Rectifier Type Instrument Construction and Advantages

A rectifier type instrument measures alternating electrical signal by means of D.C measuring instrument. As the name implies, this instrument first rectifies an A.C signal to D.C then measures. Although it measures the rectified A.C signal (D.C signal), but the scale of the instrument is calibrated for A.C. The sensitivity of D’ Arsonval instrument is quite high. But a D’Arsonval instrument can only measure to D.C. So, to utilize the sensitivity of D’Arsonval movement for A.C, we use a rectifier type instrument.

Advantages of Rectifier Type Instrument

  1. This instrument can measure an electrical signal of very low frequency to radio frequency.
  2. The instrument is also capable of measuring electrical signals up to several mega Hz.
  3. The sensitivity of the instrument is much higher than that of any other type of A.C measuring instruments. Actually, for achieving very high sensitivity in A.C measurements, we use rectifier type instruments.

rectifier type instrument

Rectifier Elements for Rectifier Type Instrument

To convert A.C. to D.C, a rectifier instrument must have some rectifier elements.

At the low-frequency range, the instrument uses copper oxide or selenium cells for rectification purpose. Again at higher frequencies, we use germanium or silicon diodes.

The copper oxide rectifier element has a peak inverse voltage (PIV) about 2 volts. On the other hand, the selenium element has PIV of 10 volts. Also, both of these rectifier elements have very low current handling capacity. Therefore, for rectification purpose, these elements have become obsolete in the modern era.

Besides it, a germanium diode has a peak inverse voltage (PIV) of about 300 volts. In addition to that, the current carrying capacity of a germanium diode is about 100 mA. Also, PIV of silicon diode is about 1000 volts with a current rating of 5000 mA. Therefore these germanium and silicon diodes have become most suitable choice as rectifier elements for the purposes.

Characteristic of Rectifier Type Instrument

  1. A rectifier type instrument is an economical and suitable mean of A.C measurement.
  2. It can measure the electrical signal at an audio frequency range.
  3. The sensitivity of the instrument is much higher than a commonly used A.C measuring instrument. It has been found that the sensitivity of a general rectifier type instrument is around 50 times higher than of a dynamometer type instrument or a moving iron type instrument.
  4. Generally, D’ Arsonval movement is uniform; therefore a rectifier type instrument generally has a linear scale.
  5. The sensitivity of the instrument is in order of 1000 to 2000 ohms/volt.
  6. The power consumption of a rectifier type instrument is high because of the resistance of the rectifier elements.
  7. Shunting of the rectifier instrument is not practical because the resistance of rectifier elements changes with the temperature and the current flowing through it.
  8. The rectifier type instrument is capable of measuring a very tiny current of microampere range. Again it can also be capable of measuring current in the milliampere range. But we do not construct a rectifier type instrument beyond for the current of 15mA. Because it requires special sized rectifier elements and the size of the instruments becomes impracticality large.

Rectifier Ammeter

A rectifier type ammeter usually consists of a full wave bridge rectifier circuit. It uses a permanent magnet moving coil instrument for measuring the rectified current. The movement of the pointer of the pmmc instrument depends on the average value of the rectified signal. So, we can calibrate the scale of the pmmc instrument for RMS value of the current to be measured. Since the RMS value of the signal is 1.11 times greater than the average value of the instrument for sinusoidal AC signals.

Rectifier Voltmeter

The construction and the working of a rectifier type voltmeter are similar to that of a rectifier type ammeter. The only difference is that we connect a rectifier type voltmeter across the circuit instead of connecting it in series.

Ranges of Rectifier Type Instrument

  1. It can measure from 0 to 1000 μA.
  2. For milliampere range, it can measure from 0 to 1000 mA.
  3. The rectifier type instrument can measure from 0 to 1 volt, although some instruments can measure a higher voltage range from 0 to 250 volts.
  4. The range of the frequencies of the signal to be measured is from 20Hz to 20 kHz (Human audio frequency range).

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