Answer / kingshuk dey

On-delay timers: The timer coil is given supply, the time is measured, the NO contact become NC (or vice-versa).

Off-delay timers: Supply to timer coil is stopped, the time is measured, then the NC contact becomes NO (or vice-versa).

The timer, in both cases, won't require power as it is spring driven (like normal watches' time counters).

Difference is that on-delay timer starts counting when supply is given and resets when supply is removed, off-delay timer starts counting when supply is removed, and resets when supply is given.

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