What is a SPST Switch? A switch may contain one movable input to contact one output connection point (a terminal). If this is the case, we are speaking about a Single Pole Single Throw (SPST) switch.
The working principle of such a switch type is very straightforward. Its only application within a circuit is to keep a single device on or off. That is why it is commonly named as an on-off switch.
To keep the circuit on, you need to close the SPST switch. On the contrary, if you need to keep the circuit off, then open the single pole single throw switch.
Single Pole Single Throw (SPST) Switch Circuit
Look at the figure that presents a one-device wiring diagram to get a full idea of the SPST switch working principle.
If you arrange the SPST switch to be closed, current runs to the device (a lamp, for example) and keeps it on. That is to say, to make the circuit. In contrast, the SPST switch that is arranged to be open breaks the circuit. Therefore, the current can’t run to the device and, thus, keeps it off.
This shows what is a SPST Switch and the basic nature and function of a single pole single throw.