#Volt meter series#

Its resistance changes from the order of few mega-ohms to few hundred ohms when it is illuminated by a light source in the dark place. Chopper Type DC VoltmeteД¬ircuit diagram of an electronic chopper employing photo diodes is shown in figure.В Photo diodes change its resistance under different illumination conditions, this property f photo diode is used in chopper amplifier. Modulator chopper and demodulator chopper act in anti-synchronism.Chopper system may be either mechanical or electronic. Firstly dc input voltage is converted into ac voltage by chopper modulator and then it is supplied to an ac amplifier, i Output of amВplifier is then demodulated toВ a dc voltage proportional to the original input voltage. Chopper Type DC VoltmeterД¬hopper type dc amplifier is used in highly sensitive dc electronic voltВmeters. This voltmeter can be used for measurement of voltages of the order of milli-volts because of sufficient gain of ampliВfier.ДЄpart from the high input impedВance, this circuit has another advanВtage that when inВput voltage exceeds its limit, amplifier gets saturated which limits the current passing through the PMMC meter. When transistors work within their dynamic region, the deflecВtion of meter remains proportional to the applied input voltage. FET is used in the input stage of amplifier because of its high input impedance so that is does not load the circuit of which voltage is to be measured and it also keeps the sensitivity of voltmeter very high.В As FET is a voltage controlled device so resistance network of attenuator is put in shunt in the circuit.В Transistors Q 2 and Q 3 form the direct coupled dc amplifier whose output is finally supplied to PMMC meter. DC Voltmeter using FETДЄnother circuit diagram of a direct coupled amplifier dc voltmeter using FET in input stage is shown in figure.В In this voltmeter, voltage to be measured is firstly attenuated with range selector switch to keep the input voltage of amplifier within its level. It controls the bucking current from the supply E to buck out the quiescent current.В 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. They form a direct coupled amplifier.В A variable resistance R is put in the circuit for zero adjustment of the PMMC. Transistors Q 1 and Q 2 are taken complement to each other and are directly coupled to minimize the number of components in the circuit. 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. So current drawn from the circuit is only 5micro Ampere. Other values of range selecting resistors are also so chosen that sensitivity remains the same for all ranges. It can be seen from figure that sensitivity of voltmeter is 200 kilo ohms/volt neglecting small resistance offered by transistor Q 1.

#Volt meter series#

A transistor is a current controlled device so resistance is inserted in series with the transistor Q 1 to select the voltage range. This instrument can be used only to measure voltages of the order of milli-volts owing to limited amplifier gain.В The circuit diagram for a direct coupled amВplifier dc voltmeter using cascaded transistors is shown in figure.В An attenuator is used in input stage to select voltage range. This type of voltmeter is very common because of its low cost. Direct Coupled Amplifier DC Voltmeter DC Voltmeter This is particularly true where voltage measurement is done in higher resistance circuit.DC voltmeters can further be divided into two categories. It should also be noted that meter with higher sensitivity gives reliable result. So in second case loading effect is not prominent as the voltmeter resistance is much higher than the circuit resistance but in first case loading effect id dominating as the voltmeter resistance is comparable to circuit resistance. See how close to the true reading we get. Mind here that the resistance of voltmeter > circuit resistance i.e 1000 kOhm >50 kOhm so definitely we are going to get voltage reading close to true value i.e 50 V. Resistance of Voltmeter R VA= SensitivityxVoltage Sensitivity of Voltmeter A = 20,000 ohm/V It should be noted that for getting good accuracy, the resistance of voltmeter > circuit resistance but here in this case both are equal. Therefore we see that voltmeter A reads much less than the true value. Now when this voltmeter A is connected across the resistor of 50 kOhm, the equivalent resistance of the parallel combination, Resistance of Voltmeter R VA = SensitivityxVoltage