What is Time Delay Relay?

Topics Covered in This Article

• Ⅰ. Introduction to the Time Relay

• Ⅱ. Types of Time Relays

• Ⅲ. Time Relay Wiring Diagram and Method

• Ⅳ. Time Relay Symbols

• Ⅴ. Time Relay Applications

Ⅰ. Introduction to the Time Relay

1. What is a Time Delay Relay?

A Time Relay (or Time Delay Relay) is an electrical control component used in circuits to create a deliberate time delay before switching a load on or off. It allows a lower voltage or lower current control signal to switch a higher voltage and larger current circuit, or to regulate complex automated processes. In industrial power supply circuits, the electromagnetic switching device is often referred to as a contactor, while the timing component is the time relay.

The primary function of a Time Relay is to act as an executive device in program control logic. It begins timing immediately upon receiving a start signal (energization or trigger). Once the preset timing duration is complete, the working contacts open or close to allow the circuit to continue its sequence.

In modern automation, time relays are designed so their delay performance can be adjusted easily within a specific range. While a single time relay usually handles one delay function (e.g., delayed closing), complex logic such as "delayed closing followed by delayed opening" (Cycle Timing) can be achieved by combining time relays with intermediate relays or by using modern multi-function digital timers.

2. What is the Working Principle of a Time Delay Relay?

Time relays utilize electromagnetic, mechanical, or electronic principles to achieve delay control. The working principle depends on the type of technology used:

• Pneumatic (Air Damping): This traditional method uses an electromagnetic coil to attract an armature. However, the movement is physically restricted by a piston rod attached to a rubber film in an air chamber. When the coil is energized, the armature tries to move, but the air chamber acts as a damper. The piston rod sinks slowly as air enters through a small inlet hole. The size of this hole is adjustable via a screw, which changes the speed of the piston and, consequently, the time delay. Once the piston reaches a set position, it triggers a lever to snap the contacts (Delay-ON or Delay-OFF).

• Electronic/Digital: Modern time relays are predominantly electronic. They use an oscillator and a counter (digital) or an RC charging circuit (analog) to measure time. When the input voltage is applied, the internal circuit counts pulses or charges a capacitor. Once the threshold is reached, a transistor or output relay is triggered to change the contact state. These offer superior precision compared to pneumatic types.

3. What is the Function of a Time Delay Relay?

Time Delay Relays are essential in both household appliances (like microwave ovens and air conditioners) and industrial machinery. They serve as "translators" that manage the timing logic between control circuits (low power) and main circuits (high power).

They are commonly used as auxiliary relays to increase the number of available contacts and contact capacity in protection systems and automatic control circuits. A key advantage is the ability to adjust the delay duration freely according to the process requirements, ensuring motors start in sequence, lights stay on for a set time after exit, or pumps run for a specific duration.

Ⅱ. Types of Time Relays

1. Classification by Working Principle

Time relays are categorized into several types based on their internal mechanisms. While pneumatic and motor types are legacy technologies, electronic types are the current industry standard.

(1) Air Damping Time Relay (Pneumatic)

This type utilizes the principle of air resistance (damping) through a small orifice to generate a delay. It consists of three main parts: an electromagnetic system, a pneumatic delay mechanism, and the contact system.    
Pros: Simple structure, relatively cheap, wide adjustable range, and unaffected by voltage fluctuations.    
Cons: Lower accuracy and bulky size compared to electronic versions.

(2) Electronic Time Relay

Analog (RC): Based on the principle that voltage across a capacitor in an RC circuit increases exponentially. The delay is determined by the resistance and capacitance values.    
Digital (Microcontroller): Uses a quartz crystal oscillator and a digital counter.    
Features: Electronic relays offer a wide delay range (from milliseconds to hundreds of hours), high precision (often better than 1%), compact size, and shock resistance. Modern digital versions often feature multi-mode capabilities (Signal On-Delay, Signal Off-Delay, Flasher, etc.) in a single unit.

(3) Motor-Driven Time Relay

This type uses a miniature synchronous motor to drive a reduction gear train. When the gear rotates to a certain angle, it trips a switch.    
Features: Extremely long delay ranges (up to 72 hours or more) with high linearity.    
Drawbacks: Complex mechanical structure, large volume, shorter lifespan due to moving parts, and higher cost. Accuracy is dependent on the mains power frequency.

(4) Electromagnetic Time Relay

This relay relies on the slow decay of magnetic flux. By using a copper sleeve (slug) around the iron core, the release of the armature is delayed after the coil is de-energized.    
Features: Only suitable for DC circuits. It has a large contact capacity but a very narrow and short delay range (usually milliseconds to a few seconds). It is primarily used for breaking DC contractor coils.

2. Classification by Delay Method

Regardless of the internal mechanism, the functional behavior of time relays falls into two main categories:

(1) On-Delay (Power-On Delay)

The timing cycle begins immediately when the input voltage (coil) is energized. The contacts remain in their default state during the timing period. Once the set time elapses, the contacts switch (open or close) and remain in that state until power is removed.

(2) Off-Delay (Power-Off Delay)

The contacts switch immediately when the input signal (or power) is applied. The timing cycle begins only when the input signal is removed (de-energized). The contacts return to their original state only after the delay time has passed. This is critical for applications like cooling fans running after a machine is turned off.

Ⅲ. Time Relay Wiring Diagram and Wiring Method

Figure: Standard 8-Pin Time Relay Wiring Diagram

The diagram above represents a standard 8-pin DPDT (Double Pole Double Throw) time relay. Here is the pin breakdown:

• Source (Coil): Pins 2 and 7 are the power supply terminals. Ensure the voltage matches the relay specifications (e.g., 24VDC, 110VAC, 220VAC).

• Contact Set 1:

• Pin 1: Common (COM)

• Pin 4: Normally Closed (NC) - Opens after delay.

• Pin 3: Normally Open (NO) - Closes after delay.

• Contact Set 2:

• Pin 8: Common (COM)

• Pin 5: Normally Closed (NC) - Opens after delay.

• Pin 6: Normally Open (NO) - Closes after delay.

Note on Terminology: The contacts are often called "Time Delay Contacts."    
The "Normally Open" contact (e.g., 1-3) is technically a "Normally Open, Timed Closed" (NOTC) contact.    
The "Normally Closed" contact (e.g., 1-4) is technically a "Normally Closed, Timed Open" (NCTO) contact.

Figure: Practical Wiring Illustration

Step-by-Step Wiring Method

1. Check Control Voltage: Identify the coil voltage required (printed on the relay housing). Connect the control power (L/N for AC or +/- for DC) to pins 2 and 7.

2. Understand the Contacts: Think of the relay as an automatic switch.

• Pin 8 is a "Common" moving arm.

• Pin 5 is the "Normally Closed" point (Connected to 8 when the timer is idle or counting).

• Pin 6 is the "Normally Open" point (Connected to 8 only after the time delay is complete).

3. Load Wiring:

• Connect the Neutral (or Negative) wire directly to the load (e.g., light bulb, motor starter).

• Connect the Live (or Positive) wire to the Common pin (Pin 8).

• Connect the wire from the Normally Open pin (Pin 6) to the other side of the load.

4. Operation: When power is applied to coils 2 and 7, the timer starts. Pins 8 and 6 remain disconnected. Once the time elapses, Pins 8 and 6 close, completing the circuit and turning on the load (similar to flipping a light switch).

Ⅳ. Time Relay Symbols

Figure: IEC Graphical Symbols for Electromagnetic Time Relays

1. Coil Symbols

In electrical schematics, the coil is the "brain" of the relay.    
On-Delay Coil: Depicted as a rectangle with a box marked with an "X" or a cross.    
Off-Delay Coil: Depicted as a rectangle with a solid black or filled section on one side. The standard text designation for a time relay is KT (Time Contactor/Relay).

2. Contact Symbols

The contacts are distinguished by the "parachute" or semicircle shapes attached to the switch arm. The direction of the curve indicates the type of delay:

• On-Delay Contact: The curve points away from the direction of movement (acting as a parachute resisting the closing).

• Off-Delay Contact: The curve points toward the direction of movement.

Tip to Remember: The semi-circle acts like a parachute. If you pull the switch to close it, and the parachute catches air (curve facing movement), it slows down the action (Delay).

Ⅴ. Time Relay Applications

Time Delay Relays are ubiquitous in automation. Here are some specific, high-value applications:

• Star-Delta Motor Starting: This is one of the most common industrial applications. A time relay is used to switch a large 3-phase motor from "Star" connection (low voltage/torque for starting) to "Delta" connection (full voltage/speed) after a few seconds, preventing massive current surges that could trip breakers.

• Conveyor Belt Sequencing: In a factory with multiple conveyor belts, belt #2 must start before belt #1 to prevent material pile-up. Time relays ensure the correct start-up and shut-down sequence.

• Lighting Control: Staircase timers or hallway lights that remain on for a set period after a button is pressed, then automatically turn off to save energy.

• Furnace Safety Purge: In gas furnaces, a time relay ensures the fan runs for a specific time to "purge" any combustible gases from the chamber before the igniter is allowed to fire.

• Traffic Lights: Older or simple traffic control systems use time relays to control the duration of Green, Amber, and Red lights.

• Pump Protection: Used to prevent "short cycling" (turning on and off too quickly) which damages pump motors. The relay forces a delay before the pump can restart after stopping.