Time Delay Relay Functions


Time relays and exactly how they function can get very confusing very quickly. Below is a very generalized chart of the differences between some of the more common functions of time delay relays. When looking at products, be sure to check out the datasheets as some of these might require consistent power along with the input or control contact. There also might be some slight variations as well, especially when you only have repeating functions like flashers and repeat cycles.

This timer is most commonly used with indicator lights or alarms. When voltage is applied, the relay is energized for the set time then de-energizes for the same set time and repeats until the voltage is removed.

The relay energizes when voltage is applied and starts the timer. At the end of the set time the relay will de-energize. Reset is accomplished by removing, then reapplying voltage.

The relay is energized whenever there is input voltage. The timer will start once the input voltage is removed. The relay will remain energized until the timer expires. Reapplying voltage before the timer expires will reset the timer. Since the relay is energized after the input voltage is removed there is typically supply voltage contacts and control or input contacts.

Applying voltage to the input contact will start the timer. The relay will remain de-energized until the timer expires which will then energize the relay. To reset the relay, remove the input voltage and reapply.

One Shot
These will energize the relay for the set time. In order to restart the timer, voltage must be removed from the control or input contacts and then reapplied.

Repeat Cycle
Similar to the flasher function, this will pulse the relays on and off. The difference is these relays will have two timer controls allowing you to set both the on and off times. Some of these relays will start the on timer first and others will begin with the off timer. If that’s relevant to your design, you will have to look at the datasheet to determine which one it is.

Star Delta Controller
This relay is specifically used to help start a 3-phase motor. Starting a motor in a star (more commonly known as wye) configuration will consume 1/3 of the power but you will also reduce your torque equally. After a duration the contacts will switch it over to a delta configuration to get the full power back. This is used to reduce the initial draw from the power supply. Typically you will set the start time, or the time it will be in the star configuration. After that time expires it will completely disconnect the relay contacts for a preset time to prevent shorting the phases.