Page 309 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Fig 5

Fig 6 - 8

Shunt resistors across the meter movement bypass current in excess of 0.05 mA at fsd. A suitable value of shunt
resistor is selected through the range switch for the required range of current measurement.
A circuitry when working as a voltmeter. (Fig 7)

The voltage drop across the meter coil is dependent on the current and the coil resistance. To indicate voltages
greater than 50 mV at fsd as per the circuit, multiplier resistances of different values are connected in series with
the meter movement through the range switch for the required range of measurement.
A circuitry when working as an ohmmeter. (Fig 8)
To measure resistance, the leads are connected across the external resistor to be measured (Fig 8). This
connection completes the circuit, allowing the internal battery to produce current through the meter coil, causing
deflection of the pointer, proportional to the value of the external resistance being measured.
Zero adjustment
When the ohmmeter leads are open, the pointer is at full left scale, indicating infinite (¥) resistance (open circuit).
When the leads are shorted, the pointer is at full right scale, indicating zero resistance.
The purpose of the variable resistor is to adjust the current so that the pointer is at exactly zero when the leads
are shorted. It is used to compensate for changes in the internal battery voltage due to aging.
Multiple range
Shunt (parallel) resistors are used to provide multiple ranges so that the meter can measure resistance values
from very small to very large ones. The reading on the ohmmeter scale is multiplied by the factor indicated by the
range setting.
Remember, an ohmmeter must not be connected to a circuit when the circuit’s power is on. Always
turn the power off before connecting the ohmmeter.

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CITS : Power - Electrician & Wireman - Lesson 50-53