Use of Shunt in Galvanometer

Use of Shunt in Galvanometer

The shunt is an electrical conductor joining two points of a circuit, through which more or less of a current may be diverted. It is a device which allows electric current to pass around another point in the circuit by creating a low resistance path. In order to convert a Galvanometer into an Ammeter, a very low resistance known as “shunt” resistance is connected in parallel to Galvanometer.

Suppose a resistance S of small value is connected between the two ends A and B parallel to the galvanometer of resistance G [Figure]. This S is the shunt. Let the principal current in the circuit be ‘i’. While reaching point ‘A’ this current will be divided into two parts. A small portion of the principal current will flow through the galvanometer and large current will flow through the shunt. As a result, the galvanometer will not be damaged due to heat produced for a large flow of current.

Two currents will meet at ‘B’ and will form principal current again. Let this current through the galvanometer and the shunt be respectively i_g and i_s. Now it the potential difference between the points A and B is (V_A – V_B), then according to Ohm’s law we get,

i_g = (V_A – V_B) / G … … … (1)

i_s = (V_A – V_B) / S … …. …. (2)

Dividing equation (2) by equation (1), we get,

i_s / i_g = G/S; or, i_s = i_g x (G/S)

But, i_s + i_g = i.

Inserting the value of i_s in this equation we get,

i_g (G + S)/S = i

So, i_g = (S x i) / (G + S) = principal current x (shunt resistance/total resistance) … … … (3)

or, i = i_g x (G + S)/S; (G + S)/S, is called the power-multiplier of the shunt. .

Again, putting the value of i_g in equation (3), we get,

i_s = (S x i) / (G + S) x G/S; or, S = (i_g x G) / (i – i_g)

So, i_s = (G x i) / (G x S) and, S = (i_g x G) / (i – i_g)