A change over relay, often referred to as a Single-Pole Double-Throw (SPDT) relay, functions as an electrical switch designed to divert a single input connection to one of two possible output connections. This enables dynamic control over which circuit path receives power at any given time, making it a versatile component in various electrical systems.
Understanding the Core Mechanism
At its heart, a change over relay operates on the principle of electromagnetism. It contains an electromagnetic coil, a movable armature, and a set of contacts. When the coil is energized, it creates a magnetic field that pulls the armature, causing the contacts to switch positions. When the coil is de-energized, a spring returns the contacts to their default state.
The most common changeover function relays are specifically engineered to switch a single feed line (typically a positive power supply, often designated as terminal 30 in automotive applications) between two distinct active functions within a circuit.
Key Components of a Change Over Relay
Understanding the individual parts is crucial to grasping its operation:
- Electromagnetic Coil (Control Terminals 85, 86): This is the heart of the relay. When a control voltage is applied across these terminals, the coil becomes an electromagnet.
- Common Contact (COM / Terminal 30): This is the input terminal where the power to be switched (e.g., the main positive feed) is connected. This is the "single feed line" mentioned earlier.
- Normally Closed (NC) Contact (Terminal 87a): This output terminal is connected to the Common contact when the relay coil is de-energized. It represents the default path for power.
- Normally Open (NO) Contact (Terminal 87): This output terminal is disconnected from the Common contact when the relay coil is de-energized, but connects to it when the coil is energized. It represents the switched path.
- Armature: A movable metallic piece that is attracted by the electromagnetic coil, transferring its motion to the contacts.
- Spring: Returns the armature and contacts to their original (Normally Closed) position when the coil is de-energized.
How It Works: Step-by-Step Operation
The operation of a change over relay can be broken down into two primary states:
1. De-energized State (Default Position)
- Coil State: No voltage is applied to the electromagnetic coil (terminals 85 and 86).
- Contact Position: The armature is held by the spring, establishing a direct connection between the Common (30) contact and the Normally Closed (87a) contact.
- Power Flow: Any power supplied to the Common (30) terminal will flow directly to the Normally Closed (87a) terminal, powering the device or circuit connected to it.
2. Energized State (Switched Position)
- Coil State: A control voltage (e.g., from a switch, sensor, or control unit) is applied across the electromagnetic coil (terminals 85 and 86).
- Electromagnetic Action: The energized coil generates a magnetic field, attracting the armature.
- Contact Position: The armature moves, breaking the connection between Common (30) and Normally Closed (87a), and simultaneously establishing a connection between Common (30) and Normally Open (87).
- Power Flow: The power supplied to the Common (30) terminal is now redirected to the Normally Open (87) terminal, powering the device or circuit connected to it.
This switching action effectively allows the single input feed to be routed to one of two different output paths, depending on whether the relay's control coil is activated.
The relay's coil can be activated in various ways:
- Manually: Through a physical toggle switch or a push button that completes the circuit to the relay coil.
- Remotely: Via another part of the electrical circuit, such as a sensor output, a control module, or even another relay, providing automated or logic-driven control.
Operational Summary
The following table summarizes the contact states:
| Coil State | Connection Path | Function |
|---|---|---|
| De-energized | Common (30) to NC (87a) | Default circuit path active |
| Energized | Common (30) to NO (87) | Switched circuit path active; default path open |
Practical Applications and Benefits
Change over relays are indispensable in a wide range of applications due to their ability to efficiently switch power between two loads or control functions.
Common Use Cases:
- Automotive Systems:
- Headlight Control: Switching between high beam (NO) and low beam (NC) using a single power feed.
- Auxiliary Lighting: Routing power from the battery to either fog lights or driving lights, preventing both from being on simultaneously.
- Horn Circuits: Switching between different horn tones or ensuring the horn receives full power.
- Industrial Control:
- Motor Direction Control: In more complex setups, changeover relays can be used with other components to reverse motor direction.
- Alternating Loads: Switching between two pumps or fans to distribute wear or manage capacity.
- Home Automation:
- Lighting Control: Diverting power to different light fixtures based on time or sensor input.
- Security Systems: Activating an alarm (NO) or keeping a circuit armed (NC) depending on system status.
Advantages of Using Change Over Relays:
- Isolation: The control circuit (low current) is electrically isolated from the power circuit (high current), protecting sensitive control components.
- High Current Switching: They can switch much higher currents than most manual switches or control units.
- Remote Control: Allows a small signal from a distance or a logic circuit to control a high-power device.
- Versatility: Offers two distinct output paths from a single input, providing flexibility in circuit design.
- Reliability: Designed for repeated switching cycles, offering robust performance in demanding environments.
By understanding these principles, it's clear how a change over relay serves as a fundamental building block in both simple and complex electrical and electronic systems, providing a reliable method for directing power between two distinct paths.
