Page 264 - Electromechanical Devices and Components Illustrated Sourcebook

226 Electromechanical Devices & Components Illustrated Sourcebook

Multiply
Reading by 10
Permanent Scale 10 Volt, 1 Ma Meter
Magnet (10 K Ohm)
1 2 3 4 5 6 7 8 9
10
DC VOLTS Mirror
DC VOLTS Needle
1 2 3 4 5 6 7 8 9 10
Stops
Needle
Moving Coil Zero Adjust
Spring
Bridge − +
Pivot
N S
Insulating Iron Core
Washers − +
Pole Faces 90 k ohms
Terminals
− + 0- to 100-volt Output
Figure 13-8 Moving Coil Voltmeter Figure 13-10 Voltmeter with Single Voltage Compensa-
tion resistor

By adjusting the position of the coil/core assembly and converted to a 100-volt meter. In this way, virtually any volt-
needle, as shown in Figure 13-9, it is possible to set up a volt- age can be measured on a relatively low voltmeter.
meter to indicate the polarity of the incoming signal. If the Multirange voltmeters can be configured by setting up an
signal matches the polarity of the meter then the needle will array of resistors, as shown in Figure 13-11. In this case there
deflect to the right. If the signal has a reverse polarity, then the is a common terminal and four voltage terminals. Each termi-
needle will deflect to the left. nal is arranged with a resistor in series with the meter. The
“Times 0” terminal doesn’t require a resistor. The voltage
reading is based on the multiplier associated with each terminal.
As an example, if an 8-volt reading is shown while a voltage
is connected across the “Times 100” terminal and the com-
Permanent mon, then the actual indication would be multiplied by 100.
Magnet Scale
(8 100 800 volts)
4 3 2 1 1 2 3 4
− 5 DC VOLTS 5 + 1 Volt, 1 Ma Meter
(1 k ohm)
Needle 4
Centering Moving Coil 1 2 3 5 6 7 8 9 10
Spring Bridge DC VOLTS Mirror
Pivot
Needle
N S Stops
Insulating Iron Core
Washers Zero Adjust
Pole Faces
Terminals − +
− +

Figure 13-9 +/– Indicating Moving Coil Voltmeter
Common

Times 0 0 ohms
The range of any voltmeter can be adjusted to read higher
Times 10 9 k ohms
voltages by adding a compensation resistor, as shown in
Figure 13-10. In this example, the internal resistance of a 0 to Times 100 99 k ohms
10 volt meter is 10,000 ohm. By adding a 900,000-ohm resistor, Times 1000 999 k ohms
the effective resistance of the instrument is 10 times higher
and, therefore, will read one-tenth of the input signal. To get Multiplier Resistors
a full reading at 10 volts, the input signal must be 100 volts. Figure 13-11 Voltmeter with Multi Range Multiplying
By simply adding the resistor, the 10-volt meter has been Resistors