Voltmeter in general is divided into two namely:
- Analog Voltmeter
- Digital Voltmeter
The analog voltmeter displays the output using the deflection pointer whereas only the calibration scale varies from meter to meter.
The digital voltmeter internally consists of LED using the seven-segment display which gives the output using the ADC.
Analog voltmeters and Digital voltmeters both are meant to measure analog voltage signals.
Analog Voltmeter:
Analog electronic voltmeter types namely:
- Amplifier type voltmeter
- Rectifier voltmeter
- Thermocouple voltmeter.
In these voltmeters, the amplifier voltmeter is DC electronic voltmeter whereas the Rectifier voltmeter and the thermocouple voltmeter are the AC electronic voltmeters.
Amplifier type DC Electronic voltmeter:
PMMC voltmeter offers high resistance in Kiloohm but a highly sensitive instrument is needed to measure very low current. To measure very low current we go for high resistance circuits.
CRO offers high resistance in Mega ohm in order to amplify at the input side.
So in this amplifier type DC electronic voltmeter, an amplifier circuit is added to amplify at the input side so that it offers very high resistance which in turn it becomes a very sensitive instrument for the low voltage measurement.
The loading effect will be very less as the sensitivity is high.
For better amplification purposes and for its high resistance, MOSFET is selected rather than the BJT.
When an electronic voltmeter is very sensitive it means it can measure low-level signal detection.
Electronic voltmeters cause low power consumption.
Examples of Amplifier type analog voltmeter:
- Vacuum tube voltmeters
- Transistor-type electronic voltmeter
- Differential type voltmeter
- FET input voltmeter
Rectifier type electronic voltmeter:
In this voltmeter, the full-wave bridge rectifier will be kept at the input side whose output is fed to the PMMC voltmeter and the DC scale of PMMC will be calibrated in terms of AC.
The form factor of the Sine wave is 1.11 which is the value used for the calibration.
For a square wave, the form factor is 1.
But this voltmeter has to be designed specifically for each wave as we are calibrating this in terms of its form factor.
The input of the rectifier is the AC input sine wave and the output of the rectifier is the average output Vout which is fed to the PMMC DC voltmeter.
Vout = 2Vm / π
= 0.636 Vm
Multiplying 0.636 with the form factor 1.11 we get 0.707 Vm which is the RMS value of the AC meter.
Thereby DC voltmeter is simply calibrated by using the form factor and also it is a linear derived RMS meter.
The RMS scale of the AC electronic voltmeter is linear.
Rectifier type AC electronic voltmeter is an audio frequency voltmeter and it can be used for the measurement of AC up to 20KHz.
Hence this voltmeter works accordingly as per the calibration as it is not a true RMS meter as it is a derived RMS meter.
Thermocouple Voltmeter:
A thermocouple voltmeter is a true RMS voltmeter.
There will be a resistance heater at the input side where when there is a flow of high current it heats. Due to its power consumption, the junction temperature of the thermocouple is raised the voltage is developed across the amplifier circuit.
Therefore the amplified output is fed to the PMMC meter.
This voltmeter is a radio frequency AC voltmeter and it can be used for the measurement of high frequency up to Mega Hertz.
The disadvantage of this meter is that it cannot withstand a large value of current.
The deflection is proportional to the square of the RMS voltage.
Digital Voltmeter:
A digital electronic voltmeter is suitable for measuring analog voltage signals where the measured value will be displayed on a 7-segment display.
The digital electronic voltmeter consists of the ADC, the cascaded decade counters and some seven-segment LED displays.
Types of Analog to Digital converters used today:
- Successive approximation ADC
- Delta-sigma ADC
- Dual slope ADC
- Pipelined ADC
- Flash ADC
From these ADCs the Dual slope integrating type is selected as it is the slowest and the preferred one because of its accuracy as well as its integrating type whereas the remaining are non-integrating.
The unknown analog voltage signal being measured is first converted into digital form by ADC.
Advantages of Dual slope integrating type:
- High accuracy
- Excellent noise rejection
- Superior stability
Disadvantage:
It is slow because of its high conversion time.
Voltage to time conversion is the main principle of the dual-slope integrating type.
The ADC produces digital output in the form of the clock pulses which are then counted by the counters. The one counter can count from 0 to 9 clock pulses then it overflows on the 10th clock.
These count of the clock pulses will be displayed on the seven-segment display.
Each seven-segment display has its own decimal point. One decimal point will be turned on while measuring the particular voltage.
The MOD 10 DC counter will be used as it converts the clock pulses to the seven-segment counter.
The decimal point selector switch and the voltage range select switch will be used here and it is mechanically ganged. The decimal point will be turned on based on the position of the range switch.
3-digit rollover:
000 will be the RESET count, 001 will be the minimum count and 999 will be the maximum count. In total it consists of 999 + 1 (000) = 1000 clock counts.
The step decade is a rollover from one count to the next immediate count.
The resolution is the minimum step size of the digital electronic voltmeter.
The resolution of the three-digit digital voltmeter is 1 step size to the 1000 steps.
Resolution = 1 / 1000
= 0.001
When it moves from 000 to 001 1 clock pulse will be counted and displayed.
000
001
002
002
.
.
.
.
009
010
011
.
.
.
.
099
100
101
.
.
.
999
999 will be the maximum count in 3 digit voltmeter
001 will be the minimum count.
(0-1) Voltage range selection of voltmeter:
The smallest incremental change is known as resolution.
Resolution is defined as the voltage range of operation to the total steps.
The total number of steps will be based on the digits of the voltmeter.
If the voltmeter is a 3-digit voltmeter, then the number of steps will be 10^N which is 10^3 which is 1000.
Therefore the resolution value we get is 1V / 1000 = 1mV.
For this digital voltmeter, the minimum voltage will be 1mV.
The display will roll over like
.000 V
.001 V
.002 V
.003 V
.
.
.
.
.999 V
The minimum voltage is 1mV which is the resolution.
A digital meter cannot present the full context of the range as it resets to .000 after the .999 V but the actual voltage is 1 V
Resolution is the range of operation to the total number of steps.
(0-10) Voltage range selection of digital voltmeter:
Resolution = range of operation / total number of steps
0.00 V
0.01 V
0.02 V
.
.
.
.
.
0.99 V
.
.
.
9.99 V
The maximum voltage is 9.99 V which is equal to the 10 V range and the minimum voltage is 10mV.
Thereby a user can easily note down the readings.
(0-100)V Range selection of voltmeter:
00.0 V
00.1 V
00.2 V
.
.
.
0.99 V
.
.
.
.
.
99.9 V
The maximum voltage is 99.9 which is a 100 V range voltmeter.
The minimum voltage is 100mV which is the resolution.
From these 3 voltage ranges of voltmeters consider that each voltmeter is measuring 0.9 V. Which of the voltmeter is a precise one?
The precise voltmeter is a lower range from (0 to 1)V as it gives .999 V as a reading whereas the other voltage range of voltmeters measures 0.09 and 0.09 V.
With this statement, we can conclude that the higher the voltage range the less precision.
Sensitivity:
The sensitivity of the digital voltmeter is the lowest voltage range to the total number of steps.
S = lowest voltage range / total steps
3 1/2 digit digital voltmeter:
This voltmeter has a toggle switch in it where it displays either 0 or 1.
1/2 digit means simply it can print from 0 to 1. In total 2.
There will not be any decimal point for the toggle flipflop whereas for each digit there will be a decimal point. For each voltmeter range, the decimal point displays accordingly.
This voltage range can be varied in the digital voltmeter using the voltage range selector switch.
Rollover of 3 1/2 digit:
0000
0001
0002
.
.
.
.
0999
.
1001
1002
.
.
.
.
1999
The minimum count is 0001 and the resolution will be 1mV for 2 voltage range of operation.
As it is 3 1/2 digit, the voltage scale will be 2.
The total steps will be 2000.
Resolution = 2V / 2000 = 1mV.
When it is 1/2 digit the 1V voltage range is multiplied by 2 which gives 2 voltage range which means that the voltage range will be extended because of 1/2 digit and the sensitivity, and resolution remains unchanged.
Advantages of digital voltmeter:
- High input impedance because of the op-amp at the input side in the dual-slope integrating digital voltmeter.
- Loading errors will be less.
- No reading error as it is digital when compared to analog voltmeters there will be an observational error.
- Superior resolution.
- Programmability and computerized control can be done using microcontrollers or embedded systems.