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single pole double throw (SPDT)

A changeover switch is a switch that switches between two switching contacts. If one switching contact is closed, the other opens at the same time and vice versa. Each switching position is characterised by the fact that one switching contact is always open while the other is closed.

How the changeover switch works

The classic changeover switch is single-pole and is called Single Pole Double Throw (SPDT). It switches one of two poles (contacts) to the common center terminal, which is labeled Common (COM). One of the two contacts is generally closed, this is referred to in English as Normally Closed (NC) and the second is Normally Opened (NO). Changeover switches are well known from domestic electrics, where they are also used in a combined series circuit to switch a lamp alternately. In this technique with two changeover switches, the lamp can be switched on and off from each changeover switch. These switches are also available as logics


Switch designs

Switch designs

The simplest switch design is the on switch

, Single Pole Single Throw (SPST). In addition to the Single Pole Double Throw, there is also the double pole version, the Double Pole Double Throw (DPDT). This version corresponds to a double single pole version. The switching contacts of the two SPDT switches are mechanically connected to each other.

SPDT switch (changeover switch), circuit diagram and analogue switch DG469, diagram: Vishay

SPDT switch (changeover switch), circuit diagram and analogue switch DG469, diagram: Vishay

The designation changeover switch only says something about the function, not about the design, whether it is a mechanical switch or an electronic logic circuit. Mechanical changeover switches can be designed for manual switching or for automatic motor-controlled switching. Electronically controlled changeover switches are used in electronic devices and computers.

With them, the switching currents, the operating voltages and the switching times play a decisive role.

Informationen zum Artikel
Englisch: single pole double throw - SPDT
Updated at: 26.01.2014
#Words: 371
Translations: DE