switch, electrical device having two states: on, or closed; and off, or open. Ideally a switch offers a zero impedance to a current when it is closed, and it offers infinite impedance when open. Mechanical switches, which operate by moving contacts together and apart, are often classified by the number of connections they can make or break at once and the number of closed positions in which they can be placed. A single-pole double-throw switch can be placed in either of two closed positions, making one connection in each position. A double-pole single-throw switch can open or close two connections at once. For many operations, as in computers and digital telecommunications, the operation of mechanical switches is too slow and transistors are used instead. The PIN diode, which contains an intermediate intrinsic layer between the n-type and p-type layers, is also used as a simple switch. This device, which normally acts as a conductor, acts as a strong dielectric when the intrinsic layer is reverse biased. See relay.
See J. C. McDonald, Fundamentals of Digital Switching (1990).
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