What is a Double Throw Relay?

A double throw (DT) relay, also known as a two-way relay or changeover relay, is an electromechanical switch that can route an input signal to one of two output terminals. Unlike a single throw (ST) relay which has only one output terminal, a DT relay has two output terminals, allowing it to switch between two different circuits or devices.

DT relays are widely used in various applications such as:

• Switching between primary and backup power sources
• Controlling reversible motors
• Switching between heating and cooling modes in HVAC systems
• Switching audio signals between speakers
• Automobile headlight and wiper controls

Types of Double Throw Relays

There are two main types of double throw relays:

1. Double Pole Double Throw (DPDT) Relay
2. Single Pole Double Throw (SPDT) Relay

Double Pole Double Throw (DPDT) Relay

A DPDT relay has two sets of contacts, each with a common terminal (COM), a normally closed terminal (NC), and a normally open terminal (NO). When the relay coil is energized, both sets of contacts switch simultaneously, connecting the COM terminals to their respective NO terminals and disconnecting them from the NC terminals.

DPDT relays are commonly used in applications that require switching two independent circuits simultaneously, such as:

• Switching between two power sources for redundancy
• Reversing the polarity of a DC motor for bidirectional control
• Switching between two sets of speakers in an audio system

Single Pole Double Throw (SPDT) Relay

An SPDT Relay has a single set of contacts with a common terminal (COM), a normally closed terminal (NC), and a normally open terminal (NO). When the relay coil is energized, the COM terminal switches from the NC terminal to the NO terminal.

SPDT relays are used in applications that require switching a single circuit between two states, such as:

• Switching between heating and cooling modes in an HVAC system
• Switching between two antennas in a radio system
• Switching between two sensors in a monitoring system

How Does a Double Throw Relay Work?

A double throw relay consists of the following main components:

1. Coil: An electromagnet that generates a magnetic field when energized by an input signal (usually a DC voltage).
2. Armature: A movable metal lever that is attracted to the coil when it is energized.
3. Contacts: Stationary metal terminals that are connected or disconnected by the movement of the armature.
4. Spring: A mechanical spring that returns the armature to its original position when the coil is de-energized.

When an input signal is applied to the coil, it generates a magnetic field that attracts the armature. The armature, which is normally held in place by the spring, moves towards the coil, causing the contacts to switch states. In the case of an SPDT relay, the COM terminal disconnects from the NC terminal and connects to the NO terminal. In a DPDT relay, both sets of contacts switch simultaneously.

When the input signal is removed from the coil, the magnetic field collapses, and the spring pulls the armature back to its original position. This causes the contacts to return to their normal state, with the COM terminal connected to the NC terminal (in an SPDT relay) or both COM terminals connected to their respective NC terminals (in a DPDT relay).

Double Throw Relay Circuit Diagram

SPDT Relay Circuit Diagram

Here’s a basic circuit diagram for an SPDT relay:

           +-------------------+
           |                   |
  Input ---+                   |
           |                   |
           |                 NC|------- Output 1
           |                   |
           |                COM|
           |                   |
           |                 NO|------- Output 2
           |                   |
           |                   |
           +-------------------+
                   Relay

In this diagram:
– The input signal is connected to the relay coil.
– Output 1 is connected to the NC terminal.
– Output 2 is connected to the NO terminal.
– The COM terminal switches between Output 1 and Output 2 based on the state of the relay coil.

DPDT Relay Circuit Diagram

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

           +-------------------+
           |                   |
  Input ---+                   |
           |                   |
           |               NC1|------- Output 1A
           |                   |
           |              COM1|
           |                   |
           |               NO1|------- Output 1B
           |                   |
           |               NC2|------- Output 2A
           |                   |
           |              COM2|
           |                   |
           |               NO2|------- Output 2B
           |                   |
           +-------------------+
                   Relay

In this diagram:
– The input signal is connected to the relay coil.
– Output 1A is connected to the NC1 terminal.
– Output 1B is connected to the NO1 terminal.
– Output 2A is connected to the NC2 terminal.
– Output 2B is connected to the NO2 terminal.
– The COM1 terminal switches between Output 1A and Output 1B, while the COM2 terminal switches between Output 2A and Output 2B based on the state of the relay coil.

Double Throw Relay Usage and Applications

Double throw relays are used in a wide range of applications across various industries, including automotive, industrial automation, HVAC, and telecommunications. Some common applications include:

Power Switching and Redundancy

DT relays can be used to switch between primary and backup power sources in critical systems that require high availability. For example, in a data center, a DPDT relay can be used to switch between the main power grid and a backup generator in case of a power outage.

Motor Control

DPDT relays are commonly used for controlling reversible motors, such as those found in conveyor systems, garage door openers, and window blinds. By switching the polarity of the motor’s input voltage, the relay can change the direction of the motor’s rotation.

HVAC Systems

SPDT relays are often used in heating, ventilation, and air conditioning (HVAC) systems to switch between heating and cooling modes. The relay can switch the system’s compressor and reversing valve to change the direction of the refrigerant flow, effectively switching between heating and cooling.

Audio Systems

DT relays can be used in audio systems to switch between different sets of speakers or to mute the audio output when needed. For example, an SPDT relay can be used to switch between main and surround speakers in a home theater system.

Automotive Applications

Double throw relays are widely used in automotive applications for controlling various functions, such as headlights, wipers, and horn circuits. For instance, an SPDT relay can be used to switch between high and low beam headlights based on the driver’s input.

Advantages of Double Throw Relays

Double throw relays offer several advantages over other switching methods:

1. Isolation: DT relays provide electrical isolation between the input and output circuits, which can help protect sensitive components from voltage spikes and noise.

2. High current handling: Relays can switch higher currents than most solid-state devices, making them suitable for applications that require controlling high-power loads.

3. Low power consumption: The input signal required to energize a relay coil is generally low, which means that relays can be controlled by low-power devices such as microcontrollers.

4. Flexibility: DT relays can be used to switch a wide range of loads, including AC and DC circuits, making them versatile components in many applications.

5. Reliability: When properly selected and installed, relays are reliable switching devices that can operate for millions of cycles without failing.

Disadvantages of Double Throw Relays

Despite their many advantages, double throw relays also have some limitations:

1. Switching speed: Relays are generally slower than solid-state switches due to the mechanical movement of the armature and contacts. This can limit their use in applications that require high-speed switching.

2. Contact wear: The mechanical contacts in a relay are subject to wear and tear over time, which can lead to increased contact resistance and eventual failure.

3. Noise: The switching action of a relay can generate electromagnetic noise, which may interfere with nearby electronic devices if not properly suppressed.

4. Size: Relays are generally larger than solid-state switches, which can be a limitation in space-constrained applications.

5. Cost: While relays are relatively inexpensive, they may be more costly than solid-state switches in some applications, particularly when a large number of relays is required.

Frequently Asked Questions (FAQ)

1. What is the difference between a single throw and a double throw relay?

A single throw (ST) relay has one output terminal, while a double throw (DT) relay has two output terminals. An ST relay can switch a single circuit on or off, while a DT relay can switch between two circuits or switch the polarity of a single circuit.

2. Can a double throw relay be used to control an AC load?

Yes, double throw relays can be used to switch both AC and DC loads. However, it is essential to ensure that the relay is rated for the voltage and current of the load being switched.

3. How do I choose the right double throw relay for my application?

When selecting a double throw relay, consider the following factors:
– Coil voltage and current: Ensure that the relay coil is compatible with the input signal voltage and current.
– Contact rating: Choose a relay with contacts rated for the voltage and current of the load being switched.
– Switching speed: If the application requires high-speed switching, consider using a solid-state relay instead.
– Environment: Consider the operating temperature, humidity, and vibration levels in the application environment.

4. Can I use a DPDT relay in place of an SPDT relay?

Yes, a DPDT relay can be used in place of an SPDT relay by simply using only one set of contacts. However, using a DPDT relay in this manner may be more expensive and take up more space than using an SPDT relay.

5. How can I prolong the life of a double throw relay?

To extend the life of a double throw relay:
– Ensure that the relay is properly rated for the load being switched.
– Use a relay with a higher contact rating than required to provide a safety margin.
– Implement relay contact protection, such as RC snubbers or varistors, to suppress voltage spikes and arcing.
– Ensure proper cooling and ventilation to prevent overheating of the relay.
– Perform regular maintenance, such as inspecting and cleaning the contacts, to ensure optimal performance.

Conclusion

Double throw relays are versatile and reliable switching devices used in a wide range of applications, from power switching and motor control to audio systems and automotive applications. By understanding the working principle, circuit diagrams, and applications of DT relays, engineers and technicians can effectively incorporate these components into their designs to create robust and efficient systems.

When selecting a double throw relay for a specific application, it is essential to consider factors such as the coil voltage and current, contact rating, switching speed, and operating environment. By choosing the right relay and implementing proper protection and maintenance measures, designers can ensure optimal performance and longevity of their systems.

As technology continues to evolve, double throw relays remain essential components in many applications, offering a reliable and cost-effective solution for switching between circuits or controlling the polarity of a single circuit. By mastering the use of these relays, engineers and technicians can create innovative and efficient systems across various industries.