SPST vs SPDT Relays. Decoding Symbols & Understanding Wiring Diagrams

Decode the symbols and understand wiring diagrams for SPST and SPDT relays.

A Quick Overview

Key Points About SPST and SPDT Relays Description
Relay Function A relay is an electrical component that uses an electromagnet to control the flow of electricity between circuits. It allows a low-voltage signal to control a high-voltage circuit, turning it on or off as needed.
SPST Relay Definition An SPST (single-pole, single-throw) relay has a single input and a single output connection. It can switch one circuit on or off at a time.
SPDT Relay Definition An SPDT (single-pole, double-throw) relay has a single input and two output connections. It can switch between two circuits, connecting one and disconnecting the other.
Symbolic Representation On wiring diagrams, SPST relays are represented by a single line and a switch symbol, while SPDT relays are represented by a line and a switch symbol with a diagonal line crossing through it.
Wiring Diagram Usage Wiring diagrams are used by mechanics and electricians to identify and repair electrical problems in a vehicle or other electrical system. They provide a visual representation of the system’s wiring and components.
Wiring Colors Conventionally, electrical wiring is color-coded to indicate its function. For example, black or red wires usually indicate a positive (+) connection, while white or yellow wires typically indicate a ground (–) connection.
Proper Handling and Installation When working with relays and wiring systems, it is crucial to follow proper handling and installation procedures to prevent damage or electrical shocks. Always wear protective equipment and follow local electrical codes and regulations.

Relays are essential components in electrical circuits, acting as switches that control the flow of current. Two common types of relays are Single-Pole Single-Throw (SPST) and Single-Pole Double-Throw (SPDT) relays. Understanding the differences between these relay types and decoding the symbols used in wiring diagrams is crucial for electrical circuit design and troubleshooting. In this blog post, we’ll explore SPST and SPDT relays, decipher their symbols, and help you understand wiring diagrams involving these relays. Let’s get started!

Introduction to Relays

Relays are electromechanical devices that control electrical circuits. They consist of a coil, an armature, and contacts. When the coil is energized, it creates a magnetic field that moves the armature, thereby changing the state of the contacts and allowing or interrupting the flow of current.

Single-Pole Single-Throw (SPST) Relays

An SPST relay is the simplest type of relay. It consists of a single set of contacts that are either open or closed. Here are the key characteristics of an SPST relay:

  • Single Pole: It has one set of contacts.
  • Single Throw: The contacts are either open or closed, depending on the state of the relay.

Single-Pole Double-Throw (SPDT) Relays

An SPDT relay, also known as a changeover relay, has one common terminal and two sets of contacts. It allows the switching of a single input to one of two outputs. Here are the key characteristics of an SPDT relay:

  • Single Pole: It has one common terminal.
  • Double Throw: It has two sets of contacts, which can be in one of two positions: normally open (NO) or normally closed (NC).

Decoding Relay Symbols in Wiring Diagrams

Relay symbols are used in wiring diagrams to represent the different types of relays. Understanding these symbols is essential for accurately interpreting the circuit design. Here are the symbols for SPST and SPDT relays:

SPST Relay Symbol

The symbol for an SPST relay consists of a rectangle representing the relay with a single line extending from one side, indicating the single set of contacts:

SPDT Relay Symbol

The symbol for an SPDT relay consists of a rectangle representing the relay with a line extending from one side and branching into two, indicating the two sets of contacts:

Understanding Wiring Diagrams with SPST and SPDT Relays

Wiring diagrams depict the electrical connections between components in a circuit. Here’s how SPST and SPDT relays are represented in wiring diagrams:

SPST Relay Wiring

In an SPST relay wiring diagram, a single line represents the coil, while the single set of contacts is represented by a separate line connected to the coil:

`Coil          Contacts
     ┌─────┐         ┌─────┐
  ──>|     |────────>|     |───
     └─────┘         └─────┘`

SPDT Relay Wiring

In an SPDT relay wiring diagram, the coil is represented by a single line, while the two sets of contacts are shown as separate lines branching from the coil:

`Coil         Contacts
     ┌─────┐       ┌──────┐
  ──>|     |──────>|      |───
     └─────┘       └──────┘
 Coil         Contacts
     ┌─────┐       ┌──────┐
  ──>|     |──────>|      |───
     └─────┘       └──────┘`

Applications and Usage

The choice between SPST and SPDT relays depends on the specific circuit requirements and desired functionality. Here are some typical applications for each relay type:

SPST Relays

  • Controlling lights or devices that need to be turned on or off.
  • Activating small motors or solenoids.
  • Switching low-power circuits.

SPDT Relays

  • Reversing the direction of a motor.
  • Switching between two power sources or circuits.
  • Controlling devices that need to be turned on or off using different triggers.

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What is a relay?

A relay is an electromagnetic switch that is used to control a circuit by a low-power signal. Relays are commonly used in applications where a higher voltage or current needs to be controlled by a lower voltage or current circuit.

What is the difference between SPST and SPDT relays?

SPST (single-pole, single-throw) and SPDT (single-pole, double-throw) relays have different switch configurations. SPST relays have one contact point, while SPDT relays have two contact points. SPDT relays are also known as changeover relays since they can switch between two circuits.

What is a NO contact on a relay?

NO (normally open) contact on a relay refers to a state where the contact point is open when the relay coil is not energized. When the coil is energized, the NO contact will close and complete the circuit.

What is a NC contact on a relay?

NC (normally closed) contact on a relay refers to a state where the contact point is closed when the relay coil is not energized. When the coil is energized, the NC contact will open and break the circuit.

What do the symbols on a relay wiring diagram mean?

The symbols on a relay wiring diagram represent the different components of the relay circuit. Some common symbols include the relay coil (circle), NO contact (arrow pointing away from circle), NC contact (arrow pointing towards the circle), and diode (triangle).

What is the purpose of a diode in a relay circuit?

A diode is often used in a relay circuit to protect the circuit from voltage spikes that can occur when the relay coil is de-energized. The diode serves to dissipate this voltage by providing a low-resistance path for the current.

How do you identify the pins on a relay?

Relay pins are typically identified by numbers or letters on the bottom of the relay. The position of the pins can also help identify them, as some relays have a keying feature that keeps the pins in a specific orientation. It is important to consult the manufacturer’s datasheet for accurate pin identification.