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These terminals are usually labelled as normally open, common, and normally closed (NO-C-NC). An alternate notation for Form C is SPDT. [12] These contacts are quite frequently found in electrical switches and relays as the common contact element provides a mechanically economical method of providing a higher contact count. [12]
The internals of a micro switch. Contacts, from left to right, are common, normally open, and normally closed. In one type of microswitch, [4] internally there are two conductive springs. A long flat spring is hinged at one end of the switch (the left, in the photograph) and has electrical contacts on the other.
Some switches may also be manufactured from other plastics like polyamide, polypropylene or pvc but mainly limited to wire mounted switches (like night lamp switches) mainly due to reduced weight, and most such switches are not used in high power devices (mainly table lamps, table fans, some radios) mainly up to 2 amperes.
Each key of a computer keyboard, for example, is a normally-open "push-to-make" switch. A "push-to-break" (or normally-closed or NC) switch, on the other hand, breaks contact when the button is pressed and makes contact when it is released. An example of a push-to-break switch is a button used to release a door held closed by an electromagnet.
A typical circuit consists of a number of rungs, with each rung controlling an output. This output is controlled by a combination of input or output conditions, such as input switches and control relays. The conditions that represent the inputs are connected in series, parallel, or series-parallel to obtain the logic required to drive the output.
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The transistor can be operated as a switch. The bipolar junction transistor (BJT) cutoff and saturation regions of operation can respectively be treated as a closed and open switch. The most widely used electronic switch in digital circuits is the metal–oxide–semiconductor field-effect transistor (MOSFET). [2]
Pull switches may be either two-position (open or closed) or multi-position (allowing for different fan speeds or levels of illumination). Mounted inside a pull switch, there could be two types of two-position toggle switches. (In both cases the stable physical open- or closed -situation would be switch in outer and string in upper position.)