Part A A d\'Arsonval ammeter is shown in the first figure (Figure 1) . The large

Question

Part A

A d'Arsonval ammeter is shown in the first figure (Figure 1) . The largest allowable current and voltage for this d'Arsonval movement are indicated in the figure. Calculate the value of the shunt resistor, Ra, required to give a full-scale current reading of 2.8A.

Part B

How much resistance is added to a circuit when the 2.8A

ammeter with the shunt resistor you selected in part A is used to measure circuit current?

Part C

Suppose this same 150 mV, 3 mA d'Arsonval movement is used to construct a voltmeter as shown in the second figure (Figure 2) . Find the value of Rv required to achieve a v= 5.5V full scale reading.

Explanation / Answer

part A:

as Ra and ammeter are in parallel, they have the same voltage across them

hence 2.8*Ra=150 mV

Ra=0.0536 ohms

part B:

resistance of ammeter=150/3= 50 ohms

so net resistance=50*Ra/(50+Ra)=0.053514 ohms

part C:
current passing through Rv=3 mA

voltage across Rv=5.5-0.15=5.35 volts

so Rv=5.35/0.003=1.783 kilo ohms

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