How to Use a Voltmeter

A voltmeter is one of the most useful devices for home electrical testing if you use it correctly. If this is your first time using a voltmeter, you should learn how to set up the device and test it on a low-voltage circuit, such as a household battery.

This wikiHow teaches you how to check voltage. You’ll probably also want to learn how to use a multimeter to measure amperage and resistance.

Table of Contents

Device settings

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Set the device to the voltage measurement function. Most voltage measuring devices are also multimeters with the function of testing many aspects of electrical circuits. If the device has a knob with multiple settings, set it to the following mode: ^{[1] X Trusted Source Science Buddies Go to source}

To check the AC voltage, turn the knob to V~ , ACV or VAC mode. Household electrical circuits are mostly alternating current.
To test the voltage of DC current, you need to select V– , V— , DCV or VDC . Batteries and portable electronic devices both use direct current.

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Select a range higher than the maximum expected voltage. Most voltmeters have different options for voltage, so you can change the sensitivity of the meter for more accurate measurements and avoid damage to the device. If the electronic device does not show range, this is an “automatic modulation” device and will detect the appropriate range on its own. If the device has range, you need:

Select a setting higher than the maximum expected voltage. If you do not know this parameter, choose the highest level to avoid damage to the device.
Household batteries will have a label showing the voltage, usually 9V or less.
Car batteries have a voltage of about 12.6V when fully charged with the engine off. ^{[2] X Research Source}
In most countries around the world, the power source in the house is usually 240V. In Vietnam, the standard voltage is 220V. ^{[3] X Research Sources}
mV is the symbol for millivpt or ( ¹ / ₁₀₀₀ V), this unit is sometimes used to indicate the lowest setting level.

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Connect the probe. The voltmeter will come with two probe leads, one black and the other red. Each wire has a metal probe, the other end is a metal jack to plug into the hole on the voltmeter. Connect the jacks as follows: ^{[4] X Trusted Source Science Buddies Go to source}

The black jack always plugs into the hole labeled “COM”.
When measuring voltage, you need to plug the red jack into the hole labeled V (among other symbols). If there is no V symbol, select the hole with the smallest unit of measurement or mA .

Measure voltage

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Handle the probe carefully. Do not touch the metal tip while it is connected to the circuit. If the insulation is worn or torn, you need to wear insulating gloves or purchase a replacement conductor.

Two metal probes should never touch each other while connected to a circuit, otherwise dangerous sparks may result.

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Insert the black probe into the part of the circuit. Check the voltage of the circuit by connecting the wires in parallel. In other words, you would connect probes to two points on a closed circuit where current is flowing.

On the battery, touch the black probe to the cathode.
With a wall outlet, insert the black probe into the hole with the cold (or neutral) pin. You should use an electric tester to check first. If the light on the pen doesn’t come on, it’s a hole with a cold pin.
Check the black probe and remove the small protruding plastic knob (if any) before proceeding, as this piece of plastic can become lodged in the electrical outlet.

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Connect the red probe to the rest of the circuit. Once the parallel circuit has formed, the voltage will show on the meter.

For the battery, you need to connect the red probe to the positive terminal.
On an electrical outlet, plug the red probe into the hole with the “hot” pin. You should use an electric tester to check first. If the light on the pen comes on, it’s a hot pin hole.

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Increase the range if the measurement result is overloaded. Immediately increase the range to a higher voltage setting before the meter fails if you get one of the following readings:

The digital display shows “OL”, “overload” or “1”. ^{[5] X Trusted Source Science Buddies Go to Source} Note: “1V” is a true reading and nothing to worry about.
The analog needle jumps to the other side of the scale.

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Adjust the voltmeter if necessary. You may need an adjustment if the digital voltmeter says 0V or shows nothing, as well as when the analog voltmeter’s needle barely moves. If there is still no index, check in turn as follows:

Check that the probes are in contact with the electrical circuit.
If you are measuring DC circuits and there are no results, find the small knob or switch labeled DC+ and DC- on the unit and switch to the other setting. ^{[6] X Study Source} If the instrument does not have this option, you need to swap sides of the red and black probes.
Decrease range by level. Keep decreasing until you get a real reading.

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Read the voltmeter. The digital voltmeter will show the voltage clearly on the electronic display. With a needle voltmeter it will be a bit more complicated, but not too difficult if you know how to read. Continue reading instructions on how to read a needle voltmeter below.

Read needle voltmeter

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Determine the voltage scale on the dial of the hand. Select the scale that matches the setting you adjusted using the voltmeter’s knob. If there is no exact result, you need to do a little calculation based on the scale reading and setting level.

For example, if the voltmeter is in DC 10V mode, you need to look at the scale with a maximum reading of 10. If not, look for a scale with a maximum reading of 50.

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Estimate the index through the position of the needle relative to nearby numbers. This is a linear scale similar to a ruler.

For example, if the needle is pointing between 30 and 40, the reading is 35V.

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Divide the index if you use a different scale. Skip this step if you are reading from a scale that matches the voltmeter setting perfectly. If not, you need to recalculate correctly by dividing the maximum value of the printed scale by the setting index of the knob. Then, take the index that the needle is pointing to divide by the quotient just now to get the actual voltage.

For example, if the voltmeter is on the 10V setting but you read it on the 50V scale, take 50 ÷ 10 = 5 . If the needle is pointing at 35V, your actual voltage is 35 ÷ 5 = 7V.

Advice

The instructions for checking the voltage in the electrical outlet above assume that you are trying to determine the voltage passing through the device that will use this power source. If you want to identify a line problem, you need to measure the voltage between the ground and the other hole. If the measured voltage is low (such as 2V), this is a cold pin and you have just performed a voltage drop (voltage drop). If you get a high reading (eg 110V or 220V) then this hole is the hot wire pin. ^{[7] X Research Sources}
If testing a wall outlet, measure the vertical pins. For 3-pin sockets, the round hole is the ground wire pin, so you don’t need to check.

Warning

If used improperly, the device may be damaged, you may receive a serious electric shock, or create a spark that can cause a fire. Testing a socket or other high voltage circuit is more risky than measuring voltage on a low voltage battery. If you just want to practice measuring and reading on a voltmeter, do it on a household battery.

Steps

Device settings

Measure voltage

Read needle voltmeter

Advice

Warning

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