Relay coil after switch ON needs to voltage much lower than its nominal voltage,
We can use this property of relay to saving 85% of the power to increase the battery life in battery powered circuits.
To reduce the current / power consumption we can put a resistor through the relay coil, but since this low current not enough to running the relay at the beginning so we should short out the resistor for 1 second, then the relay switch keep going.
You have to use this formula to calculate the suitable resistor value:
For more explanations please watch this video:
