This document discusses various types of electronic instruments used to measure voltage, including voltmeters, Q-meters, and digital voltmeters. It describes the limitations of traditional mechanical voltmeters and how electronic circuits overcome these limitations with high input impedance, low output impedance, and voltage amplification. Circuit diagrams and working principles are provided for different types of DC and AC voltmeters using transistors and op-amps. Thermocouple-based true RMS voltmeters are explained as well as Q-meters for measuring coil and capacitor characteristics and digital voltmeters that directly provide a numerical voltage output.

1. Electrical and Electronic Measurement
Electronic Instruments
Parveen Malik
Assistant Professor
School of Electronics Engineering
KIIT University
parveen.malikfet@kiit.ac.in
October 30, 2019
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2. Outline
1 Why we use Electronic Instruments?
2 Voltmeters
D.C.Voltmeter Circuits
A.C.Voltmeter Circuits
3 True RMS Voltmeter
4 Q-meter
5 Digital Voltmeter
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3. Limitations
of
Electrical Voltmeters & Ammeters

4. Why we use electronic instruments ?
Limitations of electrical Voltmeters & Ammeter
The voltmeter constructed of moving coil instruments and multipliers
can not
Measure Low Voltages
Input impedance is low
These limitations are over come by use of electronic circuits that
have high input impedance.
have low output impedance.
can amplify low voltages to measurable level.
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5. Electronic Voltmeters

6. D.C.Voltmeter
D.C.Voltmeter Transistor Circuits
(a) (b)
(c) (d)
Different Voltmeter circuits based upon Transistors
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7. D.C.Voltmeter
D.C.Voltmeter Op-amp Circuits
(a) (b)
(c)
Different D.C.Voltmeter circuits based upon Transistors
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8. A.C.Voltmeter
A.C.Voltmeter Op-amp Circuits
(a) (b)
(c) (d)
Different A.C. Voltmeter circuits based upon Op-Amp
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9. True RMS Voltmeter

10. True RMS Voltmeter
The measuring circuits used in the electronic D.C. and A.C. voltmeter
are not suitable for specialized applications.
Most of them require input as pure sinusoids.
Thermocouple are used for such type of applications.
Thermocouple Principle - Junctions of dissimilar metals produces a
voltage when heated. Thermocouple voltage is proportional to the
rms value of input current.
Can measure rms levels of all voltages regardless of their distortion
level.
True rms voltmeter works on the thermocouple principle.
Thermocouple
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11. True RMS Voltmeter
Thermocouple
A junction of two dissimilar metal produces an emf when heated.
Heating effect of current is directly proportional to the rms value of
the input current regardless of input waveform. Thus emf can be used
as a measure of rms value.
Materials - iron-constantan, copper-constantan, chromel-alumel and
platinum- platinum/rhodium.
Temp - 300◦C, Thermocouple Output - 12mV, Heating current 2mA
to 50 mA.
(a) (b)
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12. True RMS voltmeter Thermocouple type
Thermocouple
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13. True RMS Voltmeter- Working
Video Amplifier Stage- The input voltage (E1) with a non-sinusoidal
waveform is amplified to Av E1 by means of video amplifier (A1). A video
amplifier operates over a wide range of frequencies (audio to very high).
The current I1 passed through resistor R1 has the same waveform as E1
and consequently the heat generated in R1 is directly proportional to the
true rms level of input voltage.
Audio Amplifier Stage- A2 works as a non-inverting d.c. amplifier which
passes direct current I2 through resistor R2. R2 is the heater for
thermocouple T2 and thermocouple voltage e2 is directly proportional to
the heating effect of I2.
Final Stage - The feedback from the output produces a voltage at the
op-amp inverting input equal to that at non-inverting input. Voltage e1 is
applied to the non-inverting input of A2 and e2 feedback to the inverting
terminal. Output voltage E2 settles at the level that makes e2 exactly
equal to e1. IF T1 and T2 are similar, heating effect of I2 is same as that
of I1. This means that I2 equals the rms value of I1. Therefore D.C. output
voltage E2 is directly proportional to rms value of a.c. input voltage E1.
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14. Q meter

15. Q meter
Q meter is used to measure characteristics of coils and capacitors.
Principle - based on the series R,L,C resonant circuit.
Series Resonant Circuit
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16. Q meter - Practical Circuit
Practical Q meter circuit
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17. Q meter - errors
Observation 1
QTrue = ω0L
R
QMeasured = ω0L
R+Rsh
QTrue = QMeasured
1 + Rsh
R
Observation 2
QTrue = 1
ω0CR
QMeasured = 1
ω0(C+Cd )R
QTrue = QMeasured
1 + Cd
C
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18. Digital Voltmeter

19. Digital Voltmeter
Measure the voltage directly giving numerical output
Versatile and accurate instruments
Superior resolution
Reduced size or compact
Less power consumption than analog voltmeter
Less costly and rugged construction
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20. Digital Voltmeter
Block Diagram
Analog to Digital Converter
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21. Digital Voltmeter
Working
Objective - Measurement of Input Voltage Vi .
When input voltage Vi is greater than ramp output voltage Vr , the
opamp output saturates to +Vsat or 1 as it is fed at the non-inverting
terminal of voltage comparator. At this moment, one input of AND gate is
1 and consequently, the clock pulses start passing through it. Once the
ramp voltage become greater than input voltage Vi at time, t1, the one
input of the AND gate becomes 0 and consequently, no clock pulses pass
through it.Thus, the decade counter counts the no. of clock pulses till Vi
is greater than Vr and the no. of pulses passed are directly proportional
the input voltage level.
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