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Electrical Tool Basics: What Is a Multimeter and How Does It Work?

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What Is a Multimeter?

In today’s world where almost everything is powered by electricity, a multimeter is an extremely useful tool. Also called a multitester or VOM (volt-ohm-milliammeter), multimeters are designed to measure voltage, current, and resistance. This enables the user to check if a connection is broken, determine the remaining capacity of a battery, and diagnose a faulty appliance, among many other applications.

There are two types of multimeters available in the market today: analog multimeters and digital multimeters. The two usually have the same functions, and only differ in terms of their display. An analog multimeter uses a thin needle to indicate the measurement, while a digital multimeter shows the value on an LCD screen. The latter is often sufficient for simple home or office repairs, but some electrical professionals may prefer the former since these are more sensitive to smaller variances and can thus offer more precise readings.Digital and Analog Multimeter

However, whichever kind of multimeter you choose, the basics of operating and reading them remain the same. Simply plug in the probes (the black probe usually goes into the “COM” or the ground port and the red probe into the mAVΩ port or whatever is the lowest current label), turn the dial to the function you need, and simply touch the probes to the circuit (make sure it is turned off) or the battery poles.

Testing for Voltage

To test for AC or DC voltage, simply turn the dial to the V section. The ⎓ symbol indicates direct current (use this setting for testing batteries) while a tilde or wavy line (~) indicates alternating current (use this setting for testing household circuits). In some models, the settings can be marked V–, V—, or VDC or V~, ACV, or VAC for direct and alternating current, respectively.

From there, you have to choose the correct value setting. This should be the value that is the next higher value immediately after the one that you are going to measure. For example, if you are testing a 9 volt battery and your multimeter has 10V, 50V, 250V, and 500V settings, the best option is 10V. With autoranging multimeters, you don’t need to choose the correct setting since the multimeter will do it for you.

Multimeter testing voltage

If you’re not sure about the expected value, start at the highest setting and adjust until you get the correct one. You will know if the setting is too low if the dial is on 1 or the display reads OL. You will also know that you have placed the probes to the incorrect terminal if the output reading is negative. (Don’t worry since this will not damage your multimeter.)

Testing for Resistance

The resistance mode is indicated by the Greek capital letter omega (Ω), the symbol used to denote ohms. If your multimeter is an older model, the resistance mode may use the label R. A multimeter can check the resistance for both complete circuits and individual, unattached components like a resistor. If you are testing a complete circuit, make sure that there is no current running through it at the time.

Multimeter testing resistance

Similar to the way you test for voltage, the setting should be a bit higher than your expected measurement. If the multimeter indicates a value that is zero or close to zero, the range is too high; if it reads 1 or OL, the range is too low. Adjust the setting one step at a time to ensure that you can get the most accurate measure. Moreover, it doesn’t matter which probe you place at each end of the circuit because resistance is non-directional. As long as you select the right range, you will get the right value.

Testing for Current

Current is measured in amperes, so the usual symbol for this in your multimeter is the capital letter A. Like testing for voltage, you should determine if you are testing direct or alternating current. Symbols can vary – among the most common ones are A–, A—, DCA, or ADC for direct current, and or A~, ACA, or AAC for alternating current.

Testing for current means your multimeter has to be a part of the (series) circuit. Once your multimeter is connected to the circuit, it will be able to measure the current as it flows through the circuit board. From here, you simply have to get the correct range. Use the 200mA setting for most basic electrical diagnoses and repairs; the 10A port is likely too much and you may end up blowing a fuse.

If you get a negative reading or if the needle doesn’t move, this means that you simply have to switch placement of the probes. Alternatively, if your multimeter has DC+ and DC- settings, you can set it to DC+ when measuring a direct current and then switch to DC- if you get no reading or a negative reading.

 

These are the most common functions of multimeters, although more modern and more expensive units may have additional settings. These include a continuity test function (indicated by ))) or similar symbols), which indicates if a circuit is electrically connected; a diode test function (an arrow pointing to the right, with a cross at the end), which tests if a one-way electrical circuit is connected; a Hertz function (Hz), which measures frequency in AC circuits; and a capacitance function (indicated by the –|(– symbol), which measures the ability of an object to store an electric charge.

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