The document discusses the principles and applications of electronic AC voltmeters, detailing the limitations of PMMC instruments for measuring AC voltage and the necessity of rectification. It introduces precision rectifier circuits that eliminate voltage drop errors, enhancing measurement accuracy. The content also covers various probes and adapters for extending the functionality of electronic meters in measurements of voltage, current, and temperature.

1. Electronic AC Voltmeter
Unit II Electronics & Digital meters
3/6/2017 1
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad

2. Electronic AC Voltmeters
3/6/2017 2
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
PMMMC instrument are polarised i.e. has +ve and –ve terminal
When a very low ac voltage (upto 0.1 Hz) is applied, pointer
tends to follow, for high frequency ac voltage, damping and
inertia forces pointer to stay at average (zero).
Thus PMMC instrument can not measure AC voltage or current
To measure AC voltage, it needs to be rectified
PMMC meter
RS
RCE
Vo
ICAttenuator
or
Volt range
selector
Amplifier
High input/
low output
impendence
Rectifier

3. Electronic AC Voltmeters
(voltage follower rectifier voltmeter)
3/6/2017 3
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
Coupling capacitor blocks dc.
Input to PMMC meter is half wave rectified waveform
Meter deflection corresponds to average value i.e. Eav=0.318EB
 Voltage drop across rectifier is a source of error in voltage
D1
Coupling Capacitor
+
+
-
RF
+VCC
-VEE
Ra
Rb
Rd
Rc
E
C1 VF
- PMMC meter
RS
RC
Vo
IC
Attenuator Voltage
follower
Meter
circuit
+EB
-EB
+EB
Input waveform
Output
waveform
Rectifier

4. Average and rms voltage of Rectifier
3/6/2017 4
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
B
B
T
B
T
Bav E
E
T
tE
dE
T
t
E
T
E 318.02cos
2
.2sin
1 2
0
2
0


















  



T
+EB
Eav
B
B
T
B
T
Brms E
E
dt
T
t
T
E
dt
T
t
E
T
E 5.0
2
.4cos1.2sin
1 2
0
22
0
22


















  
B
B
T
B
T
Bav E
E
T
tE
dE
T
t
E
T
E 637.0
2
2cos.2sin
2 2
0
2
0


















  



B
B
T
B
T
Brms E
E
dt
T
t
T
E
dt
T
t
E
T
E 707.0
2
.4cos1
2
.2sin
2 2
0
22
0
22


















  
+EB
Eav
Half wave rectifier
Full wave rectifier

5. 3/6/2017 5
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
rectifier voltage drop error can be eliminated by connecting the
rectifier between op-amp output and voltage follower feedback.
rectifier output precisely follows the +ve half cycle of input so no
voltage drop error. The circuit is called precision rectifier
Attenuator capacitors compensate amplifier input capacitance
Electronic AC Voltmeters
(voltage follower precision rectifier voltmeter)
D1
+
+
-
RF
+VCC
-VEE
Ra
Rb
Rd
Rc
E
C1
VF
-
Attenuator Precision rectifier
+EB
-EB
Input waveform
PMMC meter
RS
RC
Vo
IC
Meter
circuit
+EB
Output waveform
C2
C3
C4
C5

6. 3/6/2017 6
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
Electronic AC Voltmeters
(precision rectifier amplifier voltmeter)
Low ac voltage can be accurately amplified before rectification.
Voltage gain of ac voltage Av=(R2+R3)/R3
There is no rectifier voltage drop.
D1
+
+
-
+VCC
-VEE
VF
-
Precision rectifier
PMMC meter
RS
RC
Vo
IC
Meter
circuit
R1 R2
R3

7. 3/6/2017 7
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
Electronic AC Voltmeters
(Voltage to current converter with half wave rectifier)
Current passes through meter only in positive half cycle
Peak current IP=EP/R3
Average current through meter IAV=0.318 IP
D1
+
+
-
+VCC
-VEE
VF
-E
Meter current
R1
R3
PMMC
meter
RC
IC

8. 3/6/2017 8
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
Electronic AC Voltmeters
(Voltage to current converter with full wave rectifier)
Current passes through meter only in positive half cycle
Peak current IP=EP/R3
Average current through meter IAV=0.637 IP
+
-
+VCC
-VEE
E
R1
R3
Meter current
RC
IC
D1
D2
-
PMMC
meter
D4
D3
+

9. Current Measurement with Electronic Instrument
3/6/2017 9
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
+
-
+VCC
-VEE
RS
R3
PMMC
meter
RC
IC
Ammeter
Terminals
E
Electronic voltmeters have high input impedances while ammeter
have low input impedance
Electronic voltmeters can be used for measuring current by passing
the current through small shunt resistance.
Voltage drop across shunt resistance is amplified before applying to
PMMC meter
For AC current measurement
Electronic voltmeters having
rectifier is used

10. Probes
3/6/2017 10
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
There are many probes and adopter for use with analog/ digital
electronic meters which can extend its range or adopt instrument
for measuring other quantities
High Voltage Probe or voltage multiplier: essentially a potential
divider well insulated. Voltage to be measured is usually divided
by 1000 so that scale is multiplied by 1000.

11. Probes
3/6/2017 11
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
High Current Probe: High ac currents are
reduced by current transformers in ac
current probes.
Hall effect based high current probe use
magnetic flux to produce a measureable
voltage at the edges of a thin current
conductor. It works for both ac and dc.
Primary
conductor
To meter

12. Temperature Probes
3/6/2017 12
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
used to make contact measurements of
surface temperatures.
temperature sensor like RTD
thermistor, thermocouple produce a
voltage that varies with temperature.
Thermistor/RTD probes need electrically
stimulated to produce voltage
Voltmeters can be used in temperature
probes, that measure the probe's
output voltage, and conversion of the
voltage to temperature units is done.

13. RF Probes
3/6/2017 13
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
RF (radio frequency) can not be measured directly on multi-
meters due to their limited bandwidth.
In RF probes, RF waveform is rectified to produce equivalent dc
voltage which can be measured on the meter .
RF
waveform
meter