Electronic voltmeters use rectifiers, amplifiers and other circuit elements to produce a current. And this current is proportional to the voltage to be measure.
A conventional voltmeter takes a non negligeable current from the circuit during measurement. Although this current may be in microampere range. But still this current affects the measuring voltage. A permanent magnet moving coil instrument normally draws a current of 50 microamperes for its full scale deflection. So, when it measures the voltage across a circuit it takes proportionate current from the circuit. Since the instrument takes a current from the circuit, the voltage profile of the circuit must be changed. Although during measurement this change of voltage may be quite small. But sometimes we need very accurate measurement of original voltage across the circuit.
Working Principle of Electronic Voltmeter
In this type voltmeter there is always at least an amplifier. This amplifier amplifies the very small current taken by the electronic voltmeter during measurement. Then this amplified current is measured by a conventional PMMC instrument. That means this voltmeter takes very small current from the circuit but it amplifies the current to higher value so that the current can be measured by conventional PMMC instrument. In this way an electronic voltmeter can measure a voltage without affecting the value of the original voltage.
An electronic voltmeter does have its own power supply. So, it needs not to take any significant current from the circuit under measurement. The amplifying arrangement makes it possible to measure very tiny current with the help of conventional PMMC instrument.
Advantages of Electronic Voltmeter
There are numbers of advantages of using an electronic voltmeter. Such as,
- It consumes very low power from the circuit under measurement.
- Its accuracy level is high.
- A PMMC instrument can only measure direct currents. So, for measuring AC, electronic voltmeter, must have a rectifier circuit. Because of that this instrument can measure alternating voltage for wide range of frequencies.
- The meters are compact and portable in shape and size.
- It has low level of signal detection.
- Also it has high sensitivity.
- It offers very high input impedance. This is why it draws a very low current.
- Since current taken by the instrument is very negligible, the loading effect is quite low.
Loading effects means the change of voltage across two terminals of a circuit when we connect a voltmeter is between the terminals.
Comparison between Electronic Mechanical Voltmeter and Electronic Voltmeter
- Power consumption in an electronic voltmeter is much lower than that of an electromechanical voltmeter.
- An electro mechanical type voltmeter is much robust in construction than that of an electronic type.
- An electronic variant of voltmeters must have at least an amplifier but a conventional voltmeter does not have an amplifier.
- The electronic type of voltmeter can measure much higher voltage than a conventional type.
- The loading effect (mentioned earlier) is much lower in an electronic variant.
- We can measure very low level of voltage signal by EVM which is not possible with help of a conventional analogue voltmeter.
- The sensitivity, frequency range and input impedance are higher in an electronic voltmeter than that in an equivalent electro mechanical voltmeter.
Types of Electronic Voltmeter
There are two types of electronic voltmeters available in the market.
- Analogue Type.
- Digital Type.
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