Machine Design

Switching Sensors

Some types of sensors actuate an integral switch in response to a stimulus of some kind. Typical inputs include temperature, centrifugal force, or a simple spring.

Snap-acting switches: Snap-acting switches consist of closely spaced switches that are snapped together or apart by a spring mechanism.

For thermostatic controls, a low operating force and short movement differential permit a temperature-sensitive element to act directly on the operating button of the switch. Thus, heater and motor loads can be controlled without relays. For timing controls, the ability of the switch to operate within close limits of repeatability ensures accuracy of controlled time intervals. In household appliances, the switch can operate directly in fractional-horsepower motor circuits. In machine-tool service, the switch is widely used as a limit switch.

Switches controlling motors, relays, and solenoids must handle inrush currents which may be ten times the steady-state value. And since a lamp filament has low cold resistance in comparison to its hot resistance, inrush current can be 12 times the steady-state current. To switch these loads, the spring mechanism must have enough stored energy to ensure breaking the welds produced by inrush currents. Thus, the switch must have higher operating force and longer movement differential than a switch used for resistive loads.

Contact welding (on make) and arcing (on break) are more severe on direct than on alternating current. Switch failure caused by mechanical interlocking of contacts may result from contact material transferred during making and breaking.

The following guidelines may be applied in low-power circuits:

  • For voltages above 6 V and loads over 3 VA minimum, snap-acting switches with silver contacts are adequate.
  • When voltage is 6 V or more and the load is between 1 and 3 VA, standard snap-acting switches with silver contacts are relatively safe, provided special nonoutgassing case materials are used.
  • When voltage is less than 6 V and the load is less than 1 VA, silver contacts are not satisfactory unless plated with gold or more noble metals; for maximum reliability, solid gold or more noble metals or alloys should be used.
TAGS: Technologies
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