What types of time relays are there?

In the intricate world of automation and control, precise timing is everything. From the sequential startup of an industrial production line to the simple delay of a hallway light, controlling when an event happens is as crucial as controlling if it happens. This is where the Time Relay proves indispensable.

What is a Time Relay?

A Time Relay, also known as a Timer Relay or Time Delay Relay, is an automatic control component that, after receiving an input signal (energization or de-energization), can precisely control the output circuit to activate or reset after a preset time interval. It functions as an automated controller with a "timed switch" capability. As an indispensable fundamental component in modern industrial automation, building management, and equipment protection, its core purpose is to achieve sequential control, delayed start-up, energy-efficient operation, and safety protection for circuits or devices.

Different Types of Time Relays

Time relays come in various types, primarily classified by their timing action, operating principle, and mounting style.

By Timing Action (Most Common)

This is the most fundamental classification, determining the relay's core operating logic.

1. On-Delay Timer (Delay-on-Make)

   • Logic: Timing begins when the coil is energized. The output contacts (Normally Open close, Normally Closed open) change state only after the preset time elapses. Contacts return to their default state immediately when the coil is de-energized.

   • Symbol: Often denoted as KT or a square with "▷" inside.

   • Typical Use: Motor Star-Delta starting (star connection runs for a set time before switching to delta).

2. Off-Delay Timer (Delay-on-Break)

   • Logic: The output contacts change state immediately when the coil is energized (NO closes, NC opens). Timing begins only when the coil is de-energized. After the preset time elapses, the contacts return to their default state.

   • Symbol: Often denoted by a square with "◁" inside.

   • Typical Use: Machine tool lubrication systems (pump runs for a set time after main motor stops); safety ventilation (continues operation for a period after power loss).

3. Star-Delta Starter Timer

   • Logic: A special combination. Upon energization, one set of contacts changes state immediately and timing starts. When time elapses, that set resets and a second set of contacts activates. Specifically designed for the motor star-delta starting sequence.

   • Typical Use: Simplifies wiring in star-delta starter circuits.

4. Interval Timer & Flasher/Repeat Cycle Timer

   • Interval Timer: Upon energization or trigger, the output activates immediately, then resets automatically after the preset time, creating a single pulse.

   • Flasher/Repeat Cycle Timer: Allows separate settings for "On" time and "Off" time, cycling repeatedly to create periodic operation of a device.

By Operating Principle

This determines timing accuracy, adjustable range, and lifespan.

1. Electronic/Solid-State Timer (Mainstream)

   • Principle: Uses semiconductor integrated circuits, RC circuit charge/discharge, or quartz crystal oscillation counting to generate delay. Time is set via digital dials, potentiometers, or communication interfaces.

   • Characteristics: High precision, wide delay range (milliseconds to hours), compact size, multifunctional, no mechanical wear, long lifespan. The most common type today.

2. Motor-Driven (Analog) Timer

   • Principle: Uses a synchronous miniature motor to drive a gear train, which slows rotation to move a cam that controls contact action. A spring returns the mechanism upon de-energization.

   • Characteristics: High accuracy for long delays, very long timing ranges (up to tens of hours). However, complex construction, higher cost, and mechanical wear. Used for applications requiring very long delays.

3. Thermal (Bimetallic) Timer

   • Principle: Uses heat generated by current flowing through a heating element to cause a bimetallic strip to bend and actuate the contacts.

   • Characteristics: Simple, low-cost, but poor accuracy, susceptible to ambient temperature, and low repeatability. Now largely obsolete.

4. Pneumatic (Air-Dashpot) Timer

   • Principle: Uses air damping (air flowing through a small orifice) to create the delay. Timing is adjusted by varying the orifice size.

   • Characteristics: Simple, inexpensive, relatively large delay range (e.g., 0.4-180s), but low accuracy and affected by temperature/humidity. Common in older systems.

By Mounting & Display Style

• Panel-Mount: Features a display (LCD/LED) and buttons for easy on-site setting and monitoring. Mounts on control cabinet doors.

• DIN Rail Mount: Standard mounting on DIN rails inside enclosures, saving space. The most common form in industrial settings.

• Socket/Plug-in: Plugs into a standardized base, allowing easy maintenance and replacement.

• PCB Mount: Soldered directly onto or mounted on a printed circuit board.

How Does a Time Delay Relay Work?

Let's take the most common Electronic On-Delay Timer as an example:

1. Power & Input: The relay's operating power supply is connected. A control signal is applied to the relay's input control terminals (coil).

2. Timing Initiation: The internal power circuit supplies the timing chip and RC circuit. The set time value is loaded into the timer, which begins counting down or up.

3. Delay Period: During the countdown, the output driver circuit is locked. The output contacts remain in their default state, so the connected load is inactive.

4. Time Elapsed: When the timer reaches zero or the preset value, the timing chip sends a trigger signal.

5. Output Action: This signal drives an internal solid-state switch or miniature electromechanical relay, changing the state of the output contacts.

6. Load Control: The change in contact state connects or disconnects the main power circuit, thereby starting or stopping the load device.

7. Reset: When the input control signal is removed, the timing circuit resets instantly, and the output contacts immediately return to their default state, ready for the next cycle.

Practical Applications of Time Relays

Time relays are ubiquitous in any field requiring automatic control:

• Industrial Automation:

  • Sequential start/stop of production line equipment (e.g., start Machine B 10 seconds after Machine A).

  • Motor Star-Delta starting, soft-start control.

  • Molding cycle control in injection molding or stamping machines.

• Building Automation (HVAC/BAS):

  • Energy-saving lighting control in corridors/stairwells (delayed turn-off after motion detection).

  • Delayed operation for exhaust fans and air handling units.

  • Automatic testing cycles for fire pumps.

• Energy & Power Systems:

  • Delay logic in Automatic Transfer Switches (ATS) to avoid nuisance switching during grid fluctuations.

  • Delay for capacitor bank switching to prevent inrush current.

• Equipment Protection:

  • Mandatory cooling delay for compressors/large motors after shutdown (prevents immediate restart damage).

  • Post-lubrication delay for CNC machine tool spindles after stop.

• Home Appliances:

  • Timing for wash, rinse, and spin cycles in washing machines.

  • Cooking timers in microwave ovens and bread makers.

  • Delayed shut-off for bathroom exhaust fans.