What is Single Pole Single Throw Switch (SPST), Types, Working

SPST – Single Pole Single Throw Switch:

SPST Single pole single throw switch is nothing but a simple two-terminal switch which help us to disconnect the one terminal to another terminal (vice versa – ON or OFF operation). Example: our household lamp switch.

Single Pole Single Throw Switch
Single Pole Single Throw Switch

Here pole refers to the maximum number of input connection. In our SPST switch, you can give one input.

Also, throw refers to the maximum number of output from the switch. In our Single pole single throw switch, you can get single output.

Hence SPST uses to control a single circuit at a time.

SPST circuit diagram
SPST circuit diagram

Construction of SPST:

Construction front, the switch has two terminal and contact portion is made up of silver alloy to withstand the high circuit breaking current. It will be separated by the minimal distance to avoid internal induction current circulation. The distance between the two contacts can be varied according to its application.

The best example for high voltage SPST switch is railway 25KV DC voltage system. The negative terminal will be looed with the source through the track, but the positive will be connected through SPST switch (It is called circuit breaker).

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The contact will be insulated by Polyvinyl chloride material to avoid external contact. The insulation material can be varied according to the operating voltage.

Working principle of SPST Switch:

Just take a simple circuit. A small lamp is connected in series with the power source and with our switch. Look the circuit, at the initial condition the lamp will not glow since the switch is in off position there is no current flow through the circuit.

SPST switch Operation

But when you turn the switch from off to on, the lamp starts glowing until you do the same process again.

Turn On Operation

For high voltage application, you cannot use your hand to open or close the switch contact. There is a spring mechanism uses to ON or OFF the process.

Types of SPST Switch:

Depending upon the application convenient, It can be classified into three types such as

  • Simple SPST
  • (ON)-OFF, Push-to-close, SPST Momentary
  • ON-(OFF), Push-to-Open, SPST Momentary
  • Inches Switch SPST
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Types of Single Pole Single Throw Switch

Simple SPST:

In this, the manual action is used to connect or disconnect the circuit. Example: On/Off switch.

(ON) – OFF Push to Open – Momentary Switch:

A Momentary mechanism is used to keep the switches in a new position, in such While pushing an SPST with your hand, the switch goes to Off to ON position. While returning your hand the switch remains the same position. These switches play major roles in electrical safety circuit. Also sometimes they come with the internal key based locking system.

(Off) – ON Push to Close – Momentary Switch:

It is a reverse function of Push to Open switch. The switch will be at open condition while pushing the switch will close the circuit. It means ON to Off position.

Inches Switches:

It does not come with the momentary contact, which means, it can be used as ON-OFF or OFF to ON by changing the contact element. In this switch, while pushing the button, they become ON to off Position and while releasing it will come to its original position. These logics are used in a Crane operating system.

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Frequently Asked Questions:

What is the cost of SPST Switch:

The cost is depending upon the current carrying capacity. Normal household switches come with the capacity of 6 Amperes and it costs around Rs 20/-.

What is the best make for SPST switches?

Salzer make is the best one. I have used in many industries, good quality with lifelong.



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