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Circuits and Switches Terms Open—off position, a break in the circuit Closed—on position, circuit is complete Pole—a leg of an electrical circuit,
the number of separate circuits which can conduct current through a switch at
one time Throw—the number of positions that a switch can adopt to make a connection Example: a Single Pole Single Throw switch has one leg that can make one connection Types of Switches Single Pole Single Throw (SPST) Single Pole Double Throw (SPDT) Double Pole Double Throw (DPDT) Situation 1 There is a room in your house where you always turn the lights on when you enter and turn them off again when you leave. For this reason, you only need one light switch by the door. A SPST switch would work well in this situation because the flow of the circuit only needs to be opened or closed at one location. Situation 2 You have a light in your stairway that you would like to be able to turn on and off from both the top and the bottom of the stairs. If you used two SPST switches in this circuit, you would be able to turn the light off from either location, but both switches would have to be closed in order for the lights to be on. Instead, you can use two SPDT switches, in which two sets of contacts allow the lights to be turned on or off from either location. Situation 3 Your entryway has rooms and hallways branching off of it in three (or more) different locations. You would like to be able to turn the lights on or off from any of these places. If you wired the circuit with three SPDT switches, at least two of the switches would have to be in the on position in order for the light to be on. However, if you place a DPDT switch in between the two SPDT switches, the additional contacts allow the lights to be turned on or off from any location.
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