DC Voltmeters

  
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This type of voltmeter is very common because of its low cost. This instrument can be used only to measure voltages of the order of milli-volts owing to limited amplifier gain. The circuit diagram for a direct coupled am ­plifier dc voltmeter using cascaded transistors is shown in figure. An attenuator is used in input stage to select voltage range.
DC Voltmeters - schematic

A transistor is a current controlled device so resistance is inserted in series with the transistor Q1 to select the voltage range. It can be seen from figure that sensitivity of voltmeter is 200 kilo ohms/volt neglecting small resistance offered by transistor Q1. Other values of range selecting resistors are also so chosen that sensitivity remains the same for all ranges. So current drawn from the circuit is only 5micro Ampere. Two transistors in cascaded connections are used instead of using a single transistor for amplification in order to keep the sensitivity of the circuit high. Transistors Q1 and Q2 are taken complement to each other and are directly coupled to minimize the number of components in the circuit. They form a direct coupled amplifier. A variable resistance R is put in the circuit for zero adjustment of the PMMC. It controls the bucking current from the supply E to buck out the quiescent current. The draw-back of such a voltmeter is that it has to work under specified ambient temperature to get the required accuracy otherwise excessive drift problem occurs during operation. Another circuit diagram of a direct coupled amplifier dc voltmeter using FET in input stage is shown in figure. In this voltmeter, voltage to be measured is firstly attenuated with range selector switch to keep the input voltage of amplifier within its level. FET is used in the input stage of amplifier because of its high input...



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