Page 265 - Electromechanical Devices and Components Illustrated Sourcebook

Chapter 13 Meters 227

Another method to read higher voltages with a low volt- Multiply Reading by 10
meter is to incorporate a voltage divider, as discussed in
Chapter 4. Figure 13-12 shows a 10-volt meter configured to
1 volt,
accept a 0- to 100-volt input signal. This method is normally (100 megohm)
not used on analog meters because the current loss over the
circuit can be fairly high. This, in turn, affects the sensitivity
of the meter. As an example, the circuit shown would require
Range Selector
a 10-mA drive current to read full scale.
1 volts 1000 volts

Multiply 10 volts 100 volts
Reading by 10
10 volt, 100 _a Meter + −
(100 ohm) 90 megohm 9 megohm 900 K 100 K
1 2 3 4 5 6 7 8 9 100-Megohm Input Impedance
10
DC VOLTS Mirror Figure 13-14 Digital Voltmeter with Four Range Voltage Divider
Needle
Stops
If an extremely high input impedance is required while
Zero Adjust
using a moving coil voltmeter, then an amplifier must be
− + incorporated, as shown in Figure 13-15. The voltage divider
network is the same as with a digital voltmeter. The output of
the selector switch is fed through a calibration potentiometer
and then into an amplifier, which, in turn, drives the meter.
− + The calibration adjustment is intended to tune the input signal
1 k ohms 9 k ohms
to the amplifier so that the meter can be referenced against a
0- to 100-volt Output standard voltage.
Figure 13-12 Voltmeter with Voltage Divider Compen-
sation Resistors

3 4 5 6 7
1 2 8 9
10
Multiply Reading by 10 DC VOLTS
0 to 10 volts
10 volt, (100 megohms)

+ −
− +
2 megohms 18 megohms
0- to 100-volt Input
Amplifier
Figure 13-13 Digital Voltmeter with Voltage Divider
Resistors

Calibration
Potentiometer
Voltage dividers are more commonly used on digital
meters, as shown in Figure 13-13. Because a digital meter has Range
an extremely high input impedance, the resistors that are used Selector
for the voltage divider can be in the megohm range and, there- 1 volts 1000 volts
fore, require very low driving currents. As an example, the
circuit shown would only require a 0.5- A drive current to 10 volts 100 volts
read full scale. −
A multirange digital voltmeter can be set up using a volt- +
90 megohm 9 megohm 900 k ohm 100 k ohm
age divider network coupled with a selector switch, as
shown in Figure 13-14. In this case the input impedance is 100-megohm Input Impedance
100 megohms, which translates to an extremely low drive Figure 13-15 Amplified Analog Voltmeter with Four Range
current. Voltage Divider