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A push switch (button) is a momentary or non-latching switch which causes a temporary change in the state of an electrical circuit only while the switch is physically actuated. An automatic mechanism (i.e. a spring ) returns the switch to its default position immediately afterwards, restoring the initial circuit condition.
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.
Figure 1D illustrates a double-pole, double-throw (DPDT) switch. Contact form, or simply form, is the term relay manufacturers use to describe a relay's contact configuration. "Form A” refers to a single-pole, normally open switch. "Form B" indicates a single-pole, normally closed switch, and "Form C" indicates a single-pole, double-throw switch.
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]
Isolation valves can be in the normally open position (NO) or normally closed (NC). Normally open valves are located between pressure vessels, pumps, compressors, tanks, pressure sensors, liquid level measurement instrumentation and other components and allow fluids to flow between components, or to be connected to sensors. [3]
The above realizes the function: Door motor = Key switch 1 AND Key switch 2 This circuit shows two key switches that security guards might use to activate an electric motor on a bank vault door. When the normally open contacts of both switches close, electricity is able to flow to the motor which opens the door.
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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.