a. For this discussion, we can use the same meter as used for the ammeter.
The meter sensitivity is 1 milliampere and coil resistance is 75 ohms. Assume we
need to determine the correct multipliers for ranges of 0 to 10 volts, 0 to 100
volts, and 0 to 500 volts (fig 1-15).
Since 1 milliampere causes a full-scale
deflection, the total resistance of the meter (R1+75) must be such that the voltage
across it is 10 volts (0-to 10-volt range) when 1 milliampere current is flowing.
Thus:
Figure 1-15.
Determining multipliers
b. The total resistance is 10,000 ohms and the resistance of the multiplier
(R1) is 9925 ohms. The resistance of the other two multipliers (R2 and R3) can be
determined in a similar manner.
c. A term which you frequently encounter while troubleshooting television
systems is voltmeter sensitivity.
The sensitivity of a voltmeter is expressed in
ohms per volt and is determined by dividing the resistance of the meter and the
multiplier by the full-scale reading in volts. It is just another way of stating
what current can cause full-scale deflection.
d. A voltmeter should have very high resistance so it draws very little
current and affects the circuit as little as possible during voltage measurements.
Sensitivity, therefore, is an indication of the measuring quality of a voltmeter.
Generally, a meter with a 100-ohms-per-volt sensitivity is satisfactory. However,
for good accuracy in circuits with high resistance, you must use a meter with a
sensitivity of 20,000 (or more) ohms per volt.
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