When a DC voltage source is applied to the circuit, a current Im flows through it, with a magnitude equal to V/(Rm + RS). This current causes full-scale deflection in the instrument.
When an AC voltage is applied to the same circuit, the rectifying element converts the AC voltage into unidirectional DC voltage, yielding a rectified output through the instrument. The PMMC instrument deflects based on the average value of the current, which depends on the average voltage of the AC source.
Average Value of Voltage
The above calculation indicates that the sensitivity of the instrument for AC is 0.45 times the current sensitivity for DC.
Full-Wave Rectifier Instrument
The circuit of a full-wave rectifier is shown in the figure below.
The DC voltage applied to the circuit causes the full-scale deflection of the PMMC meter. The sinusoidal voltage applies to the meter express as
For the same voltage value, the average value of AC is 0.9 times that of DC. In other words, the sensitivity of the instrument with AC is 90% of that with DC.
The sensitivity of a full-wave rectifier instrument is double that of a half-wave rectifier instrument.
Sensitivity of Rectifier Instrument
The sensitivity of an instrument reflects how the measured quantity varies from input to output, such as the DC sensitivity of a rectifier instrument.
The sensitivity of an AC rectifier-type instrument depends on the type of rectifying element used in the circuit.
Factors Affecting the Performance of Rectifier-Type Instruments
The following factors impact the instrument’s performance when used with AC:
Effects of Waveform
Rectifier instruments are calibrated based on the RMS (root-mean-square) value of voltage and current. The form factor of half-wave and full-wave rectifier instruments is fixed for the calibrated scale. If a waveform with a different form factor is applied, reading errors due to waveform mismatch will occur.
Effect of Temperature Change
The resistance of the rectifying element varies with temperature, introducing errors into the instrument’s measurements.
Effect of High-Frequency Currents
Rectifier instruments have imperfect capacitance characteristics, allowing high-frequency currents to pass through and affect readings.、
Decrease in Sensitivity
The sensitivity of rectifier-type instruments for AC operation is lower than that for DC operation.
Advantages of Rectifying Instruments
- Expanded Frequency Range: Operates from 20 Hz to high-frequency ranges.
- Low Current Consumption: For voltmeters, the current operating range is much lower than other AC instruments.
- Uniform Scales: Provides uniform scales across a wide measurement range.
- Moderate Accuracy: Achieves ±5% accuracy under normal operating conditions.
Applications of Rectifying Instruments
- Measures voltage in the range of 50–250 V.
- Used as milliammeters or microammeters.
- Applied in communication circuits for signal measurement.
- Note: The sensitivity of an AC rectifier voltmeter is lower than that of a DC voltmeter, resulting in a higher loading effect for AC measurements.
