the male end of the cord, and the other probe to the other male prong on the same end. If the meter beeps, that indicates a short and the cord needs to be replaced.
If your meter doesn't have a continuity setting, you can test the resistance instead. Turn the selection knob to the Ω setting, and use the same procedure outlined above. The only difference is that you're looking for a reading between 0 and 1 on the display instead of listening for an audible beep. If the reading is 1 or OL (open loop), the circuit lacks continuity and the cord should be replaced.
Aside from testing continuity, measuring resistance is commonly used when checking an electrical component’s resistors (like in a speaker).
If checking a resistor, determine the resistor's resistance value. This can either be found marked on the resistor itself or in the owner's manual of the component being tested. Turn the selection knob to the Ω setting and place each of the multimeter's probes to one of the resistor’s leads. If the resistor is functioning properly, the resistance value on the meter’s display should be the same as the resistor’s rating. If not, the resistor is defective and needs to be replaced.
One of the most common reasons to measure amps is for diagnosing vehicle electrical issues. For example, a parasitic draw on the battery.
What Is a Parasitic Draw?
Also called a "drain", a parasitic draw is a term used to describe an electrical component that's consuming electricity even when the vehicle is shut off. This can lead to low battery voltage, causing a persistently dead battery and vehicle starting issues.
To diagnose this, disconnect the battery cable from the battery's negative post (marked "-", and usually black in color). Plug the red probe into the 10A port of your multimeter, and turn the selection knob to the amp setting. Connect one of the multimeter's probes to the battery post, and the other probe to the battery cable. The measurement should be between 50 and 60 mA. Anything higher indicates a parasitic draw.
If a parasitic draw is detected, remove and test the vehicle's fuses one by one until you achieve the desired reading on the multimeter. The circuit operated by the fuse that was pulled is the source of the issue.
Employee Is Electrocuted From Contact With Energized Wires. Occupational Safety and Health Administration, U.S. Department of Labor.