Why stop button is normally closed?

What is a normally open and normally closed push button

Buttons are usually made of hard materials, usually plastic or metal , but they can also be made of rubber . There are two types of buttons: the normally open button and the normally closed button (normally closed contact (CR) or normally closed contact (CT)).

In a normally open button, the electrical connection is created when the button is pressed, and in a normally closed button, the electrical circuit is opened when the button is pressed (see diagram). The button mechanism is mainly equipped with a spring or a system that automatically allows the button to return to the initial position.

Most of the time, push buttons are equipped with a mechanism that maintains the engaged position when actuated with short pressure. In most cases, pressing the button activates the process for a long time until the button is pressed again, which stops the process. In this case, the button may remain lightly depressed, indicating the actuated state of the switch. A second action on the button allows it to return to the initial position. The process can also be stopped with another button. These buttons are sometimes equipped with a timer, which allows the button to automatically return to the initial position.

Normally closed pushbutton symbol

NC pushbutton. For example, it is used to switch on the interior lamp of a cabinet or car trunk. When the cabinet door is closed, the pushbutton is actuated and the lamp is off. When the door is opened, the pushbutton returns to its normally closed state and the lamp is switched on.

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In order to do this by means of a double switch we must connect the pins together and connect the motor as shown in the figure. Then you must connect the motor, a battery and a main switch to completely disconnect the circuit. Note the direction of the current when changing the position of the bipolar switch.

The following circuit can be used, for example, to make an elevator, a garage door or a drawbridge. At the end of the travel, either in one direction or the other, the motor stops automatically thanks to the introduction of the limit switches.

When an electric current is passed through the coil or electromagnet, it generates a magnetic field around it and attracts the armature which, with its movement, causes the contacts to change position.

Emergency stop regulation

The button of an electronic device usually works as an electrical switch, i.e. inside it has two contacts, when one is pressed, it will activate the inverse function of the one it is performing at that moment, if it is an NO (normally open) device it will be closed, if it is an NC (normally closed) device it will be open.

In industrial and commercial applications, buttons can be linked together by a mechanical joint so that the act of pressing one button causes the other button to stop being pressed. In this way, a stop button can “force” a start button to be released. This method of linkage is used in simple manual operations where the machine or process has no electrical circuitry for control.

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Types of emergency stop

A practical application of medium complexity is the start and stop control of an AC motor. Before running the program in a PLC, let’s first understand the wiring of the control circuit.

In this example, the contactor coil (M) is wired in series with a normally open (NO) momentary start button and a normally closed (NC) momentary stop button and a protection contact coming from the normally closed overload relay (OL).

From the diagram it can be seen that pressing the start button closes the circuit to energize the coil of the starter contactor (M). This generates the closing of the contacts associated with the contactor (M) in this case (Ma).

The motor will run until the NC stop button is pressed unless the overload relay contact (OL) is opened. When the stop button is pressed, the path for current flow is interrupted, opening the contacts (M) and (Ma) therefore the motor stops.

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