Answer to Question #93504 in Electric Circuits for David

Question

Suppose the coil of a meter has a resistance r of 20ohms and is fully deflected by a current of 5mA. Show that it can be converted to a meter of range 0-5A

Answer 1

Let a shunt resistance of resistance $R_s$ is connected in parallel to this meter,

Let $I$ be the total current coming, $I_m$ be the current passing through this meter and $I-I_m$ be the current through shunt resistance

resistance of meter$=$ 20$\Omega$

Voltage across shunt $=$ $V_s=$ Voltage across meter$=V_m$

Now $V_s=(I-I_m)\times R_s=I_m\times 20$

For caliberating this meter for given range $I$ should be $5A$, $I_m$ should be $(5\times 10^{-3})A$

Now putting all values

$(5- 5\times10^{-3})\times R_s=(5\times 10^{-3})\times20$

$R_s=0.02\Omega$

Hence if a shunt of resistance $0.02\Omega$ is connected in parellel to meter then meter will be caliberated to   range of 0-5$A$