6. RECTIFIER INSTRUMENTS
These are not separate types of instruments, but rather a means of using a D’Arsonval movement, in conjunction with a rectifier, to change A.C. to D.C. Thus, a direct current movement can be adopted for use with alternating current as shown in Fig. 24. Rectifier type meters, Using copper oxide rectifiers, are useful at low frequencies and will give good indication upto about 20 kHz.
Fig. 24. A D.C. moving-coil meter can be used to measure A.C. voltage by putting a diode or rectifier in the meter circuit.
- Rectifier instruments can operate well into the RF. (radio-frequency) range with the proper use of silicon or germanium rectifiers.
- Measurements of current and voltage at several hundred megahertz are possible.
- Another advantage is that this type of meter is more sensitive than any other type of A.C. meter.
Characteristics of Rectifier Instruments:
The characteristics of rectifier instruments are given below:
- They provide an economical and practical means of A.C. measurements in radio and, communication circuits at audio frequencies (1000 to 10000 Hz) and in other A.C. circuits where small available power makes it necessary to use sensitive low-energy instruments,
- A rectifier instrument is, in general, approximately fifty times more sensitive than either an electrodynamometer or a moving-iron instrument.
- In most instances rectifier instruments have essentially linear scales.
- They commonly possess sensitivity of the order of 1000 to 2000 ohms per volt and more.
- The power consumed by a rectifier instrument is generally several times as high as its permanent magnet moving coil (PMMC) mechanism because of the resistance of its rectifier.
- Rectifier instruments are manufactured as microammeters and milliammeters in ratings from 100 µ A to 15 mA for full scale deflection. The higher rating of 15 mA is more or less governed by the special size of the rectifier in comparison with the mechanism with which it is to be used. –
- Shunting of rectifier instruments is not practical because of the change in resistance of the rectifier with both temperature and the amount of current.
Rectifier Ammeters. The rectifier ammeters usually consist of four rectifier elements L, M, N and P arranged in the form of a bridge (as shown in Fig. 25) and A represents a moving-coil ammeter. The apex of the black triangle (shown in the bridge) indicates the direction in which the resistance is low.
Fig. 25. Bridge circuit for full-wave rectification.
As shown in Fig. 25, during the half-cycles current flows through elements Land N from left to right as shown by the full arrows. During other half-cycles, the current flows through M and N, as shown by the dotted arrows. The waveform of the current flowing through A is therefore as shown in Fig. 26. The deflection of moving-coil ammeter A, thus depends upon the average value of the current; and the scale of A can be calibrated to read the r.m.s. value of the current on the assumption that the waveform of the latter is sinusoidal with a form factor of 1.11.
Fig. 26. Waveform of current through moving-coil ammeter.
Rectifier Voltmeters. In a rectifier voltmeter, A is a milliammeter and the bridge circuit of Fig. 25 is connected in series with a suitable resistor.
Advantages of rectifier instruments:
- The primary advantage of the rectifier voltmeter is that it is far more sensitive as compared to other types of voltmeter .suitable for measuring A.C. voltages.
- Metal rectifiers can be incorporated in universal instruments, such as ammeter, thereby enabling a moving-coil milliammeter to be used in combination with shunt and series resistances to measure various ranges of D.C. and D.C. voltage, and in combination with a bridge rectifier and suitable resistors to measure various ranges of A.C. and A.C. voltage.