AC Analog Electronic
Voltmeters and
Multimeters
Electrical and Electronic Measurements (EE2101)
1
Half-Wave Rectifier Voltmeters
The IC op-amp voltage-follower voltmeter is a dc instrument.
Connecting a rectifier in series with the meter circuit of this instrument
converts it into a half-wave rectifier voltmeter.
The output from the voltage follower is exactly the same as the input.
So the voltage fed to the meter circuit is simply a half-wave rectified version
of the input voltage 𝐸𝐵 from the attenuator.
The coupling capacitor ( 𝐶1 ) is usually provided at the input of an ac
voltmeter to block unwanted dc voltages.
2
Half-Wave Rectifier Voltmeters
3
Figure 1
Half-Wave Rectifier Voltmeters
The voltage drop (𝑉𝐹 ) across rectifier is a source of error in the circuit.
𝑉𝐹 can be taken into account in design calculations when the instrument is
indicating full scale.
However, at other points on the scale an error occurs due to 𝑉𝐹 .
Also, the rectifier voltage drop is not always exactly 0.7 V, as usually
assumed for a silicon diode, and it varies with temperature change.
4
Precision Rectifier Voltmeters
To avoid these errors, the voltage follower feedback connection to the
inverting terminal is taken from the cathode of rectifier 𝐷1 instead of from
the amplifier output (see Figure 2).
The result is that the half-wave-rectified output precisely follows the (+ve)
half-cycle of the input voltage.
So there is no rectifier voltage drop from input to output.
The circuit is known as precision rectifier.
Capacitors 𝐶2 , 𝐶3 , and 𝐶4 , connected across the attenuator resistors, are
normally employed in order to compensate for the input capacitance of the
amplifier.
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Precision Rectifier Voltmeters
Figure 2 6
Working of Precision Rectifier
Figure 2.1 7
Calculation of Series Resistance 𝑅𝑠
** A PMMC instrument with 𝐹𝑆𝐷 = 100 𝜇𝐴 and 𝑅𝑚 = 1 𝑘Ω is to be employed as
an ac voltmeter with 𝐹𝑆𝐷 = 100 𝑉 (𝑟𝑚𝑠). Silicon diodes are used in the bridge
rectifier circuit of Figure 2.2. Calculate multiplier resistance value required.
Figure 2.2
Precision Rectifier Amplifier Voltmeter
Low-level ac voltages should be accurately amplified before being rectified
and applied to a meter circuit.
Amplification is combined with half-wave rectification in the circuit shown in
Figure 3.
With the diode omitted, the op-amp circuit is a non-inverting amplifier.
Inclusion of 𝐷1 causes the (+ve) half-cycles of the input to be amplified by a
factor 𝐴𝑣 = (𝑅2 + 𝑅3 )/𝑅3 .
The amplification is precise and here again there is no rectifier voltage drop
involved.
In this case, the peak voltage applied to the meter circuit is 𝐴𝑣 𝐸𝑝 , and again
rectifier voltage drop does not enter into the calculations.
9
Precision Rectifier Amplifier Voltmeter
Figure 3 10
Voltage-to-Current Converter
The circuit in Figure 4 is a voltage-to-current converter with half-wave
rectification.
Only the (+ve) half-cycles of the ac input are effective in passing current
through the meter.
During the (-ve) half-cycle, the diode is reverse biased and no current flows
through the meter or through resistance 𝑅3 .
The meter peak current is 𝐼𝑝 = 𝐸𝑝 /𝑅3 , and the average meter current is 𝐼𝑎𝑣 =
0.5(0.637𝐼𝑝 ).
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Voltage-to-Current Converter
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Figure 4
Voltage-to-Current Converter
Problem 4: The half-wave rectifier electronic voltmeter circuit in Figure 4
uses a meter with a FSD current of 1 𝑚𝐴. The meter coil resistance is 1.2 𝑘Ω.
Calculate the value of 𝑅3 that will give meter full-scale pointer deflection
when the ac input voltage is 100 𝑚𝑉 𝑟𝑚𝑠 . Also determine the meter
deflection when the input is 50 𝑚𝑉.
13
Analog Electronic Multimeter
14
Figure 5
Analog Electronic Multimeter
The Hewlett-Packard (HP) model 427A electronic voltmeter is a
representative of laboratory-type analog electronic instruments.
The front panel illustrated in Fig. 5 shows that it can measure dc voltage
(±DCV), ac voltage (ACV), and resistance (OHMS).
A knife-edge pointer and mirror are provided for precise reading on two
voltage scales: 0 to 1 and 0 to 3.
The ohmmeter scale has its 1 position at the scale center.
A decibel scale (DBM) is also provided.
Voltage measurements are made using the VOLTS and COM (common)
terminals.
For resistance measurements, the COM and OHMS terminals are employed.
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Analog Electronic Multimeter
16
Figure 6
Analog Electronic Multimeter
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Analog Electronic Multimeter
An electrical zero control (DC ZERO/Ω ∞) is included on the front panel, as
well as a mechanical zero control for the pointer.
The HP427A has nine dc voltage ranges, from 0.1 V (full scale) to 1000 V. The
±DCV positions on the FUNCTION SWITCH permit either positive or negative
polarity voltages to be measured.
The measurement accuracy is ±2% of full scale, and the input resistance is 10
MΩ on all ranges.
There are 10 ac voltage ranges, the lowest being 10 mV, and highest 300 V.
The frequency range for ac voltage measurements is 10 Hz to 1 MHz.
However, this can be extended by the use of a high-frequency probe that has
a peak detector circuit.
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Analog Electronic Multimeter
The measurement accuracy for ac voltages is ±2% of full scale.
The input impedance is stated as 10 MΩ shunted by a 40 pF capacitance on
ranges upto and including 1 V, and 10 MΩ shunted by a 20 pF on 3 V and
greater ranges.
There are seven resistance-measuring ranges on the HP427A, starting at 10 Ω
(center scale) and going to a maximum of 10 MΩ.
The accuracy of resistance measurements is ±5% of the reading at midscale.
19
Procedure for DC Voltage Measurements
1. With the instrument switched off, check the pointer zero position. Adjust
the mechanical zero as required.
2. If the instrument is battery operated, set the FUNCTION switch to BATT, and
check that the battery voltage is a minimum of 1.5 V. For instruments with
an internal power supply and line cord, this step is not necessary.
3. Set the FUNCTION switch to DCV+ or DCV- as required.
4. Set the RANGE switch to 0.1, and short-circuit the VOLTS and COM
terminals.
5. Adjust the DC ZERO/Ω ∞ control to set the pointer precisely to zero on the
scale, then remove the short-circuit connection.
6. Select a voltage range greater than the voltage to be measured. Where the
approximate value of the voltage is not known, rotate the RANGE switch to
the highest range.
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Procedure for DC Voltage Measurements
7. Connect the input voltage to the VOLTS and COM terminals, and adjust the
range switch to give the greatest on-scale pointer deflection.
(**Where there is a grounded point in a circuit, the COM terminal should be
connected to that point. Where there is more than one electronic
instrument involved, all of the COM terminals should be connected to a
single point.)
The procedure for ac voltage measurements is exactly as for dc voltage, but
with the FUNCTION switch set to ACV and no need for DC ZERO adjustment.
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Analog Electronic Multimeter (Repeat)
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Figure 7
Resistance Measurement Procedure
1. With the instrument switched off, check the pointer zero and adjust the
mechanical zero control as necessary.
2. Check the battery voltage as explained for dc voltage measurements.
3. Set the FUNCTION switch to OHMS, and with the instrument terminal open-
circuited, adjust the DC ZERO/Ω ∞ control until the pointer indicates infinity
(∞) on the resistance scale.
4. Select a resistance range to suit the approximate value of the resistance to
be measured.
5. Connect the resistance to the COM and OHMS terminals, and adjust the
RANGE control to give a resistance reading as close as possible to center
scale.
23
Decibel (dB) and Decibel-Milliwatt (dBm)
Measurements
Decibel measurement is essentially the same as AC voltage measurements.
On the HP427A the decibel scale is read directly when the instrument is set
to the 1 V (ACV) range.
Since each range position above 1 V is 10 dB above 1 V, 10 dB must be added
to the scale reading.
For each range position below 1 V, 10 dB must be subtracted from the dB
measurements.
For example, if the pointer is indicating -1 dB on the 10 V range, the dB
measurement is -1 dB+ 20 dB = 19 dB.
A -5 dB scale reading on the 0.3 V range represents (-5 dB -10 dB) = -15 dB.
24
Decibel (dB) and Decibel-MilliWatt (dBm)
Measurements
The decibel scale on the HP427A is based on 0 dB as 1 mW dissipated in a 600
Ω load resistance.
Where the load resistance is other than 600 Ω, the scale readings must be
corrected to obtain the absolute dB measurements.
However, scale changes can be read directly as changes in dB levels.
The Decibel-Milliwatt (dBm) term is sometimes employed because the
absolute dB measurements are measurements of changes in power level from
a starting point of 1 mW power dissipation. 25
Multimeter Probes
There are many probes and adapters available for use with multimeters (and
for use with analog and digital electronic instruments).
They can extend the ranges of measurement, or adapt the instrument for
measurement of temperature or other quantities. Some are illustrated here.
High-Voltage Probe
The high-voltage probe (also known as a voltage multiplier) is essentially a
potential divider, well insulated for safety.
The voltage to measured is usually divided by a factor of 1000, so that the
instrument scales are effectively multiplied by 1000; a 50 V scale becomes 50
kV scale.
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Multimeter Probes
Figure 8
27
Multimeter Probes
High-Current Probe (based on Current Transformer)
High levels of alternating current can be reduced by the use of a current
transformer, and this principle is used in the ac current probe.
The transformer core opens to close around a conductor carrying the current
to be measured.
The conductor can be treated as a single-turn primary on the transformer,
and the secondary winding then determines the measurable current level.
Typically, the 1 mA ac scale on the multimeter is converted into a 1 A scale
by the use of the current probe.
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Multimeter Probes
Figure 9
29
Multimeter Probes
High-Current Probe (based on Hall-Effect Principle)
The Hall-effect probe has a similar appearance to the transformer-type ac
current probe.
However, it produces an output voltage for both dc and ac currents in a
conductor.
The Hall-effect transducer contained in the probe operates on the principle
that a small voltage is produced at the edges of a flat current carrying
conductor in the presence of a magnetic field.
The output is connected to the meter voltage terminals, typically resulting in
the mV scale being read as a current scale with 1 mV representing 1 A.
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Multimeter Probes
Figure 10
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