A switch is an electrical device used to manually or automatically create or break an electrical circuit. Switches can be mechanical or electronic. Mechanical switches are operated physically, while electronic switches are activated by semiconductor action. Generally speaking, switches are classified into four types: Single Pole Single Throw (SPST), Single Pole Double Throw (SPDT), Double Pole Single Throw (DPST), and Double Pole Double Throw (DPDT).
The SPST is a basic on/off switch used to connect or break a connection between two terminals. The power supply for the circuit is controlled by the switch. SPST switches are commonly used as light switches and are also known as toggle switches. They have a single input and output. It therefore controls one wire and makes one connection. When the SPST switch in a circuit is open, the current flows through the terminal. When the switch is closed, the current is blocked from flowing through the terminals. The SPDT switch is a three terminal switch, wherein one switch is used as an input and the remaining two are used as outputs. They join a mutual terminal to one of the two terminals. SPDT switches, instead of using other terminals, use COM (common) terminals. SPDT switches are primarily used in three-way circuits to control a device from two locations. An example of this is a light switch at two ends of a hallway controlling the same light.
In a DPST switch, there are two inputs and two outputs. Each input corresponds to an output. The terminals of a DPST can either be in the on (closed) or off (open) positions. A DPST switch is very versatile. They are able to accept two inputs which, in turn, make it able to drive two different outputs in a circuit. What the switch drives depends on the design of the circuit and what the circuit is intended to do. However, DPST switches have a broad range of applications within circuits. The DPDT switch has two inputs and four outputs. Each input has two corresponding outputs to which it can connect. Each of the terminals can be in one of two positions, making the DPDT switch a highly versatile option. With two inputs, it can connect to four different outputs and can reroute a circuit into two different modes of operation. A DPDT switch is two SPDT switches.
While these are the four main classifications of switches, other types exist. For example, transistors, SCRs, MOSFETs, and IGBTs are also examples of switches. A transistor operates similarly to a mechanical switch, either blocking a current or allowing it to pass. A MOSFET (metal oxide semiconductor field effect transistor) is a high power switching device found in many applications. It features three terminals for output, idle, and input. The IGBT, or insulated gate bipolar transistor, is a combination of other types of switches. It is a voltage controlled device with a low voltage. It is also a three-terminal device turned on and off via a voltage between the gate and emitter. Lastly, Silicon Controlled Rectifiers (SCR) are switches for high-speed devices. It is controlled via its gate input and the control of conditions in the anode and cathode terminals.
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