SPST Relay Vs SPDT Relay, Symbol, Wiring Diagram Explained

SPST And SPDT Relay Diagram

A relay is a switch that is used to turn a on or off a circuit. It is operated by an electromagnet that is activated by a current flowing through the coil. The electromagnet then either opens or closes the switch, depending on the direction of the current. It is used to control multiple circuits by using a single trigger. The different types of relay configurations are useful in different types of circuits.

Suppose you want an inactive circuit to activate, then you can use a single pole single throw (SPST) relay. If you want the live circuit to be inactive, and activate another circuit at the same time, then you can use a single pole double throw (SPST) relay.

In this powerful article, you are going to learn the difference between Single Pole Single Throw (SPST) Relay, and Single Pole Double Throw (SPDT) Relay, diagrams, and symbols in less than four minutes.

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First Know The Wiring Diagram Of The Relay

4-Pin-Relay-Wiring-Diagram
4-Pin-Relay-Wiring-Diagram

First of all, you should know the relay terminals and wiring diagrams before understanding the single-pole and double-pole relays. A relay consists of two circuits, a coil, and high amperage circuit.

The electromagnetic coil circuit consists of two terminals 85, and 86. Terminal 85 is negative, while terminal 86 is a positive terminal. The high amperage circuit consists of two terminals 30, and 87. Terminal 30 is a common and power-in terminal, while terminal 87 is a normally open and power-out terminal.

5-Pin-Relay-Wiring-Diagram
5-Pin-Relay-Wiring-Diagram

A five-pin relay has one extra terminal 87a in high amperage circuit. Terminal 87a is used for circuits when power is required when the relay is inactive.

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What Is Pole

Pole And Throw
Pole And Throw

Pole indicates the number of circuits controlled by a relay, a single pole controls one circuit and a double pole controls two different circuits. In the car relays, the pole refers to the common terminal 30 of the relay.

What Is Throw

Throw indicates the number of contact points, single throw means one contact, and double-throw means two contacts. In the car relays, the throw refers to the high amperage circuit’s terminals 87 and 87a.

This is an Info
Poles and Throws have nothing to do with the relay’s coil circuit. It is only concerned with the relay’s high amperage circuit. A single-pole means one common terminal 30. A single throw means the relay’s high amperage circuit’s terminal 87 and a double throw means the relay’s high amperage circuit’s terminal 87 and 87a.

SPST Relay

Single Pole Single Throw Relay also called SPST relay, or A Single Pole Relay consists of one pole and one throw. A single pole single throw has one pole, which is a common terminal 30, and one throw, which is a terminal 87.

If a relay controls one circuit for “One Operation” is called Single Pole Single Throw. In fact, a four-pin relay is a Single Pole Single Throw (SPST) relay. Because It controls one circuit (High Amperage Circuit Terminal 30 and 87) in a single operation.

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SPST Relay Symbol

SPST Relay Symbol
SPST Relay Symbol

Single Pole Single Throw or SPST Relay symbol has only one contact either NO or NC. An SPST relay has a total of four terminals including common terminal 30. Above, you can see the symbol of the SPST Relay.

SPST Relay Diagram

SPST Relay Diagram
SPST Relay Diagram (Normally Open)

A normally open four-pin relay has a normally open contact (terminal 87), which closes the contacts by energizing the relay and is called a single pole single throw NO (Normally Open) Relay. A single pole single throw NO (Normally Open) relay is mostly used in the car applications, such as air conditioners, radiator fans, horns, etc.

Normally Closed Relay Diagram
Single Pole Single Throw NC (Normally Closed) Relay Diagram

A normally closed four-pin relay has a normally closed contact (terminal 87a), which opens the contacts by energizing the relay is called a single pole single throw NC (Normally Closed) Relay. If you see in the above normally open and normally closed relays, the relay control the high amperage contacts (Opens and Closes) in one operation, that’s why it is called a single pole single throw relay.

Related Post: Normally Open & Normally Closed Relay, Diagrams, Symbols

SPDT Relay

A Single Pole Double Throw Relay also called SPDT relay, or double pole relay consists of one pole and two throws. A single-pole double-throw has one pole which is terminal 30 and two throws which are terminal 87 and 87a.

When a relay controls two circuits in “One Operation” is called Single Pole Double Throw (SPDT). A double pole relay connects one circuit (Terminal 30 and 87) and disconnects another circuit (Terminal 30 and 87a) at the same time in one operation, that’s why it is called a single pole double throw relay.

SPDT Relay Symbol

Single Pole Double Throw Relay Symbol
Single Pole Double Throw Relay Symbol

Single Pole Double Throw relay or SPDT Relay has a total of five terminals. Two for the coil circuit and three for the high amperage circuit. An SPDT relay has two terminals in which one terminal is normally open (NO) and the other one is normally closed (NC) at the same time. Above you can see the symbol of the SPDT Relay.

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SPDT Relay Diagram

A Single Pole Double Throw Relay (NO & NC Relay) Diagram
A Single Pole Double Throw Relay (NO & NC Relay) Diagram

Practically, a five-pin relay is called Single Pole Double Throw because It controls two circuits (Terminal 30 and 87) and (Terminal 30 and 87a) at one operation. A five-pin relay has one common control terminal 30 and two contacts 87 and 87a.

When the relay has not energized terminals 30 and 87a have connectivity. By energizing the five-pin relay, the common terminal contact 30 shifts from terminal 87a to 87, making the connection between terminals 30 and 87.

This is an Info
If a relay controls one circuit in “One Operation” is called an SPST relay. If a relay controls two circuits in “One Operation” is called an SPDT relay and if a relay controls three circuits in “One Operation” is a Single Pole Triple Throw (SPTT) relay.


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