Electronic AC Voltmeter Unit II Electronics & Digital meters

3/6/2017 1NEC 403 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad

Electronic AC Voltmeters

3/6/2017 2NEC 403 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad

PMMMC instrument are polarised i.e. has +ve and –ve terminalWhen a very low ac voltage (upto 0.1 Hz) is applied, pointer

tends to follow, for high frequency ac voltage, damping andinertia forces pointer to stay at average (zero).Thus PMMC instrument can not measure AC voltage or currentTo measure AC voltage, it needs to be rectified

PMMC meter

RS

RCEVo

ICAttenuatoror

Volt range selector

AmplifierHigh input/low output

impendence

Rectifier

Electronic AC Voltmeters(voltage follower rectifier voltmeter)

3/6/2017 3NEC 403 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad

Coupling capacitor blocks dc. Input to PMMC meter is half wave rectified waveformMeter 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

RCVo

IC

Attenuator Voltage follower

Meter circuit

+EB

-EB

+EB

Input waveform

Output waveform

Rectifier

Average and rms voltage of Rectifier

3/6/2017 4NEC 403 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad

BB

T

B

T

Bav EE

T

tEdE

T

tE

TE 318.02cos

2.2sin

1 2

0

2

0

T

+EB

Eav

BB

T

B

T

Brms EE

dtT

t

T

Edt

T

tE

TE 5.0

2.4cos1.2sin

1 2

0

22

0

22

BB

T

B

T

Bav EE

T

tEdE

T

tE

TE 637.0

22cos.2sin

2 2

0

2

0

BB

T

B

T

Brms EE

dtT

t

T

Edt

T

tE

TE 707.0

2.4cos1

2.2sin

2 2

0

22

0

22

+EB

Eav

Half wave rectifier

Full wave rectifier

3/6/2017 5NEC 403 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad

rectifier voltage drop error can be eliminated by connecting therectifier 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 rectifierAttenuator 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

RCVo

IC

Meter circuit

+EB

Output waveform

C2

C3

C4

C5

3/6/2017 6NEC 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

RCVo

IC

Meter circuit

R1 R2

R3

3/6/2017 7NEC 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 cyclePeak current IP=EP/R3

Average current through meter IAV=0.318 IP

D1

++

-

+VCC

-VEE

VF-E

Meter current

R1

R3

PMMC meter

RCIC

3/6/2017 8NEC 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 cyclePeak current IP=EP/R3

Average current through meter IAV=0.637 IP

+

-

+VCC

-VEE

E

R1

R3

Meter current

RCIC

D1

D2

-

PMMC meter

D4

D3+

Current Measurement with Electronic Instrument

3/6/2017 9NEC 403 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad

+

-

+VCC

-VEERS

R3

PMMC meter

RCIC

AmmeterTerminals

E

Electronic voltmeters have high input impedances while ammeterhave low input impedanceElectronic 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 meterFor AC current measurement

Electronic voltmeters havingrectifier is used

Probes

3/6/2017 10NEC 403 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad

There are many probes and adopter for use with analog/ digitalelectronic meters which can extend its range or adopt instrumentfor measuring other quantitiesHigh Voltage Probe or voltage multiplier: essentially a potential

divider well insulated. Voltage to be measured is usually dividedby 1000 so that scale is multiplied by 1000.

Probes

3/6/2017 11NEC 403 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad

High Current Probe: High ac currents arereduced by current transformers in accurrent probes.

Hall effect based high current probe usemagnetic flux to produce a measureablevoltage at the edges of a thin currentconductor. It works for both ac and dc.

Primary conductor

To meter

Temperature Probes

3/6/2017 12NEC 403 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad

used to make contact measurements ofsurface temperatures.

temperature sensor like RTDthermistor, thermocouple produce avoltage that varies with temperature.

Thermistor/RTD probes need electricallystimulated to produce voltage

Voltmeters can be used in temperatureprobes, that measure the probe'soutput voltage, and conversion of thevoltage to temperature units is done.

RF Probes

3/6/2017 13NEC 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 dcvoltage which can be measured on the meter .

RF waveform

meter