How to Use A Multimeter
You may have seen a multimeter being used by your local electrician or even a stereo installation shop. This universal tool is a must-have in all homes for many reasons. You do not need to have a degree in Electrical Engineering to find a use for a multimeter, also known as a volt/ohm meter (VOM). A VOM is an electronic measuring device that combines several different measurement functions into one handheld, easy to use device. No worries, I understand if you fear that you won’t be able to understand any of the weird symbols and numbers! Hopefully I can quell your fear here and now with helpful information and instructions on how to use a multimeter. Enough talk, let’s get to it!
There are two types of meters, auto ranging meters and manual meters. Auto ranging meters have a few selections on the dial; the meter will guess what your using it for based on the information it receives from the probes. Then there are manual ranging meters that make you select the input range it is going to use. The auto ranging meters still have the same functions as the manual meters; except auto ranging meters guesses the input range for you. This is similar to how automatic and manual cars essentially do the same thing, except one requires more user input.
The first thing you will normally see on all multimeters is the circular mode dial. The mode dial allows you to choose from many different measurements. The most common are AC/DC voltage, resistance and current. Occasionally multimeters test continuity, and they may also have diode testing functions.
The next thing you will notice are the input jacks. The black jack is called the common. It never moves from its slot. Then you have two red jacks, you choose the jack based on what you’re going to do. Measuring voltage and ohms tend to be on one jack, while measuring amperes tends to be on another jack. The reason amp measurement requires a different jack is because it lets the electrical signal flow through the meter to test it, whereas voltage/ohm measuring sends the signal out from the meter. A red and black probe connects to these jacks on the multimeter. These probes will be used to collect these different measurements. Higher end meters will have more measuring options and jacks.
Lastly, at the top of most multimeters is the display. The most important function of the display is to show as much information as possible. Some show just digits and decimal points, some show the unit of measurements as well, such as volts (V) or milivolts (mV). Higher quality units display much more information.
Now that you have the basic idea of the various components of the multimeter, lets dig a bit deeper into the mode dial and explain what all of the numbers and symbols mean. The picture displayed is a basic multimeter which I will reference throughout this article. Multimeters come in many different configurations, but all serve the same function and have similar symbols. The standard user should be able to pick up any standard multimeter and use it without a headache.
DC Voltage (V with a straight line and a dashed line under it): In the meter pictured, DC Volt is written, not expressed as a symbol. The range of this meter is 200 milivolts to 1000 volts. The 1000 is highlighted in red to remind you that working with voltage is extremely dangerous. The Car DC Volts setting is commonly used to monitor a circuit to see how it’s working. If you reverse the probes you will get a negative measurement. You can measure DC voltage while the circuit is working. The most common use of this setting is to test remaining battery voltage, alternator output, power to a car radio, amplifier, etc.
AC Voltage (V~ with a wavy line): In the meter pictured, AC Volts is written, not expressed as a symbol. This meter has a 200V setting and a 750V setting. The 750 is highlighted in red to remind you that working with that voltage is extremely dangerous. The Home AC Volts setting is used to measure wall voltage, unregulated power supplies, etc. AC voltage has no polarity; if you reverse the probes you will still get the same reading.
DC Amperage (A with a straight line and a dashed line under it): In the meter pictured, DC Amps is written, not expressed as a symbol. To test amps, the red plug needs to be removed from POWER and plugged into the slot above it. Other multimeters might have this slot labeled or in a different location. This meter measures from 2000 microamps(µ) to 200 miliamps(m).
Resistance(Ω): In the meter pictured, resistance in ohms is expressed as a symbol Ω. This setting is used to measure the resistance of a circuit in ohms (Ω). Ohm measuring is typically used to determine the resistance that a subwoofer is wired at. Make sure the device is turned off before taking a resistance measurement to avoid damage to your multi-meter.
Continuity(>+): In the meter pictured, continuity is displayed with a symbol. Continuity is used to measure the resistance of a closed circuit; a beep will sound to indicate solid continuity. Think of it as a way to test if a wire or circuit is continuous, meaning electricity flows from one end to the other. If there is no continuity, the circuit is broken or you are not on the correct circuit.
Here are a few examples where a multimeter proves handy and how exactly to use the meter. Once you understand the basic concepts, you can easily start working on more than what is listed here.
Testing Batteries: Testing batteries is a great way to practice with a multimeter and it is useful in this day and age considering all of our electronics have a thirst for energy that is quenched with battery power. The most common battery types are AAA, AA, C and D, all of which will have different voltage ratings usually listed on the battery. Some meters have a 2 Volt DC setting for the lower voltage batteries. If your meter does not, start high and work your way down. So if you’re testing a 9V Alkaline battery, put your multi-meter on 20V DC and place the red(+) probe on the positive side of the battery (usually marked) and the black(-) probe on the negative side of the battery. If it’s a new battery and your reading higher than 9V, let’s say 9.53V, that is not a mistake. The voltage listed on batteries is the average voltage you can expect from it, as it dies it will slowly drift down past 9V. When it reaches its lower operating range it is safe to assume the battery is almost used up and that it’s time to replace it.
Testing a Wall Socket: This is originally what a multimeter was designed to do and is the easiest test. Make sure you have a house rated multi-meter that can withstand 120V. House current is 120V AC, so switch your multi-meter to 200V AC. Usually the narrow prong in the wall is the positive (hot) and the wide prong is the negative (neutral). With AC voltage it does not matter which probe is plugged into which slot. Plug one probe into the hot and one into the neutral, the meter should read between 110-120V AC. If your wall socket also has a circular prong, that is the ground. If you test the hot and the ground with the same 200V AC setting, it should read 110-120V. If you test the neutral and the ground, it should read 0V. Readings that deviate slightly from the average are ok.
Testing Speaker Wires: You have your dash open and the radio pulled out with the door panels off. You removed your speakers, but as you get ready to put in new ones, it doesn’t say which is positive and which is negative. Multimeter to the rescue! Simply turn your radio on to low volume and pull the speaker out, and you are ready to test for voltage. Turn your volt meter to 20V DC and put the negative probe on a chassis ground and put the positive probe on one of the speaker terminals. The one that has a voltage is going to be positive. Most vehicles have a black or white stripe on the positive wire, but there’s always the chance that they are incorrectly marked.
Determining Subwoofer Ohms: You bridged your subwoofers together, then sit back and pray, hoping that they equal the ohm rating the amplifier needs. If you connect the positive and negative speaker wires from the sub to the amp, it could end in disaster if you did not wire them correctly. A multimeter can tell you exactly the ohm level those subwoofers are at. Connect the two ends of the multimeter to the speaker terminals of the amplifier. Set your multimeter to 200 ohms (Ω) and connect the red probe to the positive and the black probe to the negative speaker wire.
Testing a Car Battery: If you have a sneaking suspicion that your car battery is running low on juice, a quick test with a multimeter can put your suspicions to rest! Wear safety glasses and gloves to protect you from any harmful chemicals on the battery. Locate your battery under the hood and follow the manufacturer’s directions to properly disconnect the cables from the battery. Set your multimeter to 20 Volts DC to give an accurate reading. Connect the red probe from the multimeter to the positive battery terminal. Attach the black probe from the multimeter to the negative battery terminal. After a few seconds read the meter, it should read at least 9.6 volts for the battery to accept a charge if you have a 12 volt battery. Even then it may not be worth keeping. A fully charged 12V battery should test at 12.4 – 12.7 volts when it is not attached to the car.
Testing a Car Alternator: You tested your battery and it’s functioning properly, but your car’s electrical components are having problems. It’s time to test the alternator. Wear safety glasses and gloves, and keep away from moving components. Set your multimeter to DC 20 Volts. Leave your battery connected to the car, and with the car off, connect the red probe of the multimeter to the positive lead on the battery. Then connect the black probe to the negative lead on the battery. With the car off, it should read 12.5-12.8V if your battery is good. Now start the engine with the multimeter still hooked up. It should now read 13.8-14.8V if the alternator is working properly. Finally, have someone turn on the headlights and heater blower on high. The volts will drop slightly at first but then return to 13.8-14.8V if the alternator is working properly.
About the Author
Written by Kyle Duffy