The precautions to observe when using an ammeter are summarized as follows:
The voltmeter uses the same type of meter movement as the ammeter but employs a different circuit external to the meter movement.
As shown before, the voltage drop across the meter coil is a function of current and the coil resistance. In another example, 50ľA × 1000O = 50mV. In order for the meter to be used to measure voltages greater than 50mV, there must be added a series resistance to drop any excess voltage greater than that which the meter movement requires for a full scale deflection. The case of the voltmeter, this resistance is called multiplier resistance and will be designated as RM. Figure 10- 145 illustrates a basic voltmeter. This voltmeter only has one multiplier resistor for use in one range. In this example, the full scale reading will be 1 volt. RM is determined in the follow way:
The meter movement drops 50mV at a full scale deflection of 50ľA. The multiplying resistor RM must drop the remaining voltage of 1V - 50mV = 950 mV. Since RM is in series with the movement, it also carries 50ľA at full scale.
RM =950 mV/50ľA = 19k O
Therefore, for 1 volt full scale deflection, the total resistance of the voltmeter is 20 k O. That is, the multiplier resistance and the coil resistance.
Voltmeter sensitivity is defined in terms of resistance per volt (O/V). The meter used in the previous example has a sensitivity of 20 k O and a full scale deflection of 1 volt.
Multiple Range Voltmeters
The simplified voltmeter in Figure 10-145 has only one range (1 volt), which means that it can measure voltages from 0 volts to 1 volt. In order for the meter to be more useful, additional multiplier resistors must be used. One resistor must be used for each desired range.
For a 50ľA movement, the total resistance required is 20 k O for each volt of full scale reading. In other words, the sensitivity for a 50ľA movement is always 20 k O regardless of the selected range. The full-scale meter current is 50ľA at any range selection. To find the total meter resistance, multiply the sensitivity by the full scale voltage for that particular range. For example for a 10 volt range, RT = (20k O/V)/(10 V) = 200k O. The total resistance for the 1 volt range is 20k O, so RM for a 10 V range will be 200k O - 20k O = 180k O. This two-range voltmeter is illustrated in Figure 10-146.
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