Capacitive proximity switch

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Capacitive proximity switch with PTFE cap, which can also be used as a level monitor

A capacitive proximity switch is a proximity switch ( sensor ) that is contact-free - i. H. without direct contact - reacts to the approach of a conductive or non-conductive object, but also of liquids, with an electrical switching signal. It uses the changing electrical capacitance of a measuring electrode to the environment or a reference electrode.

Such sensors work with an oscillator whose frequency-determining capacitance is partly formed by the medium to be detected or the environment.

In influencing the field of the probe capacitance through a non-conductor , the capacitance change due to a change of the effective permittivity in the region of the electrodes; the achievable switching distance is small (depending on the size of the sensor up to approx. 40 mm).

If the field is influenced by a conductive object, the switching distance increases to approx. 60–80 mm.

Capacitive proximity sensors usually have a calibration option ( potentiometer ) in order to adapt the sensitivity or the switching thresholds to the application conditions. If the sensitivity is too high, however, the influence of disturbance variables (e.g. humidity, dirt) increases.

Another circuit concept works with three electrodes. In addition to the ground electrode and a measuring electrode, an additional excitation electrode is used. The advantages of this principle are a higher sensitivity with higher switching distances and a lower susceptibility to failure. Media with very low permittivity can also be detected.

features

In addition to metals and non-ferrous metals , capacitive proximity switches also detect liquid, granular and powdered materials, e.g. B. plastics, wood, stone, oil, water or cement. In addition to object detection or proximity switches, they can therefore also be used for level control (in the case of non-conductive tanks or pipes, also from the outside through the wall).

Like other proximity switches, capacitive proximity sensors are also characterized by a hysteresis of the response behavior. They require an operating voltage ( nominal value usually 24 V direct or alternating voltage , but also up to 60 V direct voltage or for mains voltage ), often have a status display with light-emitting diodes . The switching output consists of relay contacts or an electronic switch formed by transistors .

In addition to the maximum switching current and the type and level of the switchable voltage, the characteristic values are the switching hysteresis, the repeatability and temperature drift as well as the switching frequency.

See also

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