Time Delay Relays 101

Time delay relays have become indispensable in emergency vehicle operations, offering precise control over critical functions with enhanced durability and reliability. By selecting the right relay type based on application needs and environmental conditions, automotive technicians and upfitters can optimize vehicle performance and ensure operational safety in demanding scenarios.

Practical Applications

Time delay relays are used to delay the activation or deactivation of electrical circuits for a predetermined period. Work trucks, buses, construction vehicles and other mobile equipment require delayed circuit operation for dozens of applications ranging from engine idle management to automatic door locking and light shutoff. 

Time delay relays also play an important role in police and emergency vehicles for timing control of lights, sirens, and other auxiliary systems: 

• Siren Control: Emergency vehicles often have multiple sirens or different tones (such as wail, yelp, or phaser). Time delay relays can be used to sequence through these tones or to delay the activation of a siren after other systems are turned on, ensuring that the vehicle's lights are fully activated before the siren starts.
• Lighting Sequencing: Emergency vehicles have several types of lights (strobes, LEDs, spotlights) that need to activate in a specific sequence or delay. Time delay relays ensure that these lights come on in the correct order or with the necessary delay to optimize visibility and safety.
• Preset Timing: Time delay relays can also be used to preset the duration for which certain lights or sirens remain active after being triggered, helping to conserve battery power or comply with regulations regarding noise pollution. 
• Control Logic: Time delay relays can integrate with other vehicle systems to ensure that lights and sirens operate in coordination with other actions, such as doors closing or ignition being activated, enhancing operational efficiency and safety.

All these capabilities enable designers and upfitters to create an electrical system that is custom-made to the requirements of the end user. Proper use of time delay also protects the electrical accessories, enhancing system longevity and reducing maintenance costs and time in the shop. 

Key Considerations for Selection

Time delay relays are available in three key functionality types: On-Delay, Off-Delay and One-Shot. Select the functionality type that is best suited to the application you have in mind.

On-Delay: An On-Delay relay activates a circuit after a specified delay once the initiating signal (i.e. power take-off, also known as PTO) is received. An on-delay relay in an ambulance could be used to manage the activation of critical onboard medical equipment. When the vehicle's engine is started, the on-delay relay introduces a brief delay before powering up sensitive medical devices like defibrillators or ventilators. This delay allows the vehicle's electrical system to stabilize, preventing power surges that could damage the equipment.

• Tip: On-delay timer relays are used when you need one circuit function to start a delay before activating another function. 

• Practical application: An on-delay relay can be used to activate emergency lighting a few seconds after a vehicle’s emergency systems are engaged, ensuring readiness without overwhelming the electrical system initially. 

Right: Many On- and Off-Delay Solid State Relays from InPower include an adjustable time delay feature (lower right corner). 

Off-Delay: An Off-Delay relay keeps a circuit active for a set period after the initiating signal. Off-Delay is ideal for applications such as maintaining interior lighting for a specified time after the door is closed on a vehicle. The door's open switch connects to the off-delay timer relay's input, while the light connects to its output. When the door opens, the timer activates the light. When the door closes, the timer starts counting. After the preset time (for example, 10 seconds) the timer deactivates the light.

• Tip: Off-delay timer relays are used when you need one function to trigger another function, which then remains active for a set time after the first function stops.

• Practical application: Employ off-delay relays to maintain ventilation or heating operation for a short period after the vehicle is turned off, enhancing comfort and safety for occupants.

One-Shot: A One-Shot relay triggers a circuit for a fixed duration once the initiating signal is received, regardless of the signal duration. For instance, timing can be set to ensure that a rear-view mirror heater runs for only 15 minutes. The heater switch connects to the one-shot timer's input, and the timer controls the heater's operation. When you turn on the switch, the timer starts and activates the heater. After 15 minutes, the timer automatically shuts off the heater, regardless of whether the switch is still on.

• Tip: One-shot timer relays are used when you need one action to trigger another function, which then stays active for a specific duration.

• Practical application: Utilize one-shot relays to manage accessories, ensuring they operate for a specified time to conserve power and prevent unnecessary operation.

Pictured at right: A One-Shot time delay relay with optional On-Delay from Egis Mobile Electric. 

Voltage, Current, and Durability

Check the relay’s voltage and current ratings to make sure it can handle the electrical specifications of the vehicle’s system without risk of overheating or electrical failure. Ideally, the relay should be a solid-state model. Controlled by an electronic microprocessor, solid state relays are more resilient when subjected to the vibration, moisture, and temperature fluctuations commonly encountered by emergency vehicles and other heavy-duty vehicles and equipment.

Power Contactors: A 3-in-1 Option 

In addition to time delay relays, check out the InPower line of HD2VCM power contactors, pictured at right, which enable configuration of On-Delay, Off-Delay or One-Shot delay in a single device. 

Conclusion

Time delay relays in emergency and heavy-duty vehicles play a crucial role in ensuring that equipment operates effectively and in compliance with operational protocols, enhancing the vehicle’s functionality during critical responses. By integrating time delay relays into vehicle systems, manufacturers and upfitters can enhance the performance, safety, and longevity of various components, leading to more reliable and efficient vehicle operation.

Learn More about the wide selection of time delay relays available at Waytek. For more information, call 800.328.2724.