Time Delay Relay circuits

       A special class of electromechanical relays called time-delay relays provide delayed action, either upon power-up or power-down, and are commonly denoted in ladder logic diagrams by "TD" or "TR" designations near the coil symbols and arrows on the contact symbols. Here is an example of a time-delay relay contact used in a motor control circuit:

In this circuit, the motor delays start-up until three seconds after the switch is thrown to the "Run" position, but will stop immediately when the switch is returned to the "Stop" position. The relay contact is referred to as normally-open, timed-closed, or NOTC. It is alternatively referred to as a normally-open, on-delay contact.

Time-delay relay contact type symbols and labels:

>>Normally-closed, on-delay

>>Normally-open, on-delay

>>Normally-open, off-delay

>>Normally-closed, off-delay

Practice Sum:

Time-delay relays are important circuit elements in many applications. Determine what each of the lamps will do in the following circuit when pushbutton Ä" is pressed for 10 seconds and then released.

Timing diagram:

ANS:

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