Let the resistance of a permanent magnet moving coil instrument be R Ω. What is the shunt resister required to measure a maximum current which is 5 times the full scale deflection current of the instrument?

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ISRO (VSSC) Technical Assistant Electrical 2017 Official Paper
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  1. R Ω 
  2. 0.25 RΩ 
  3. 0.5R Ω 
  4. 0.75R Ω 

Answer (Detailed Solution Below)

Option 2 : 0.25 RΩ 
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Detailed Solution

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Formula:

In a PMMC instrument

M = Multiplying factor = (Present FSD) / (Previous FSD)

Where,

Rsh = Shunt resistance

Rm = Meter resistance

Vm = Potential difference across meter

Im = Meter current

Calculation:

Given-

Rm = R Ω

Let the previous FSD current of PMMC is I, then present FSD current of PMMC is 5I

Rsh = 0.25 R Ω

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