Pushbuttons are used to manually open or close electrical contacts. Push buttons can be used in a number of ways to control motors and manufacturing processes, for instance: start-stop control, reversing control, two-speed control, jogging control, thermostat-controlled motor, and ground detection with push-to-test pilot lights. They come in a variety of styles, colors, and features such as buttons with flush head, extended head, and with or without guards.
Working principle of a pushbutton
A pushbutton contains at least two contacts, which close or open when the button is pressed. Usually, a spring restores the button to its original position when external pressure is released.
The pushbutton has a single steel return spring, to create resistance to downward force on the button, and a pair of springs above a pair of contacts, to hold each contact in place and make a firm connection when the button is pressed. The two upper contacts are electrically linked.
Unlike a switch, a basic pushbutton does not have a primary contact that can be identified as the pole. However, a single pushbutton may close or open two separate pairs of contacts, in which case it can be referred to, a little misleadingly, as a double-pole pushbutton.
Parts of a pushbutton
Pushbutton assemblies are manufactured in both 30-mm NEMA and smaller 22-mm IEC types. The size is related to the diameter of the circular hole the push button is mounted in—either 30 millimeters or 22 millimeters in diameter. The button assembly consists of three parts: Operator, Legend plate, and contact blocks.
The operator is the part that is pressed, pulled, or twisted to activate the contacts. Operators come in many different colors, shapes, and sizes designed for specific control applications.
The legend plate is the label of the button (e.g., START). Legend plates come in many sizes, colors, and languages. Examples of label text include START, STOP, FWD, REV, JOG, UP, DOWN, ON, OFF, RESET, and RUN.
The contact block is where the electrical contacts are enclosed.
There are N.O. and N.C. contacts. In the case of the N.O. type, pressing the button closes the otherwise open contacts. This is sometimes described as a make-to-make connection, or as a Form A.
The N.C. type works in the opposite way in that pressing the button opens the contacts that are otherwise closed. Contacts are normally closed by default and are open only while the button is pressed. This is sometimes described as a make-to-break connection, or as a Form B.
The electrical contacts in the contact block are spring-loaded. They return to their normal state when the operator is released.
Operators are available for momentary contact or maintained-contact operation. Momentary pushbutton operators are spring-loaded. As soon as it is released, the operator goes back to its normal state. For example, if a momentary pushbutton operator is mounted on an N.O. contact block, then we will have a switch that is normally open. When the button (the operator) is pressed, the contacts will be closed. As soon as the button (operator) is released, the contacts will be opened. If the same operator is mounted on an N.C. contact, then we will have a switch that is normally closed. Pressing the button will open the contacts. When released, the contacts will close again.
The maintained-contact operators remain in their activated state. For example, an E-Stop button will remain pressed even after it is released. This maintains the state of the contacts until the button is physically reset. Typical two-position E-Stop buttons have either push/pull-release or push/twist-release action.
Pushbutton’s current ratings range from a few mA to 20A or more. Many pushbuttons have their current ratings printed on them but some do not. Current ratings are usually specified for a particular voltage and may differ for AC versus DC.
Possible failures of pushbuttons
No button: When ordering a pushbutton switch, read data sheets carefully to determine whether a cap is included. Caps are often sold separately and may not be interchangeable between switches from different manufacturers.
Mounting problems: In a panel-mount pushbutton that is secured by turning a nut, the nut may loosen with use, allowing the component to fall inside its enclosure when the button is pressed. Conversely, overtightening the nut may strip the threads on the pushbutton bushing, especially in cheaper components where the threads are molded into the plastic. Consider applying a drop of Loc-Tite or similar adhesive before completely tightening the nut. Nut sizes vary widely, and finding a replacement may be time-consuming.
LED issues: When using a pushbutton containing an LED, be careful to distinguish the LED power terminals from the switched terminals. The manufacturer’s datasheet should clarify this distinction, but the polarity of the LED terminals may not be clearly indicated. If a diode-testing meter function is unavailable, a sample of the switch should be tested with a source of 3 to 5VDC and a 2K series resistor.
Briefly touching the power to the LED terminals, through the resistor, should cause the LED to flash dimly if the polarity is correct, but should not be sufficient to burn out the LED if the polarity is incorrect.
Other problems: Problems such as arcing, overload, short circuits, wrong terminal type, and contact bounce are generally the same as those associated with a switch, and are summarized in that entry in this encyclopedia.
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