What is the difference between an AC voltmeter and a DC voltmeter?

Short Answer:

An AC voltmeter is used to measure alternating current (AC) voltages, while a DC voltmeter is designed to measure direct current (DC) voltages. The main difference lies in how they handle the type of voltage—AC voltmeters are built to respond to changing polarities, whereas DC voltmeters work with a steady, unidirectional voltage.

AC voltmeters usually include rectifier and filtering circuits to convert AC into a measurable form, while DC voltmeters can directly measure the voltage using sensitive moving coil mechanisms. Both types are calibrated differently to suit their specific voltage types.

Detailed Explanation:

Difference between AC voltmeter and DC voltmeter

Voltmeter is a device used to measure electrical potential difference between two points. However, depending on the nature of the voltage, we use either an AC voltmeter or a DC voltmeter. The construction, internal circuits, and operation of both are different because AC and DC voltages behave differently.

DC voltmeter:

A DC voltmeter is used to measure the voltage of direct current sources such as batteries, solar panels, or regulated DC power supplies. In DC voltage, the current flows in a single direction with a constant polarity.
The simplest form of a DC voltmeter uses a moving coil galvanometer with a high resistance in series (multiplier resistor) to measure voltage. When DC voltage is applied, it causes a steady current through the coil which interacts with a magnetic field and deflects a pointer to indicate the voltage. The scale is linear and directly proportional to the applied voltage.

AC voltmeter:

An AC voltmeter is designed to measure the voltage of alternating current sources like household electrical outlets or AC generators. In AC voltage, the current changes direction periodically, usually following a sine wave pattern.
Since the direction of current constantly changes, the average value of pure AC over a complete cycle is zero. Therefore, a simple galvanometer cannot measure AC directly. Instead, an AC voltmeter uses a rectifier circuit (usually diodes) to convert AC into pulsating DC, followed by a filtering process to smooth the signal. This converted voltage is then measured like DC. The reading is calibrated to show RMS (root mean square) voltage, which is equivalent to the DC value that produces the same heating effect.

Main Differences:

  1. Voltage Type:
    • DC voltmeter measures steady voltage in one direction.
    • AC voltmeter measures changing voltage that reverses direction.
  2. Circuit Design:
    • DC voltmeter uses a moving coil with a multiplier.
    • AC voltmeter uses a rectifier and filter before measurement.
  3. Current Handling:
    • DC voltmeters can directly respond to input.
    • AC voltmeters require conversion of signal to measure accurately.
  4. Measurement Scale:
    • DC voltmeter scale is usually linear.
    • AC voltmeter is calibrated in RMS values.
  5. Applications:
    • DC voltmeters are used in battery testing, DC power supplies, and electronic circuits.
    • AC voltmeters are used in electrical grids, home wiring, and AC equipment testing.

Important Considerations:

  • Using a DC voltmeter on an AC signal will give incorrect or no readings.
  • Using an AC voltmeter on DC without a blocking circuit may damage the instrument.
  • Some modern digital voltmeters are dual-purpose and can measure both AC and DC, depending on the mode selected.
Conclusion

The main difference between an AC voltmeter and a DC voltmeter lies in the type of voltage they are designed to measure and the internal circuit design. DC voltmeters are simpler and measure steady voltage, while AC voltmeters require additional rectifying and filtering stages to measure alternating voltage accurately. Understanding these differences is essential for choosing the correct instrument in electrical and electronic applications.