Off Delay Timer Working Principle

Off delay timers have wide applications in the field of automation and control. The timer is an electrical device that controls the operation of the equipment by switching on and off according to the set time of the timer.

There are mainly two types of timer- on delay timer and off delay timer. The star-delta starter uses a delay timer for delayed switching of the delta contactor. The timers are basic building blocks for the batch operation where the timer ensures the sequential operation.

We will discuss the on delay timer and off delay timer in detail.

Off-delay Timer Working

Off-delay timer as its name suggests, the timer change over its state of contacts after a lapse of a set time period. On energization of the timer coil, the timer activates and changes its contact state from NO to close or NC to open. The timer keeps its contact in the same state as long as the power supply is available to its coil.

For example, when the timer coil receives a power supply its NO contact changes to NC, and it will remain NC till the power supply is available to the timer’s coil. Now, if the supply to the timer’s coil is cut off, then the NC contact will change its state from NC to NO.

Now, take an example of an off delay timer. Let the set delay time in the timer is 30 seconds. On powering the timer the contacts of the timer change from NO to NC and NC to NO instantly. The contacts will remain in this state as long as the timer receives the power. On removing power, the timer’s contacts will not changeovers, but remain in the same state till the lapse of the off-delay timing setting. In this case, it is 30 seconds. After off time over, the timer will change its contacts from NC to NO and vice versa.

How does the off delay timer keep working after removing of the power supply?

The off delay timer has capacitors in the timing circuit. On powering the timer. the capacitors charge and hold the charge till power is available to the timer. On removing the power of the timer, the capacitor charge continues to provide the power to the timer till the capacitor discharges to a particular level. The off delay timer setting in nothing but the rate of capacitor discharging. When the capacitor discharges up to the set time of the timer, the timer changes its contact position i,e from NC to NO and vice versa.

Timing diagram of off-delay timer

Off Delay Timer timing diagram

Initially, when the power supply is not available to the timer, the contacts remain in their natural state. At point ‘A’ the timer receives a power supply. The contacts of the timer change from NC to NO and vice versa. At point ‘B’ the power supply to the timer is withdrawn. The timer will keep its contacts in the same state till point “C” is reached. The time difference between points B and C is the off delay time of the timer.

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