In the tutorial on Types of Voltmeters, we discussed the types of Voltmeters. Now we will know about the different types of DC voltmeter, which are as follows:
The DC voltmeter measures the DC voltage by moving the pointer against a fully calibrated scale. The calibrated scale is obtained by a PMMC meter.
The voltage to be measured is passed through the attenuator and then through the DC gain amplifier.
When DC voltage is measured with a DC voltmeter it is fed into the attenuator and then the output from the attenuator is passed through DC coupled amplifiers.
Types of DC Voltmeters
There are two types of DC Voltmeter.
- Direct Coupled Cascade Amplifier DC voltmeter.
- Chopper Type DC Voltmeter.
Direct Coupled Cascade Amplifier DC voltmeter Using BJT
The attenuator consists of various resistors of suitable value. The transistors used are PNP and NPN, both transistors are directly cascaded so that the maximum number of components in the circuit is minimized. It has less complexity in the components used in the circuit.
Transistor is a current controlled device. Any device that controls current requires a series connection with resistors. Thus a resistance of 14MΩ is connected in series to transistor Q1.
The attenuator network is helpful in selecting a particular range of voltages. And the network of transistors connected to each other develops a high-gain amplifier. The amplified voltage from the cascade amplifier when the PMMC enters the meter begins to show deflection on the pointer calibrated scale. In this way, the voltage applied to it is measured.
- It can provide the measurement of voltage in the milli-volt range.
- The sensitivity is about 200kΩ/volt.
- It is a Low-Cost device.
- It is suitable to operate in a particular temperature range otherwise it creates drift problems.
- There is a restriction on the use of multiple transistors to increase the gain as this will also cause drift problems.
Related Tutorial: What are BJT and FET?
Direct Coupled Cascaded Amplifier DC Voltmeter using FET
Direct coupled amplifier using BJT provides results in low sensitivity and also develops a high load in the input circuit. Therefore, to overcome these problems, FETs were used in direct-coupled amplifiers. The most important advantage of using a FET is its high impedance.
Due to the high input impedance of the FET, no load is created on the input circuit connected to the DC voltmeter. As a result of this, the sensitivity provided by the circuit is very high.
The connection architecture of a DC voltmeter using a FET is the same as that of a DC voltmeter using a BJT. The only difference is that the FET is a voltage controlled device. And the resistance network connected to the voltage controlled device remains in the shunt.
- It is a High input impedance.
- The problem of burnout of PMMC in the voltmeter is also reduced. This is because whenever the input voltage exceeds its limit, the cascade DC amplifier enters saturation mode. This allows no additional current to flow through the PMMC which prevents it from burning.
- It is High Sensitivity.
- Drift problem.
- It cannot measure voltage in the milli-volt range because it requires a high-gain DC amplifier. Which requires more transistors to cascade together. But because of the drift problem, more transistors cannot cascade.
Chopper Type DC Voltmeter
Chopper-type DC voltmeter was designed to overcome the drawbacks of the Direct Coupled Amplifier DC Voltmeter. The problem of drift present in Direct Coupled Amplifier DC Voltmeter was eliminated in Chopper type DC Voltmeter by using capacitor along with modulator.
The DC voltage to be measured is entered through a chopper modulator which converts it into an AC signal and this AC signal is then applied to the AC amplifier. Capacitors used before and after amplifier circuits eliminate the problem of drift. Chopper uses a demodulator to convert an AC signal to a DC signal, which is then passed through a low pass filter to dissipate any AC ripple current.
The neon lamp is turned on and off with the help of the oscillator for alternating half-cycles. These neon lamps illuminate the photodiode which modifies the resistance of the modulator and demodulator. When they are illuminated the resistance decreases.
The DC voltage to be measured is obtained from a low-pass filter. This signal then passes through the PMMC meter for measurement.
- it provides high sensitivity.
- It is measurement range is about 10μV.
- it can measure voltage in the micro-volt range which is not possible with a direct coupled DC voltmeter.
- Its circuit is quite complex.
- The use of modulators and demodulators increases the cost of DC voltmeters.