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Design & Technology

Switches, transistors and relays

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An important function that needs to be carried out in many circuits is connecting or disconnecting parts of the circuit to the power supply. Depending upon the task being carried out, this might be achieved by using mechanical switches, transistors or relays.

Switches

Switches are devices for connecting and disconnecting circuits. Different types of switches have different numbers of poles [poles: Output terminals or contacts in a switch. ] and throws [throws: Circuits or conducting positions controlled by each pole of a switch. ].

Mechanical switches

There are four main types of mechanical switches used in circuits. The description given to these depends on the function of the switch.

 

Switch typeDiagram
Single-pole single-throw (SPST)
Single-pole double-throw (SPDT)
Double-pole single-throw (DPST)
Double-pole double-throw (DPDT)

The single-throw switches are either open or closed. Double-pole single-throw switches are often used to switch between two outputs.

The double-throw switches are used to change from one part of a circuit to another. The double-pole double-throw can be used to reverse the direction of a motor. The dotted lines on the symbols indicate that both sides move together.

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