ESSENCE OF CONNECTING A RESISTOR IN PARALLEL WITH AN INDUCTOR

Fig 3.9: Decay of current in an RL series circuit

Let us consider the circuit shown in figure 3.9. A discharge resistor RD is connected in parallel with the inductive circuit. When the switch S is connected to A, the current  becomes E/R. When the switch is moved to position B, the circuit is disconnected from the supply.

When the position of the switch is change from A to B, the circuit in the inductive circuit is switched off and a voltage is induced in it. Arcing is produced at the time of charging the position of the switch and there is a possibility of damaging the insulation because the induced voltage may be many times greater than the applied voltage. To avoid this arcing, a discharge resistor RD is connected as shown in figure 3.9