Electronic DC Voltmeter
The electronic voltmeters are most widely used now a day, with the advancement of semiconductor devices they are becoming increasingly popular. Now, why do we use electronic voltmeters instead we have cheap electromechanical voltmeter available?? The answer is that they posses certain advantages so that they are popular. There are many types of electronic voltmeters available and accordingly we choose what we want. These types of voltmeters are constructed by the use of PMMC meters, rectifiers, voltage dividers, amplifiers, so that the current is directly proportional to the deflecting torque.
Description of the Components used in DC Voltmeter
DiodeThe diode is a two terminal device which is capable of blocking the current when it is in reverse biased condition. It conducts when it is in forward biased state. So this arrangement is used when we want to find out the average of the AC voltage.
TransistorThe transistor is a three terminal device which is used for amplification purpose. When a signal is applied to its base, then the signal is amplified by it as it has low input resistance and high output resistance when connected to active region.
It can also be operated as a switch. It has three modes of operation, namely-
- Active Region.
- Saturation Region.
- Cut Off Region.
In the active region it has its input as forward biased and the output is reversed biased. In the saturation region, both input and output are in forward biased mode. In the cut-off region, input and output are in reversed biased state. However the transistor has a considerable loss of power for that we use the transistor which has high input impedance. The types of transistors use when the loss has to be low are- Field effect transistor or FET and Metal oxide semiconductor field effect transistor or MOSFET.
AmplifierFor a small signal we have to use amplifier in order to measure it. The amplifier generates the power required for measuring instruments so that there will not be any loading effect in the circuit and the original signal is retained after measurement also. Without the use of amplifiers we cannot measure very small signals typically used in communication circuits where the voltage level and power levels are very small.
Voltage DividerThe voltage divider is provided in order to have the voltage tapping. Suppose the voltage we want is less then we go for voltage divider by connecting series resistance and output is taken across the desired resistor. Also, if we want to adjust the offset voltage, then also we can provide a potentiometer type arrangement in order to nullify the offset.
PMMC MeterThe instrument is used to indicate movement of the instrument that is to measure the current in the circuit. In this type of circuit, we use a taut suspension but in case of the other measurement types use pivot and jewel bearing arrangements. This is called the analog mode of operation, however if we want digital data we have to use A/D converter in order to have the digital reading. The main advantage of taut and suspension arrangement is that we do not require jewel bearings and the moving element is balanced. Jewel bearing has certain disadvantages such as friction is more due to damaged pivots or distorted control springs.
Average Reading Diode Vacuum Tube VoltmeterThe circuit is used to measure the average of AC voltage. Value of R should be very high in order to have almost all the voltage drop across the resistor so that it can be fed to amplifier and meter. Advantages
- Simple Construction.
- High resistance at the input so the power consumption is lower.
- Low bandwidth.
- Non linear operation when the voltage is low.