Double Throw Relay – Working, Circuit Diagram, and Usage

What is a Double Throw Relay?

A double throw relay, also known as a changeover relay or a DPDT (Double Pole Double Throw) relay, is an electromechanical device that allows switching between two circuits. It consists of a coil, which when energized, causes the switch contacts to change their position, redirecting the current flow from one circuit to another.

Double throw relays are widely used in various electrical and electronic applications where switching between two circuits is required. They offer a reliable and efficient way to control and automate the switching process.

Types of Double Throw Relays

There are two main types of double throw relays:

1. DPDT Relay (Double Pole Double Throw): This type of relay has two sets of contacts, each with a common terminal and two switching terminals. When the coil is energized, both sets of contacts change their position simultaneously, allowing the switching of two independent circuits.

2. SPDT Relay (Single Pole Double Throw): An SPDT relay has a single set of contacts with a common terminal and two switching terminals. It can switch a single circuit between two different paths.

How Does a Double Throw Relay Work?

The working principle of a double throw relay is based on the interaction between an electromagnet and a set of switch contacts. Here’s a step-by-step explanation of how it operates:

1. Coil Energization: When a voltage is applied to the coil terminals of the relay, it creates a magnetic field around the coil. This magnetic field generates a force that attracts the armature, which is a movable metal piece.

2. Armature Movement: As the armature is attracted towards the coil, it causes the switch contacts to change their position. The armature is mechanically linked to the switch contacts, so its movement directly affects the state of the contacts.

3. Contact Switching: In a DPDT relay, there are two sets of contacts, each with a common terminal (COM) and two switching terminals (NO – Normally Open, NC – Normally Closed). When the relay is not energized, the COM terminal is connected to the NC terminal. When the coil is energized, the armature moves, and the COM terminal switches its connection to the NO terminal.

4. Circuit Redirection: The switching of the contacts allows the redirection of current flow from one circuit to another. For example, in a DPDT relay, when the coil is energized, the current flow in both circuits is redirected from the NC terminals to the NO terminals.

5. De-energization: When the voltage is removed from the coil terminals, the magnetic field collapses, and the armature returns to its original position due to the force of a spring. This causes the switch contacts to revert to their initial state, with the COM terminal connected to the NC terminal.

Double Throw Relay Circuit Diagram

Here’s a typical circuit diagram for a DPDT relay:

       COM1   NC1   NO1
        |      |     |
        |      |     |
       +-+----+-+---+-+
       |           |
+------+  Coil     +------+
|      |           |      |
+------+           +------+
       |           |
       +-+----+-+---+-+
        |      |     |
        |      |     |
       COM2   NC2   NO2

In this diagram:
– COM1 and COM2 represent the common terminals of the two sets of contacts.
– NC1 and NC2 are the normally closed terminals, connected to the common terminals when the relay is not energized.
– NO1 and NO2 are the normally open terminals, connected to the common terminals when the relay is energized.
– The coil is represented by the rectangular symbol in the center, which is energized by applying a voltage to its terminals.

Applications and Usage of Double Throw Relays

Double throw relays find applications in various fields where switching between two circuits is required. Some common use cases include:

1. Automotive Systems: Double throw relays are used in automotive applications for switching between different circuits, such as headlights, fog lights, and horn circuits. They allow the control of multiple electrical components using a single switch.

2. Industrial Control: In industrial settings, double throw relays are employed for controlling motors, pumps, valves, and other machinery. They enable the switching between different operating modes or the reversal of motor direction.

3. HVAC Systems: Double throw relays are used in heating, ventilation, and air conditioning (HVAC) systems to control the switching of heating and cooling elements. They allow the system to switch between different temperature settings or modes of operation.

4. Automation and Robotics: Double throw relays are utilized in automation and robotics applications to control the switching of actuators, sensors, and other components. They enable the precise control and coordination of multiple devices in an automated system.

5. Telecommunications: In telecommunications systems, double throw relays are used for switching between different communication channels or for routing signals between different devices. They ensure reliable and efficient signal transmission.

Advantages of Double Throw Relays

Double throw relays offer several advantages over other switching methods:

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

2. High Current Capacity: Double throw relays can handle higher currents compared to solid-state switches like transistors. They are suitable for switching high-power loads or inductive loads such as motors and solenoids.

3. Reliability: Double throw relays are known for their reliability and long operational life. They can withstand multiple switching cycles and operate in harsh environments, making them suitable for industrial and automotive applications.

4. Flexibility: Double throw relays offer flexibility in terms of contact configuration. They can be used to switch between two different circuits or to control the direction of current flow in a single circuit.

5. Cost-effectiveness: Double throw relays are relatively inexpensive compared to other switching solutions, especially for high-current applications. They provide a cost-effective way to implement switching functionality in various systems.

Considerations When Using Double Throw Relays

When using double throw relays in your circuits, there are a few important considerations to keep in mind:

1. Coil Voltage and Current: Ensure that the coil voltage and current ratings of the relay match the specifications of your control circuit. Applying the correct voltage and current to the coil is essential for proper operation and to avoid damage to the relay.

2. Contact Ratings: Pay attention to the contact ratings of the relay, including the maximum switching voltage, current, and power. Make sure that the switched circuits do not exceed these ratings to prevent contact damage or failure.

3. Switching Speed: Double throw relays have a certain switching speed, which is the time taken for the contacts to change their state after the coil is energized or de-energized. Consider the switching speed requirements of your application and choose a relay with an appropriate response time.

4. Contact Bounce: When the relay contacts switch, there may be a brief period of contact bounce, where the contacts rapidly open and close before settling into their final state. This bounce can cause unwanted transient signals in the switched circuits. If necessary, use debounce circuits or software debounce techniques to mitigate this issue.

5. Interference and Noise: The switching action of relays can generate electromagnetic interference (EMI) and noise. Take appropriate measures to suppress these effects, such as using snubber circuits, shielding, or proper grounding techniques.

6. Mounting and Wiring: Ensure proper mounting and wiring of the relay in your circuit. Use appropriate wire gauges and connectors to handle the required current levels. Securely fasten the relay to avoid vibrations or mechanical stress on the connections.

Frequently Asked Questions (FAQ)

1. What is the difference between a DPDT and an SPDT relay?
   A DPDT (Double Pole Double Throw) relay has two sets of contacts, each with a common terminal and two switching terminals, allowing the switching of two independent circuits. An SPDT (Single Pole Double Throw) relay has a single set of contacts with a common terminal and two switching terminals, capable of switching a single circuit between two different paths.

2. Can a double throw relay be used for AC and DC circuits?
   Yes, double throw relays can be used for both AC and DC circuits. However, it’s important to choose a relay with the appropriate voltage and current ratings for the specific application. AC relays may have different contact ratings compared to DC relays due to the presence of inductive loads and the potential for arcing.

3. How do I select the appropriate double throw relay for my application?
   When selecting a double throw relay, consider the following factors:

4. Coil voltage and current ratings: Choose a relay with a coil voltage and current that match your control circuit.
5. Contact ratings: Ensure that the relay’s contact ratings (voltage, current, and power) are suitable for the switched circuits.
6. Switching speed: Consider the required switching speed and response time for your application.

7. Environment: Take into account the operating environment, such as temperature, humidity, and vibration, and choose a relay that can withstand those conditions.

8. What is the purpose of the common (COM) terminal in a double throw relay?
   The common (COM) terminal in a double throw relay serves as the pivot point for switching between the normally closed (NC) and normally open (NO) terminals. When the relay is not energized, the COM terminal is connected to the NC terminal. When the relay is energized, the COM terminal switches its connection to the NO terminal, allowing the redirection of current flow.

9. Can I control a double throw relay using a microcontroller or a PLC?
   Yes, double throw relays can be controlled using microcontrollers or programmable logic controllers (PLCs). The microcontroller or PLC can provide the necessary control signals to energize or de-energize the relay coil based on the desired switching logic. However, ensure that the microcontroller or PLC’s output can provide the required voltage and current to drive the relay coil effectively.

Conclusion

Double throw relays are versatile and reliable electromechanical devices used for switching between two circuits. They offer advantages such as electrical isolation, high current capacity, reliability, flexibility, and cost-effectiveness. Understanding the working principle, circuit diagram, and usage considerations of double throw relays is essential for effectively incorporating them into various electrical and electronic applications.

When selecting and using double throw relays, pay attention to the coil voltage and current ratings, contact ratings, switching speed, and environmental factors. Proper mounting, wiring, and interference suppression techniques should also be employed to ensure optimal performance and reliability.

Double throw relays find extensive use in automotive systems, industrial control, HVAC systems, automation and robotics, telecommunications, and many other fields where switching between circuits is required. By leveraging the capabilities of double throw relays, designers and engineers can create efficient and robust switching solutions for a wide range of applications.