Buzz
A voltmeter is one of the most useful tools for testing electricity at home when used correctly. If this is your first time using a voltmeter, it's recommended to learn how to set up the device and test it on a low-voltage circuit, such as a household battery.
Today, Mytour will guide you through how to check voltage. You may also want to explore how to use a multimeter to measure current and resistance.
Steps
Set up the device
Set the device to measure voltage. Most voltage testers are also multimeters, which can check various aspects of the electrical circuit. If your device has a dial with several settings, set it as follows:
- To measure alternating current (AC) voltage, turn the dial to V~, ACV, or VAC. Most household circuits use AC power.
- To measure direct current (DC) voltage, select V–, V---, DCV, or VDC. Batteries and portable electronic devices typically use DC power.
Select a range higher than the expected maximum voltage. Most voltmeters offer multiple options for voltage, allowing you to adjust the meter's sensitivity for more accurate readings and to prevent damaging the device. If the electronic device doesn't display a range, it is an "auto-ranging" device that will detect the appropriate range automatically. If the device has a range setting, you should:
- Select a setting higher than the expected maximum voltage. If you're unsure, choose the highest setting to avoid damaging the device.
- Household batteries typically show a voltage label, often 9V or lower.
- Car batteries have a voltage of about 12.6V when fully charged and with the engine off.
- In most countries, home electrical voltage is usually 240V. In Vietnam, the standard voltage is 220V.
- mV stands for millivolt, or (1/1000 V), often used for the lowest setting.
Connect the probes. The voltmeter comes with two probes, one black and the other red. Each probe has a metal tip, and the other end has a metal jack for plugging into the voltmeter. Connect the jacks as follows:
- The black jack always plugs into the "COM" port.
- For voltage measurements, the red jack should be plugged into the port labeled V (or other symbols). If there's no V label, use the port with the smallest unit or mA.
Measure voltage
Hold the probes carefully. Avoid touching the metal tips while they are connected to the circuit. If the insulation is worn or torn, wear rubber gloves or replace the cables.
- Never let the metal probes touch each other while connected to the circuit, as this could cause dangerous sparks.
Place the black probe in a part of the circuit. To check the voltage of a circuit, connect the probes in parallel to the circuit. In other words, place the probes at two points on a live circuit.
- For a battery, place the black probe on the negative terminal.
- For a wall outlet, insert the black probe into the neutral (or common) hole. Use a test pen to check first. If the pen's light doesn't turn on, the hole is neutral.
- Check the black probe and remove any small plastic tip (if present) before proceeding, as this plastic part may get stuck in the socket.
Connect the red probe to the other point of the circuit. Once the parallel circuit is established, the voltage will be displayed on the meter.
- For a battery, connect the red probe to the positive terminal.
- For an outlet, insert the red probe into the hole marked 'hot.' It is advisable to use a voltage tester first. If the light on the tester turns on, it indicates the 'hot' terminal.
Increase the range if the measurement result shows overload. If you see any of the following results, immediately increase the range to a higher voltage setting to avoid damaging the meter:
- The digital display shows 'OL', 'overload', or '1'. Note: '1V' is a real value and poses no concern.
- The analog needle swings past the scale.
Adjust the voltmeter if necessary. You may need to adjust the voltmeter if the digital display shows 0V or nothing at all, or if the analog needle barely moves. If there's still no reading, follow these steps:
- Check if the probes are properly contacting the circuit.
- If measuring a DC circuit with no result, look for a small knob or switch labeled DC+ and DC- on the device and switch to the other setting. If the device lacks this option, swap the red and black probes.
- Reduce the range step by step. Keep reducing until you get a proper reading.
Read the voltmeter. The digital voltmeter will clearly display the voltage on the electronic screen. Using an analog voltmeter is slightly more complex but still manageable if you know how to read it. Continue reading for guidance on how to read an analog voltmeter below.
Read the analog voltmeter
Identify the voltage scale on the meter's dial. Select the scale that matches the setting you've adjusted using the voltmeter's dial. If an exact reading isn't possible, you'll need to perform some calculations based on the scale's maximum value and the set level.
- For example, if the voltmeter is set to DC 10V, look for the scale with a maximum of 10. If none is found, choose a scale with a maximum of 50.
Estimate the value based on the needle's position relative to nearby numbers. This is a linear scale similar to a ruler.
- For instance, if the needle points between 30 and 40, the reading is approximately 35V.
Adjust the reading if using a different scale. Skip this step if you're reading directly from the scale that exactly matches the voltmeter's setting. If not, you'll need to recalculate by dividing the maximum value of the scale printed by the dial's setting. Then, divide the needle's reading by this result to get the actual voltage.
- For example, if the voltmeter is set to 10V but you're reading from a 50V scale, divide 50 by 10 = 5. If the needle shows 35V, the actual voltage is 35 ÷ 5 = 7V.
Advice
- The guide for checking voltage in an outlet assumes you're determining the voltage supplied to a device using that outlet. To check for line issues, measure the voltage between the ground and a different hole. If the reading is low (e.g., 2V), this indicates the neutral terminal, and you've measured voltage drop (voltage loss). If the reading is high (e.g., 110V or 220V), this is the hot terminal.
- When checking a wall outlet, measure the vertical prongs. For a three-pronged outlet, the round hole is the ground terminal, so no testing is necessary.
Warning
- Improper use of the device may lead to damage, severe electric shocks, or sparks that could cause fires. Checking high-voltage outlets or circuits carries greater risks compared to measuring the voltage across low-voltage batteries. If you simply want to practice measuring and reading voltmeter readings, start with a household battery.
