Pushbuttons - Basic Properties, Terminology and Theory
What is a Pushbutton?
Pushbutton is designed to help someone change how a machine works, or understand its current state. Humans control the machine, initiating changes based on work assignments or advancing the machine to the next step of the process.
Pushbuttons consist of operators and contact blocks. Illustrations in Fig. 7 show how your "finger push" is converted into a circuit connection (or disconnection). The operator is the portion your finger operates. Inside is a companion part that translates the finger movement into the movement of a plate or bar. The plate pushes the plungers down in the contact block and a set of contacts actually moves, opening and closing the electrical circuits.
Activation for pushbuttons can be momentary or maintained. Pressing a doorbell is an example of a momentary pushbutton. When the doorbell is released, the button returns to its original position. A maintained pushbutton keeps its new position. The light switch on a wall is a common maintained connection. Once it is switched to "on" or "off", it stays that way until it is switched back.
Momentary pushbuttons are often wired to a relay, or auxiliary contact, that holds the electrical loop even after the momentary pushbutton is released. The result is that the machine continues to run, even though the "start" button has returned to its original "off" position. The reason for this momentary action is safety.
Consider if an electric saw has a momentary pushbutton that when the button is pushed another circuit holds the electrical loop. This enables the saw to run even when the button is released. If power is lost, the additional circuit opens. When power returns, the button will have to be pressed again to start the saw. This is because the button returned to its original "off" position. If the start button were a maintained version, the instant power would "jump start" the saw, with possible injury to saw operators.
Maintained pushbuttons continue in the position they were set. These devices are necessary for safety purposes. Maintained pushbuttons can "freeze" assembly lines or saws for maintenance. A maintained setting overrides any default settings that would otherwise automatically turn the device on.
Pushbuttons are available in a variety of styles including illuminated, non-illuminated, e-stop and miniature. Styles offered range from flush, extended, mushroom, incandescent, LED, compact, modular, finger-safe in many NEMA and approval ratings. Work with a pushbutton specialist to determine the best product for your application.