Rectifier Type Instrument

Definition

Instruments that utilize a rectifying element to measure voltage and current are known as rectifying instruments. The rectifying element converts alternating current (AC) into direct current (DC), which is then indicated by a DC-responsive meter. A Permanent Magnet Moving Coil (PMMC) instrument is commonly used as the indicating device.

Rectifying instruments exhibit higher sensitivity compared to moving coil and electrodynamometer instruments, making them suitable for current and voltage measurements. The circuit arrangement of a rectifier instrument is shown in the figure below, featuring four diodes that function as the rectifying element.

The multiplier resistance

Rs

is used to limit the current such that its value does not exceed the rating of the PMMC instrument.

Rectifying Element

A rectifying element converts alternating current (AC) to direct current (DC), ensuring unidirectional current flow through the PMMC instrument. Common materials for rectifying elements include copper oxide, selenium cells, germanium diodes, and silicon diodes.

The rectifying element exhibits zero resistance in the forward-biased state and infinite resistance in the reverse-biased state, a property critical for rectification.

Characteristic Curve of Rectifying Element

The characteristic curve of a rectifying circuit is shown in the figure below. Ideally, the rectifying element has no voltage drop in the forward direction and blocks all current in the reverse direction.

But practically, this is not possible. The real characteristic curve of the rectifying element shown in the figure below.

Half-Wave Rectifier Circuit

The figure below illustrates a half-wave rectifying circuit. The rectifying element is connected in series with the voltage source, resistance multiplier, and permanent magnet moving coil (PMMC) instrument. The forward resistance of the diode is assumed to be negligible.

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.