The AC voltmeter is designed for the measurement of AC voltage. the only difference between AC and DC voltmeter is that rectifier is used for the measurement of AC voltage and DC voltage in the AC voltmeter. Since the rectifier can convert ac voltage to dc voltage. The block diagram of the AC voltmeter is similar to the block diagram of the DC voltmeter.
According to the appropriate block diagram, the input to be measured is supplied to the attenuator circuit by which a particular range of voltage is selected. The output received from the attenuator is fed into the rectifier by which AC voltage is converted into pulsed DC voltage. And then the output obtained from the DC amplifier is shown to the PMMC meter.
Rectifiers can be used before multistage amplifiers or after amplifiers. It depends on the type of amplifier used in the AC voltmeter. If a multistage AC amplifier is being used, the rectifier circuit is used after the amplifier. Conversely, if the multistage amplifier DC is being used, then the rectifier is used before that.
An amplifier is an important component, it increases the efficiency of the signal appropriated during the measurement process. If the design is economical, then a multistage DC amplifier should be used.
Types of AC Voltmeters
AC voltmeters can be of three types depending on the type of measurement. as written below:
- Average Reading AC Voltmeter.
- Peak Reading AC Voltmeter.
- True RMS Reading AC Voltmeter.
Average Reading AC Voltmeter
The average reading voltmeter displays the average value measurement of AC voltage. The average reading is with reference to the calibrated scale RMS value in the voltmeter. Most of the AC voltages to be measured above are in sinusoidal form. It is effective to measure the average value of the AC voltage rather than its actual RMS value.
Average reading voltmeters are designed in different ways. Vacuum diodes, semiconductor diodes, or full-wave rectifiers are used in this. It should have linear voltage-current properties since a change in voltage is measured by a change in current.
Using Vaccum Tube
The essential factor in such a voltmeter is that the resistance of the plates of the vacuum tubes should be low so that the plates do not cause variation in the linear characteristics of voltage and current.
The resistance used in series with a vacuum tube must have a high enough value to maintain the linear characteristics. The current across the resistor is fed to an amplifier which is connected to the PMMC meter used for measurement.
Now, let us design an average reading AC voltmeter using a semiconductor diode. This semiconductor diode operates for an alternating half cycle as it works on the principle of a half-wave rectifier.
Using Semiconductor Diode
The average current flowing through the diode can be obtained by the given equation. In this, the value of 1.11 represents the form factor. The multiplication factor of 2 represents the half-wave rectification event.
The average voltmeter can also be designed with the help of a full wave rectifier. In the case of a full wave rectifier, the average current is twice that of a half wave rectifier.
Using Full Wave Rectifier
Advantages of Average Reading AC Voltmeter
- low power consumption.
- high input impedance.
- simple construction.
- Uniform scaling.
Disadvantages of Average Reading AC Voltmeter
- The circuit under measurement must have low resistance otherwise it will load the circuit.
- Linear characteristics should be strictly maintained as non-linear characteristics cause problems during low voltage measurement.
- It is suitable only in the audio-frequency range, since in the radio frequency range, errors may occur due to distributed capacitance.
Peak Reading AC Voltmeter
Peak reading voltmeters can also be designed in two ways, one using a vacuum tube and the other using a semiconductor diode. The only difference between peak reading and average reading circuit arrangement is that capacitors are used in peak readings whereas capacitors are not used in case of average readings.
Using Vaccum Tube Diode
The above circuit is shown in an AC voltmeter using a vacuum tube to which a capacitor is connected in parallel across a resistance of high magnitude. The resistance is connected in parallel to the amplifier and the PMMC meter.
Peak reading AC voltmeter can also be designed by semiconductor diode. In this, instead of a vacuum tube, a semiconductor diode is added.
Using Semiconductor Diode
Peak reading voltmeter working as per the above two circuits, when input voltage is applied. The capacitor starts to charge and holds charge until it reaches its peak value. When the capacitor reaches the peak voltage, it starts to discharge through the resistor.
The major disadvantage of using a peak reading AC voltmeter is that it is not suitable for the measurement of low voltage signals. Since in the case of low input voltage, high velocity electrons still contribute to current flow in the circuit which can lead to errors in measurement.
True RMS reading AC Voltmeter
Average reading voltmeter is economical and provides RMS value of sinusoidal waves hence AC voltmeter requires correct RMS reading.
True RMS reading voltmeter costs more than average reading AC voltmeter. Since in the case of non-sinusoidal waveforms, the actual rms voltage needs to be calculated which is not possible by averaging the readings by AC voltmeter. As such, a true RMS reading voltmeter is required.
Principle of True RMS Reading AC Voltmeter
The heating power associated with the wave varies directly with the square of the RMS value of the voltage. Thus, if the thermal power of the wave can be measured in some way, it is easy to measure the RMS value associated with the input waveform.
For this purpose a thermocouple is a device used to measure the thermal power of waves.
Working
The thermocouple in the True RMS ac voltmeter takes the input voltage from the AC amplifier. Since the magnitude of the signal must be sufficiently high to obtain the correct measurement. The thermocouple receives the input waveform by its input terminal. This thermocouple is called the main thermocouple.
The non-linearity associated with the main thermocouple needs to be canceled out. Otherwise, it generates errors in the final measurement. For this purpose, balance thermocouples are used. The non-linearity produced by balancing the thermocouple and the non-linearity produced by the main thermocouple cancel each other out.
In this way it acts like a balancing bridge circuit. The AC uses the feedback current supplied by the amplifier to balance it in case any error signal is produced. When the input is not applied to the main thermocouple, the output of both thermocouples will be equal and produce a zero voltage signal.
Thus, the actual RMS reading of an AC voltmeter is helpful in measuring the RMS value of a non-sinusoidal waveform. It should be noted that the dynamic range of the AC amplifier must be sufficiently high because the peak excursion produced by the input waveform does not exceed the range of the AC amplifier.
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