What is SPST Switch : Working & Its Applications

A switch is a mechanical or controlling device that changes the flow of current direction or interrupts the flow of current within a circuit. Switches are designed in different configurations based on the requirements. They include various sets of contacts that are controlled through a similar knob. These contacts may function sequentially, alternately, or simultaneously. Almost all electronic or electrical systems include a minimum of one switch to turn ON/OFF the device. There are different types of switches available like DPST, DPDT, SPDT, and SPST switch. This article discusses one of these types namely the SPST switch and its working with applications.

What is SPST Switch?

The term “SPST” in an SPST switch stands for “Single Pole Single Throw” which includes a single input and a single output. Here, a single input is directly connected to a single output. The main function of this switch is to control the circuit by turning ON/OFF.

Once the switch in the circuit is closed, then the circuit will be turned ON whereas the switch is not closed or open, then the circuit will be turned off.  The SPST switch example is the 25KV railway DC voltage system & a household lamp switch. The schematic symbol of the SPST switch is shown below.

Switch Symbol
Switch Symbol

The connections of this switch are two types like the Normally Open (NO) & the common (C). Once the switch is enabled, then the circuit is closed. So the flow of current will be from the common (C) terminal to the normally open (NO) terminal. When the switch is disabled, the circuit will be an open circuit, so there is no flow of current within the circuit.

SPST Switch Construction

The construction of this switch can be done through two metal plates that can touch each other to create a physical contact to allow the flow of current. This contact separates the two metal plates from each other to interrupt the current flow.

In the Single Pole Single Throw switch, the pole refers to the maximum input connections whereas throw refers to the maximum output connections from the switch. So in this switch, we can give single input and gets single output. This switch controls a single circuit at a time. In this switch, the contact portion can be designed with silver alloy material to oppose the high current.

This contact can be insulated through Polyvinyl chloride material to keep away from exterior contact. So this insulation material can be changed based on the operating voltage.


How does SPST Switch Works?

This switch works like a one-way switch to ON/OFF the circuit. Once a user pushes the switch button, then the switch plates will be connected. So that current starts flowing within the circuit.

For example, The light in the room generally uses this switch. Once the switch is turned “OFF”, the circuit will break to turn off the light. Similarly, once the switch is turned ON then the light will be turned ON.

SPST Switch Circuit Diagram

The circuit diagram of a simple SPST switch is shown below. This circuit can be built with an SPST, two incandescent types of lamps like L1 & L2. Here L1 is a 1-watt lamp & L2 is a 100-watt lamp. The main intention of this circuit is to control two lamps by using a simple switch.

The operating of this switch is, once the switch is closed, then the first lamp like ‘L1’ will glow. The flow of current throughout the second lamp will be so small as to generate insignificant heating the L2 lamp will be turned off. Once the switch is closed, then the L2 lamp will get full voltage and turned ON and the L1 lamp will be turned off.

SPST Switch Circuit to Control Lamps
SPST Switch Circuit to Control Lamps

For instance: a 100-watt lamp at 120v has 120 ohms of resistance. A 1watt lamp at 120v has 12,200 ohms of resistance. By using two lamps that are connected in series, the voltage across the 100-watt lamp will be

V = (120/120+12,200) ×120 => 0.0097 X 120 = 1.164 volts

The dissipated power in the 100w lamp will be

P=V2/R= (1.164)2/120 = 0.011 watts

SPST Toggle Switch

The SPST toggle switch is an electrical switch that includes a lever or handles to control the current flow from a power supply to a device. These switches use a rocker-type contact mechanism. Its operation is very simple, so used in both household & commercial applications. This switch uses a flip cover as a switch guard for protecting from false switching.

SPST Toggle Switch
SPST Toggle Switch

This toggle switch includes a metal contact that connects the two pins. Once the metal contact is moved at the left side, then there is no connection among the pins. Similarly, when it is moved at the right side, then the two pins are connected & allow the flow of current from one pin to another pin.

SPDT Switch into SPST Switch Conversion

An SPDT switch can be converted as an SPST switch by simply using the common terminal & either of the remaining terminals. So you could use A & C or B & C.


The advantages of the SPST switch include the following.

  • The circuit design is very simple
  • Simple wiring
  • Cabling is less required
  • Low cost
  • Capable to work with high voltages & currents.
  • Reliable


The disadvantages of the SPST switch include the following.

  • Once the label of this switch is faded then it creates confusion for the user.
  • Not strong

What is an SPST switch used for?

The applications of the SPST switch include the following.

  • Advanced process control systems can use these switches in place of two-wire sensors. As is commonly done, one can use two of the three contacts of a standard SPDT switch to mimic an SPST.
  • This is an ON/OFF switch, used to break or connect the terminal connection. So this switch provides a power supply for the entire circuit.
  • These switches are used in applications that only require ON/OFF state
  • These switches are used in simple electronic projects.

How many circuits can an SPST switch control?

The single pole single throw switch can control only one circuit.

How do you control a lamp from two places?

By using a 2- two-way switch & one intermediate switch, a lamp can be controlled from two places.

Thus, this is all about an overview of the SPST switch. Many features need to consider while choosing a switch like contacts (single pole, double pole), ratings like maximum current & voltage, method of operation like key, slide, toggle. Switch contacts are generally described by using different terms like pole, way, throw, open, momentary and closed. Here is a question for you, what is the rating generally choosed for SPST switch used in household?

Add Comment