Time Delay Relays

Relays are switches that open and close circuits. They control one electrical circuit by opening or closing contacts in another circuit. When a relay contact is normally open (NO), there is an open contact when the relay is not energized. When a relay contact is normally closed (NC), there is a closed contact when the relay is not energized. Applying electrical current to the contacts will change their state. Many relays use an electromagnet to mechanically operate a switch, but other operating principles such as solid-state relays are also used. Relays are used where it is necessary to control a circuit by a low-power signal or where several circuits must be controlled by one signal. Depending on the structural features and operating principle, relays are of different types such as monitoring, regulating, auxiliary and protective. Classification of the types of relays depend on the function for which they are used. 

Time delay relays are used in industrial applications that require time-delayed switching, such as starting a motor or controlling a load. In a standard control relay, contacts close immediately when voltage is applied to a coil, and open immediately when voltage is removed. In some applications, it is desirable to have the operation of the contacts delayed following voltage being applied or removed. On-delay timers begin when voltage is applied. When the time has expired, the contacts close and remain closed until the voltage is removed from the coil. If the voltage is removed before time-out, the time delay resets. When using off-delay timers, nothing happens when voltage is applied. Closing the control input causes the contacts to transfer. Opening the control input causes time to begin and the contacts remain closed. On time-out, the contacts transfer. Closing the control input prior to time-out causes timing to reset. Removing voltage prior to time-out resets the timing and opens the contacts.