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Machine Design

Mechanical Switches

Mechanical switches can be divided into two basic types. The first, commercial and appliance switches, are used in fairly clean environments such as offices or homes. They are not sealed and are generally used for light, low-current applications.

The second type, industrial switches, actuate magnetic contactors and remote-operated controllers. These switches must be ruggedly constructed because they are frequently exposed to oil, solvents, chemicals, and dust. And their contacts must handle the high inrush current drawn by electromagnets in the controllers. Industrial switches are available in five basic types: standard duty, heavy duty, heavy-duty oiltight, miniature oiltight, and multilight-control oiltight. The terms standard duty and heavy duty are derived from the Standards for Industrial Control Equipment of Underwriters' Laboratories Inc. for normal current and inrush current.

Switching techniques: Manual switches can have one or a combination of switching actions. In momentary-action switches, the operator pushes (pushbutton or toggle) or twists (rotary) the actuating device and contacts move to transfer the circuits to the second set of contacts. When the actuating force is removed, the actuating device and the contacts return to the original position.

When a maintained-action switch is actuated, the contacts move to transfer the circuits to the second set of contacts. No change takes place until the operator actuates the switch a second time. Then the circuit moves to another set of contacts or returns to the original position.

Mechanical-bail switches have separate switching assemblies, which are interlocked so that actuation of one switch deactivates another.

A capacitive touch switch consists of two conductive layers on opposite sides of an insulating material such as glass or a printed-circuit board. The conductive layers create a capacitance that decreases when a layer is touched. Interface circuitry converts the capacitance change into a usable switching action to drive logic systems or to switch analog signals. There are several types of touch-switch interface circuits available.

Membrane switches are simple devices in which conductive leads on the underside of a flexible membrane are, at a keystroke, pushed through a hole in a spacer to make contact with conductive leads on a base. Most membrane switches handle loads up to about 1.5 VA at 5 to 15 V.

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