Rectifier Type Instrument | Construction Principle of Operation

A rectifier type instrument measures alternating voltage or current by rectifying the input and applying the resulting unidirectional current to a permanent magnet moving coil type of instruments. It is most often used as an AC voltmeter. Compared with electrodynamic and thermocouple type instruments, this arrangement offers the following practical trade-offs.

1. Electrodynamometer instruments are generally more complex and expensive. A rectifier instrument is therefore a practical choice when its accuracy and waveform limits are acceptable.
2. The thermocouple instruments can measure at higher frequencies, while diode capacitance and switching behaviour limit the upper frequency of a rectifier instrument.

To understand the construction and working of rectifier type instruments, first consider the voltage-current characteristic of the rectifying diode.
An ideal diode has zero forward resistance and infinite reverse resistance. It therefore conducts in one direction and blocks current in the other.

Rectification converts an alternating quantity into a unidirectional quantity. The following circuit shows the basic diode arrangement.

In this circuit, the ideal diode is in series with the voltage source and load resistance. With its anode positive relative to its cathode, the diode is forward biased and acts as a short circuit in the ideal model. Current then flows through the load.

Reversing the connections so that the negative terminal is at the anode and the positive terminal of the battery is at the cathode reverse biases the ideal diode. It then acts as an open circuit.

A practical diode differs from this ideal model. It has a forward voltage drop and small reverse leakage current. Its forward current rises rapidly after the applied voltage reaches the knee region. Under reverse bias it blocks current only up to its rated reverse voltage; reverse breakdown occurs if that limit is exceeded. These non-ideal characteristics affect meter sensitivity, linearity, temperature response and overload limits.

Rectifier type instruments commonly use either half-wave or full-wave rectification.

Half Wave Rectifier Circuits of Rectifier Type Instruments

In the half wave rectifier circuit, the diode is in series with a sinusoidal source, a permanent magnet moving coil instrument and a multiplier resistor.

The multiplier electrical resistance limits movement current and sets the voltage range. It prevents the PMMC coil from exceeding its rated current. The following analysis treats the diode as ideal and compares the response to DC and sinusoidal AC.

Let the multiplier resistance be R and the PMMC resistance be R₁. For an applied DC voltage V, the steady current is I = V/(R + R1). For a sinusoidal input v = Vm sin(ωt), the diode conducts during one half-cycle and blocks during the other. The PMMC receives a pulsating unidirectional current, and its mechanical inertia produces a steady deflection proportional to the average current.

For an ideal half-wave rectified sine wave, the average value over a full cycle is Vm/π. Since the RMS value V of the original sine wave is Vm/√2, the average rectified value is about 0.45V. The half-wave AC current sensitivity is therefore about 45% of the corresponding DC sensitivity, before practical diode effects are included.

Full Wave Rectifier Circuits of Rectifier Type Instruments

The following full wave rectifier uses a diode bridge.

The bridge directs both half-cycles through the PMMC in the same direction. A series multiplier resistance limits current from the voltage source. Let R be the multiplier resistance and R₁ the PMMC resistance. With ideal diodes, an applied DC voltage V gives I = V/(R + R₁). For the sinusoidal input v = Vm sin(ωt), the mean current is about 0.9V/(R + R₁), where V is the RMS value of the original sine wave. The AC sensitivity is therefore about 90% of the corresponding DC sensitivity. A practical bridge also includes the forward drops of the two conducting diodes.

The following factors affect the performance of rectifier type instruments.

1. A rectifier instrument responds to the average rectified value but is normally calibrated to display the RMS value of a sine wave. A non-sinusoidal waveform with a different form factor therefore causes a reading error.
2. Diode forward voltage, dynamic resistance and non-linearity introduce errors, especially when the measured voltage is low.
3. Temperature changes the diode forward voltage and circuit resistance. Accurate designs use suitable temperature-compensation components and stable multiplier resistors; a high temperature coefficient alone does not provide compensation.
4. Diode junction capacitance and switching behaviour affect the current at high frequency. The resulting frequency-response error limits the useful measurement range.
5. The rectifier voltage drop and the lower average current make AC sensitivity lower than the PMMC movement’s DC sensitivity.

Advantages of Rectifier Type Instruments

Following are the advantages of the rectifier type of instruments:

• The circuit is economical and can provide suitable accuracy when the waveform, level, frequency and temperature remain within its specified limits.
• Its useful frequency range can exceed that of some electromechanical AC instruments, subject to the rectifier’s frequency limits.
• The PMMC movement provides an approximately uniform scale.
• The PMMC movement needs relatively little operating current.

An AC rectifier voltmeter, whether it uses half-wave or full wave diode rectifier operation, has lower sensitivity than the same PMMC movement on DC. For a given voltage range, this usually means lower input resistance and greater loading of the measured circuit than a comparable DC voltmeter.