Analog electronic voltmeter
Earlier analog electronic voltmeters used vacuums tube and were known as Vacuum tube voltmeter (VTVM). With the introduction of transistor and other semiconductor devices, Now vacuum tube voltmeters have been replaced by transistors voltmeter (TVM). In general, these devices are now known as electronic voltmeter (EVM). An electronics voltmeter has a very high internal resistance (around 10Mohm or so).
An electronic voltmeter uses rectifier amplifiers and other circuits to generate a current proportional to the voltage being measured. In an electronic voltmeter a measured alternating voltage is rectified using a diode rectifier or detector and a rectified direct current is produced which is the measured of the original alternating voltage . the current so produced is measured by a conventional measuring such as permanent magnet moving –coil (pmmc)
A generalized block diagram of the most modern electronic voltmeter is shown if fig It consists of mainly three part: input switching and ranging circuit and analog-to –digital conversion ( rectification) circuit.
As shown in fig an input voltage to be measured is first supplied to the signal conditioner unit that comprises of input switching and ranging circuits and the amplification circuit. If the input voltage to be measured is a d.c voltage, the signal conditioner is composed of an attenutor for the higher voltage ranges and a d.c amplifier for lower voltage ranges. If the input voltage to be measured is an a.c voltage, a converter is used to change the a.c voltage to its equivalent d.c value using analog- to-digital converter circuit (ADC).
Advantages of electronic voltmeters
- Low level signal detection:- sensitivities corresponding to full scale deflection with an applied alternating potential of less than 1 volt are readilyobtainable.
- Low Power Consumption :- electronic voltmeters present very high resistance across the circuit whose voltage is to be measured. Thus, they consume very little or practically no power from the circuit whose voltage is being measured.
- Hight frequency range :- the response of electronic voltmeters can be made practically indepent of frequency within wide limits. They can be calibrated at a low frequency and then can be used upto very radio frequencies without any correction being made. Some electronic voltmeters permit the measurement of voltage from d.c to freq uencies of the order of hundreds of MHz.
- Low input capacitance:- Electronics voltmeter have very low input capacitance that may be of the order of a few pico Farad (pF).
- No Loading errors:- electronic voltmeters avoid the loading errors by suppying power required for measurement by using external circuits like amplifiers. The amplifiers not only supply power required for the operation but also make it possible for low level signals. These low level signals deflection, otherwise, cannot be detected in the absence of amplifiers.
The transistor voltmeter (TVM), also called the electronic voltemeter(EVM)<
Has the following advanteges over vacuum tube voltmeters (VTVM):
- It requires no warm-up time because of the absence of a heater element .
- Transistorized instruments are portable and are, thus well suited for field work.
- Power consumption is negligible.
But, the input resistance of transistor is low compared to a vacuum tube and therefore, TVM is used in conjunction with field effect transistor (FET) that has very high input impedance.
Analog electronic voltmeter are generally of the following types:
- AC Electronic voltmeters (ACEVM)
- DC Electronics voltmeters (DCEVM)