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How to Choose the Right Time Delay Relay

Posted by G. Christianson on Jun 23, 2014 6:17:46 AM


Time delay relays are designed to control an event based on time. Work trucks, buses, emergency vehicles and other mobile equipment require delayed circuit operation for several applications. The solid state design of time delay relays withstands the electrical environments typically found in on and off-road vehicles and equipment. The key is selecting the right time delay relay for your application.

When designing circuits using time delay relays, you should consider the following questions:

  • What initiates a time delay relay?
  • Does the timing start with the application or release of voltage?
  • When is the output relay energized?


Three industry standard time delay relay functions are available: On-Delay, Off-Delay and One-Shot.

On-delay timer relays are used when one circuit function must activate a time delay after another function. For example, you want to start an air compressor five seconds after the PTO is activated. The circuit that activates the PTO would be connected to the on-delay timer relay's timer input. The timer's output would operate the compressor's start circuit. When the PTO is activated the timer starts. Five seconds later the timer's output would start the compressor, and would stay on until the timer's input is removed.

Off-delay timer relays are used when one function must start a second function and the second function must stay on for a time period after the first function is deactivated. For example, a school bus has an egress light that must be started when the door is opened, and must stay on for 10 seconds after the door closes. The door open switch is connected the off-delay timer relay's timer input and the egress light is connected the timer's output. When the door is opened the timer's output is activated, turning on the light. When the door switch indicates the door is closed the timer starts. After 10 seconds the timer's output is deactivated, turning off the light.

One-shot timer relays are used when one function must start a second function and the second function must stay on for a fixed time period. For example, a rear view mirror heater must only operate for 15 minutes to prevent damage to the mirror. The mirror heater switch is connected to the one-shot timer's input and the timer's output operates the mirror heater. When the switch is turned on the timer starts and its output is activated, turning on the mirror heater. After 15 minutes the timer's output turns off regardless of if the switch is on or off.

For more information on time delay relays, call 800.328.2724.

Learn More about time delay relays.

Topics: How To, Buying Guides, Relays


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