What is SPST Relay? Single Pole Single Throw Relay

An SPST Relay or Single Pole Single Throw Relay is a device that has only one input and one output. It only connects or disconnects only one contact when it is operated. Including the coil terminals, it has a total of four terminals. The SPST relay can control a single electrical or electronic circuit. 

It generally provides a switching function or on/off function as an electrical switch provides, just the difference is, it is controlled by an electrical or electronic signal. When a voltage is applied across the relay coil, it makes its contact closed and when the voltage is removed from the coil it makes its contacts open. Generally, relays are designed to control a high voltage and high current circuit by applying a low voltage or low current power supply. For, example, a 230V AC circuit can be controlled by a 6V DC power supply. Anyway, we are going to know the concept of SPST or Single Pole Single Throw.

Here, you can see the symbol of single pole single throw relay.

Now, let's see the connection diagram of SPST Relay, so it will help to understand the better working principle of the SPST Relay.

Here, you can see the contact terminals of the relay are connected in series with the battery and LED. Also, the coil of the relay is connected to a battery by connecting a switch in series.

It is an NO type SPST Relay. So, when the switch S1 is off condition, the relay coil will get not any power supply, so the contacts also be in the open condition and the LED will not glow.

When the switch S1 is turned on, the relay coil will get the power supply and it will close its contact. So the LED will be connected to the battery and it will glow.

There are very few SPST relays are available in the market. And it has a few applications also. Most of the relays come with SPDT types and they have a wide range of applications.

Read Also: 

• What is SPDT Relay? Single Pole Double Throw
• What is DPST Relay? Double Pole Single Throw
• What is DPDT Relay? Double Pole Double Throw
• What is Relay Switch? Circuit Diagram and Working Principle
• Undervoltage Relay Connection Diagram and Wiring Procedure

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