For example if the meter movement is rated 1 ma 50 it

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Unformatted text preview: ection of the pointer, which is registered on a scale attached to the meter movement. For example, if the meter movement is rated 1 mA, 50 , it would take 1 mA to cause a full-scale deflection of the meter movement. By introducing additional circuitry to the d’Arsonval meter movement, an ammeter, voltmeter, or ohmmeter can be constructed. Consider Fig. 2.59, where an analog voltmeter and ammeter are connected to an element. The voltmeter measures the voltage across a load and is therefore connected in parallel with the element. As shown in Fig. 2.60(a), the voltmeter consists of a d’Arsonval movement in parallel with a resistor whose resistance Rm is deliberately made very large (theoretically, infinite), to minimize the current drawn from the circuit. To extend the range of voltage that the meter can measure, series multiplier resistors are often connected with the voltmeters, as shown in Fig. 2.60(b). The multiple-range voltmeter in Fig. 2.60(b) can measure voltage from 0 to 1 V, 0 to 10 V, or 0 to 100 V, depending on whether the switch is connected to R1 , R2 , or R3 , respectively. Let us calculate the multiplier resistor Rn for the single-range voltmeter in Fig. 2.60(a), or Rn = R1 , R2 , or R3 for the multiple-range voltmeter in Fig. 2.60(b). We need to determine the value of Rn to be connected in series with the internal resistance Rm of the voltmeter. In any design, we consider the worst-case condition. In this case, the worst case occurs when the full-scale current Ifs = Im flows through the meter. This should also correspond to the maximum voltage reading or the fullscale voltage Vfs . Since the multiplier resistance Rn is in series with the An instrument capable of measuring voltage, current, and resistance is called a multimeter or a volt-ohm meter (VOM). A load is a component that is receiving energy (an energy sink), as opposed to a generator supplying energy (an energy source). More about loading will be discussed in Section 4.9.1. Ammeter scale I A spr...
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This note was uploaded on 07/16/2012 for the course KA KA 2000 taught by Professor Bkav during the Spring '12 term at Cambridge.

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