Voltmeter
A voltmeter is an instrument used to measure the electrical potential difference, or voltage, between two points in an electrical circuit. It is one of the most commonly used measuring instruments in electronics, electrical engineering and troubleshooting.
What is a Voltmeter?
A voltmeter measures the voltage across a component or power source.
Voltage is the electrical "pressure" that pushes current through a circuit and is measured in volts (V).
Modern digital multimeters include a voltmeter function, while dedicated analog and digital voltmeters are also available.
How a Voltmeter Works
A voltmeter measures the difference in electrical potential between two points.
To avoid affecting the circuit being tested, a voltmeter has a very high internal resistance, typically several megaohms.
Because of this high resistance, only a tiny current flows through the meter.
How to Connect a Voltmeter
A voltmeter is always connected in parallel with the component or voltage source being measured.
Never connect a voltmeter in series with a circuit, as it will not measure the voltage correctly.
| Correct Connection | Reason |
|---|---|
| Parallel | Measures the voltage across the component. |
| Series | Incorrect connection. |
Measuring DC Voltage
- Select the DC Voltage (Vā) range.
- Insert the black lead into the COM terminal.
- Insert the red lead into the VĪ© terminal.
- Connect the probes across the circuit or battery.
- Read the displayed voltage.
Examples:
- AA Battery ā approximately 1.5 V
- 9 V Battery ā approximately 9 V
- Car Battery ā approximately 12.6 V
- USB Supply ā approximately 5 V
Measuring AC Voltage
- Select the AC Voltage (V~) range.
- Choose a range higher than the expected voltage.
- Place the probes across the AC source.
- Read the RMS voltage.
Examples:
- Household mains supply ā 230 V AC (or 120 V depending on the country).
- Transformer secondary output.
- Audio signal generators.
Analog vs Digital Voltmeters
| Analog Voltmeter | Digital Voltmeter |
|---|---|
| Pointer and scale. | Digital numerical display. |
| May be affected by parallax error. | Easy to read accurately. |
| Useful for observing changing values. | Higher accuracy and resolution. |
| Usually lower input resistance. | Very high input resistance. |
Applications
- Checking batteries.
- Testing power supplies.
- Troubleshooting electronic circuits.
- Automotive electrical systems.
- Solar power systems.
- Amplifier repair.
- Arduino and microcontroller projects.
- Industrial electrical maintenance.
Safety Tips
- Always select the correct AC or DC mode.
- Start with the highest voltage range if unsure.
- Use probes with undamaged insulation.
- Never exceed the maximum voltage rating of the meter.
- Keep fingers behind the probe guards when measuring high voltages.
- Be especially careful when measuring mains electricity.
Common Mistakes
- Connecting the voltmeter in series instead of parallel.
- Selecting AC mode when measuring DC voltage.
- Selecting DC mode when measuring AC voltage.
- Using the wrong measurement range.
- Reversing the probes on DC circuits (digital meters simply display a negative value).
Advantages of Digital Voltmeters
- High measurement accuracy.
- Very high input impedance.
- Easy-to-read display.
- Automatic polarity indication.
- Auto-ranging on many models.
- Often combined with resistance, current and continuity measurements.
Key Points
- A voltmeter measures electrical potential difference.
- Voltage is measured in volts (V).
- A voltmeter is always connected in parallel.
- Digital voltmeters have very high input resistance.
- Most modern multimeters include a voltmeter function.
- Always select the correct AC or DC voltage range before measuring.