Switches and Their Types

Posted on April 20, 2020 Josh Parker Aircraft Component

A switch is a device used to interrupt the flow of a circuit. Every single electrical application features at least one switch, to turn the device on and off. Switches can be mechanical or electronic. Mechanical switches are operated physically, by the moving, pressing, or releasing of its contacts. Electronic switches do not require manual operation to control a circuit, and are instead activated by semiconductor action. However, there are a myriad of different types of switches used in a wide range of applications. This blog will explain many of the types of mechanical and electronic switches and provide details about their characteristics and functions.

The first type of mechanical switch is the Single Pole Single Throw (SPST) switch. This is a basic on/off switch that consists of one input contact and one output contact. It switches a single circuit to either make a connection (turn on) or break a connection (turn off). The contacts within an SPST switch are either normally open or normally closed configurations. The Single Pole Double Throw (SPDT) is a switch with three terminals - one input and two output, meaning it consists of two ‘on’ positions and one ‘off’ position. Generally speaking, switches of this type are used as changeovers to connect the input between one of two outputs.

More types of mechanical switches include the Double Pole Single Throw (DPST) and Double Pole Double Throw (DPDT) switches. A DPST consists of four terminals that behave like two separate SPST switches operating simultaneously. In the ‘off’ position both switches are at an open state. This type of switch is typically used for controlling two different circuits at the same time. The DPDT switch is a dual on/off switch that consists of two ‘on’ options. These switches feature six terminals, two being input and the rest being output. Other types of mechanical switches include push-button, toggle, limit, pressure, and rotary switches.

Electric switches are generally called solid state switches due to their lack of moving parts and contacts, instead being controlled by semiconductors. Electronic switches include transistors, SCRs, MOSFETs, and IGBTs. A transistor is a switch that either allows a current to pass or blocks it. This is similar to the function of a mechanical switch. When a transistor is intended to block a current, it is considered to be in the ‘off’ position. The ‘on’ position is also called saturation mode and refers to the transistor actively allowing currents to move through it. A MOSFET (metal oxide semiconductor field effect transistor) is a high power switching device found in common electronic application. It features three terminals for output, idle, and input.

The IGBT, or insulated gate bipolar transistor, is an amalgamation 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. 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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