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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.
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]
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.
Time-delay relay contacts must be specified not only as either normally open or normally closed, but whether the delay operates in the direction of closing or in the direction of opening. The following is a description of the four basic types of time-delay relay contacts. First, we have the normally open, timed-closed (NOTC) contact.
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.
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multi position switch, in 83a out position 1 83b out position 2 Relay; 85 relay coil - Important if relay coil has flyback diode in parallel 86 relay coil + Important if relay coil has diode in parallel Relay contacts 87 common contact 30/51 87a normally closed contact 87b normally open contact 88 common contact 2 30/51 (relay) 88a