Introduction to Double Throw Relay

A double throw relay, also known as a changeover relay or a SPDT (Single Pole Double Throw) relay, is an electrically operated switch that allows switching between two circuits. It consists of a coil, an armature, and a set of contacts. When the coil is energized by applying a voltage, the armature is attracted to the coil, causing the contacts to switch from one position to another. Double throw relays are widely used in various applications, such as automotive systems, industrial control, and home automation, where switching between two circuits is required.

Types of Double Throw Relays

There are two main types of double throw relays:

1. SPDT (Single Pole Double Throw) Relay: This type of relay has a single pole (common terminal) and two throw contacts (normally open and normally closed). When the coil is energized, the common terminal switches from the normally closed contact to the normally open contact.

2. DPDT (Double Pole Double Throw) Relay: This type of relay has two poles (common terminals) and four throw contacts (two normally open and two normally closed). It essentially consists of two SPDT Relays that are actuated simultaneously.

How a Double Throw Relay Works

A double throw relay operates on the principle of electromagnetism. It consists of the following components:

• Coil: A coil of wire that generates a magnetic field when an electric current flows through it.
• Armature: A movable metal lever that is attracted to the coil when it is energized.
• Contacts: A set of stationary contacts that are connected to the circuits being switched.

When a voltage is applied to the coil, it creates a magnetic field that attracts the armature. The armature, which is mechanically linked to the contacts, moves and causes the contacts to switch from one position to another. This allows the current to flow through the normally open contact while breaking the connection with the normally closed contact.

When the voltage is removed from the coil, the magnetic field collapses, and the armature returns to its original position due to the force of a spring. This causes the contacts to switch back to their initial state, with the current flowing through the normally closed contact.

Normally Open and Normally Closed Contacts

Double throw relays have two types of contacts:

1. Normally Open (NO) Contact: This contact is open (not connected) when the relay is in its de-energized state. When the relay is energized, the NO contact closes, allowing current to flow through it.

2. Normally Closed (NC) Contact: This contact is closed (connected) when the relay is in its de-energized state. When the relay is energized, the NC contact opens, breaking the connection.

The common terminal (pole) is connected to either the NO or NC contact, depending on the state of the relay.

Double Throw Relay Circuit Diagram

A typical SPDT relay circuit diagram consists of the following components:

• Power Supply: A source of voltage to energize the relay coil.
• Relay Coil: The coil of the relay that generates the magnetic field when energized.
• Switch: A switch or a control device to activate the relay.
• NO Contact: The normally open contact of the relay.
• NC Contact: The normally closed contact of the relay.
• Common Terminal: The pole of the relay that switches between the NO and NC contacts.

Here’s a simple SPDT relay circuit diagram:

            +------+
            |      |
            |  NC  |
            |      |
            +------+
            |      |
            |  COM |
+-------+   |      |
|       |   +------+
| Power |   |      |
| Supply|   |  NO  |
|       |   |      |
+-------+   +------+
            |      |
            |  Coil|
            |      |
            +------+
            |      |
            |Switch|
            |      |
            +------+

In this diagram, when the switch is closed, the relay coil is energized, and the common terminal switches from the NC contact to the NO contact. When the switch is opened, the coil is de-energized, and the common terminal returns to the NC contact.

Applications of Double Throw Relays

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

1. Automotive Systems: Double throw relays are used in automotive systems for functions such as headlight control, horn control, and fan control. They allow switching between different circuits based on the vehicle’s requirements.

2. Industrial Control: In industrial control systems, double throw relays are used for switching between different power sources, controlling motors, and managing safety circuits. They provide reliable switching in harsh industrial environments.

3. Home Automation: Double throw relays are used in home automation systems to control lighting, heating, ventilation, and air conditioning (HVAC) systems. They allow switching between different modes of operation based on user preferences or sensor inputs.

4. Battery Charging Systems: Double throw relays are used in battery charging systems to switch between the charging and discharging modes. They ensure proper charging and prevent overcharging of the batteries.

5. Backup Power Systems: In backup power systems, double throw relays are used to switch between the main power source and the backup power source (e.g., generator or UPS) in case of a power failure.

Advantages of Double Throw Relays

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

1. Isolation: Double throw relays provide electrical isolation between the control circuit and the switched circuits. This isolation helps protect sensitive electronic components from high voltages or currents.

2. High Current Handling: Double throw relays can handle high currents, making them suitable for switching high-power loads such as motors, heaters, and lighting systems.

3. Remote Switching: Double throw relays allow remote switching of circuits. The relay coil can be energized from a remote location, enabling control of the switched circuits from a distance.

4. Flexibility: Double throw relays offer flexibility in terms of contact configuration. They can be used as SPDT or DPDT relays, depending on the requirements of the application.

5. Reliability: Double throw relays are known for their reliability and long operational life. They can withstand numerous switching cycles and operate in harsh environments.

Selecting a 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 power supply. Common coil voltages include 5V, 12V, 24V, and 48V.

2. Contact Rating: Ensure that the relay contacts are rated for the required current and voltage of the switched circuits. The contact rating should exceed the maximum expected load current.

3. Switching Capacity: Consider the switching capacity of the relay, which is the maximum power that the contacts can handle. It is determined by multiplying the maximum current and voltage ratings of the contacts.

4. Mounting Type: Select a relay with a suitable mounting type (e.g., PCB mount, panel mount, or socket mount) based on the physical requirements of the application.

5. Environmental Factors: Consider the environmental factors such as temperature, humidity, and vibration that the relay will be exposed to. Choose a relay with appropriate ratings and protection features.

Troubleshooting Double Throw Relays

If a double throw relay is not functioning properly, consider the following troubleshooting steps:

1. Check the Power Supply: Ensure that the relay coil is receiving the correct voltage from the power supply. Use a multimeter to measure the voltage across the coil terminals.

2. Inspect the Contacts: Visually inspect the relay contacts for any signs of damage, corrosion, or debris. Clean the contacts if necessary using a contact cleaner spray.

3. Test the Continuity: Use a multimeter to test the continuity between the common terminal and the NO/NC contacts in both the energized and de-energized states of the relay. Ensure that the continuity matches the expected switching behavior.

4. Check the Control Circuit: Verify that the control circuit (e.g., switch or sensor) is functioning correctly and providing the necessary signal to energize the relay coil.

5. Replace the Relay: If the relay is found to be faulty after performing the above steps, replace it with a new one of the same specifications.

Safety Considerations

When working with double throw relays, follow these safety guidelines:

1. Disconnect Power: Always disconnect the power supply before working on the relay or the associated circuits to avoid electrical shock hazards.

2. Use Appropriate Ratings: Ensure that the relay and the associated components are rated for the expected voltage and current levels to prevent overheating, fire, or damage to the equipment.

3. Provide Adequate Insulation: Use properly insulated wires and terminals to prevent short circuits and electrical shocks.

4. Follow Manufacturer’s Guidelines: Refer to the manufacturer’s datasheet and application notes for specific installation, operation, and maintenance guidelines for the relay.

5. Use Protective Equipment: When working with high voltages or currents, use appropriate personal protective equipment (PPE) such as insulated gloves and safety glasses.

Frequently Asked Questions (FAQ)

1. What is the difference between a double throw relay and a single throw relay?
   A double throw relay has two sets of contacts (normally open and normally closed) that switch between two circuits, while a single throw relay has only one set of contacts (either normally open or normally closed) that switches a single circuit.

2. Can a double throw relay be used as a single throw relay?
   Yes, a double throw relay can be used as a single throw relay by using only the normally open or normally closed contact and leaving the other contact unused.

3. How do I determine the coil voltage of a double throw relay?
   The coil voltage of a double throw relay is typically specified in the manufacturer’s datasheet. It is the voltage required to energize the relay coil and cause the contacts to switch.

4. What happens if the relay contacts are overloaded?
   If the relay contacts are subjected to a current or voltage higher than their rated capacity, it can lead to overheating, welding of the contacts, or permanent damage to the relay. Always ensure that the relay contacts are rated for the expected load current and voltage.

5. Can a double throw relay be controlled by a microcontroller?
   Yes, a double throw relay can be controlled by a microcontroller. The microcontroller can provide the necessary voltage to energize the relay coil through a transistor or a relay driver circuit.

Conclusion

Double throw relays are versatile switching devices that allow switching between two circuits. They find applications in various fields, including automotive systems, industrial control, home automation, and power management. By understanding the working principle, circuit diagram, and selection criteria of double throw relays, engineers and technicians can effectively utilize them in their projects.

When using double throw relays, it is crucial to consider factors such as coil voltage, contact rating, switching capacity, and environmental conditions. Following proper safety guidelines and troubleshooting techniques ensures the reliable and safe operation of the relays.

As technology advances, double throw relays continue to play a significant role in switching and control applications. With their robustness, reliability, and flexibility, they remain an essential component in many electrical and electronic systems.