Double Throw Relay – Working, Circuit Diagram, and Usage

What is a Double Throw Relay?

A double throw relay, also known as a changeover relay, is an electromechanical switch that allows the switching between two different circuits. It consists of a coil, a common terminal (COM), a normally closed (NC) terminal, and a normally open (NO) terminal. When the coil is energized, the relay switches from the NC terminal to the NO terminal, effectively changing the circuit path.

Types of Double Throw Relays

There are two main types of double throw relays:

1. Single Pole Double Throw (SPDT) Relay: This type of relay has one common terminal, one NC terminal, and one NO terminal. It allows the switching between two circuits using a single relay.

2. Double Pole Double Throw (DPDT) Relay: This type of relay has two sets of common, NC, and NO terminals. It allows the switching between two circuits simultaneously, effectively acting as two SPDT Relays in one package.

How Does a Double Throw Relay Work?

A double throw relay operates based on the principle of electromagnetism. When the coil is energized by applying a voltage across its terminals, it generates a magnetic field that attracts the armature. The armature, which is a movable metal contact, is pulled towards the coil, causing it to switch from the NC terminal to the NO terminal.

Energized State

In the energized state, the following events occur:

1. The coil is supplied with the rated voltage, creating a magnetic field.
2. The armature is attracted to the coil, moving towards it.
3. The armature breaks the connection with the NC terminal and establishes a connection with the NO terminal.
4. The circuit connected to the NO terminal is now active, while the circuit connected to the NC terminal is open.

De-energized State

When the voltage is removed from the coil, the relay returns to its de-energized state:

1. The magnetic field generated by the coil collapses.
2. The armature, no longer attracted by the magnetic field, returns to its original position due to the force of the spring.
3. The armature breaks the connection with the NO terminal and re-establishes the connection with the NC terminal.
4. The circuit connected to the NC terminal is now active, while the circuit connected to the NO terminal is open.

Double Throw Relay Circuit Diagram

A typical circuit diagram for a double throw relay is shown below:

         +------+
         |      |
         |  NC  |
         |      |
         +------+
            |
            |
         +------+
     +---+  COM +----+
     |   |      |    |
     |   +------+    |
     |               |
+----+----+     +----+----+
|         |     |         |
|  Coil   |     |   NO    |
|         |     |         |
+---------+     +---------+

In this diagram:
– The coil is represented by the two terminals on the left side.
– The common terminal (COM) is the middle terminal.
– The normally closed (NC) terminal is the top terminal.
– The normally open (NO) terminal is the bottom terminal.

When the coil is energized, the armature switches from the NC terminal to the NO terminal, changing the circuit path.

Applications and Usage of Double Throw Relays

Double throw relays find numerous applications in various fields, including:

1. Industrial Control Systems: Double throw relays are used in industrial control panels to switch between different circuits based on process requirements. They can be used for switching power supplies, motors, valves, and other equipment.

2. Automotive Electronics: In automotive systems, double throw relays are used for switching between different circuits, such as headlights, fog lights, and other electrical loads.

3. Home Automation: Double throw relays are used in home automation systems to control lighting, appliances, and other electrical devices. They can be controlled remotely using microcontrollers or smart home hubs.

4. Safety Systems: Double throw relays are used in safety-critical systems, such as emergency stop circuits and fire alarm systems. They provide a reliable means of switching between normal operation and safety shutdown modes.

5. Telecommunications: In telecommunications systems, double throw relays are used for switching between primary and backup power supplies, as well as for signal routing and switching.

Choosing the Right Double Throw Relay

When selecting a double throw relay for a specific application, consider the following factors:

1. Coil Voltage: Choose a relay with a coil voltage that matches the available control voltage in your system.

2. Contact Rating: Ensure that the relay’s contact rating (current and voltage) is suitable for the load you intend to switch. The contact rating should exceed the maximum expected load current and voltage.

3. Switching Speed: Consider the required switching speed for your application. Some relays offer faster switching times than others.

4. Mounting Type: Double throw relays come in various mounting options, such as PCB-mounted, panel-mounted, or DIN rail-mounted. Choose the appropriate mounting type based on your system’s physical layout.

5. Environmental Factors: Consider the operating environment of the relay, including temperature, humidity, and vibration. Select a relay that is rated for the expected environmental conditions.

Advantages of Double Throw Relays

Double throw relays offer several advantages, making them a popular choice for many applications:

1. Isolation: Double throw relays provide electrical isolation between the control circuit (coil) and the switched circuits. This isolation helps protect sensitive electronic components from voltage spikes and noise.

2. High Current Switching: Double throw relays can switch high currents, making them suitable for controlling power-hungry loads such as motors, heaters, and lighting systems.

3. Reliability: Double throw relays are known for their reliability and long service life. They can withstand numerous switching cycles without significant degradation in performance.

4. Flexibility: Double throw relays allow the switching between two different circuits, providing flexibility in circuit design and control.

5. Cost-effective: Double throw relays are relatively inexpensive compared to other switching solutions, making them a cost-effective choice for many applications.

Limitations of Double Throw Relays

While double throw relays offer many benefits, they also have some limitations:

1. Switching Speed: Double throw relays have a finite switching speed, typically in the range of milliseconds. This may not be suitable for applications requiring extremely fast switching, such as high-frequency power supplies.

2. Contact Bounce: When the relay switches, the contacts may bounce briefly before settling into their final state. This contact bounce can cause momentary interruptions in the switched circuit and may need to be addressed using debounce circuits or software debouncing techniques.

3. Power Consumption: The coil of a double throw relay consumes power when energized. This power consumption should be considered when designing the control circuit and selecting the appropriate power supply.

4. Mechanical Wear: Double throw relays have moving parts (armature and contacts) that are subject to mechanical wear over time. Regular maintenance and replacement may be necessary to ensure reliable operation in critical applications.

Frequently Asked Questions (FAQ)

1. What is the difference between a single pole double throw (SPDT) and a double pole double throw (DPDT) relay?

2. An SPDT relay has one common terminal, one NC terminal, and one NO terminal, allowing the switching between two circuits using a single relay. A DPDT relay has two sets of common, NC, and NO terminals, allowing the switching between two circuits simultaneously, effectively acting as two SPDT relays in one package.

3. Can a double throw relay be used for AC and DC loads?

4. Yes, double throw relays can be used for switching both AC and DC loads. However, it is important to ensure that the relay’s contact rating is suitable for the specific load type and voltage.

5. How do I choose the right coil voltage for my double throw relay?

6. The coil voltage of the relay should match the available control voltage in your system. Common coil voltages include 5V, 12V, 24V, and 48V DC, as well as 120V and 240V AC.

7. What is the purpose of the normally closed (NC) and normally open (NO) terminals in a double throw relay?

8. The NC terminal is connected to the common terminal when the relay is in the de-energized state. The NO terminal is connected to the common terminal when the relay is energized. This arrangement allows the relay to switch between two different circuits based on the state of the coil.

9. How can I protect a double throw relay from voltage spikes and noise?

10. To protect a double throw relay from voltage spikes and noise, you can use various techniques such as:
    • Adding a flyback diode across the coil to suppress voltage spikes when the coil is de-energized.
    • Using an RC snubber circuit across the relay contacts to reduce contact arcing and prolong the relay’s life.
    • Implementing proper grounding and shielding techniques to minimize noise coupling into the relay circuit.

Conclusion

Double throw relays are versatile and reliable electromechanical switches that allow the switching between two different circuits. They find extensive use in various applications, including industrial control systems, automotive electronics, home automation, safety systems, and telecommunications.

When selecting a double throw relay, it is important to consider factors such as coil voltage, contact rating, switching speed, mounting type, and environmental conditions. Double throw relays offer advantages such as electrical isolation, high current switching capability, reliability, flexibility, and cost-effectiveness. However, they also have limitations, including finite switching speed, contact bounce, power consumption, and mechanical wear.

By understanding the working principle, circuit diagram, and usage considerations of double throw relays, engineers and technicians can effectively incorporate them into their designs and applications, ensuring reliable and efficient switching between circuits.