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Proximity Switches

Advantages of Proximity Switches

An alternative to momentary-contact proximity switches in which position detection is performed by mechanical activation by an external object, proximity switches detect position without any physical contact. As such these types of switches virtually operate without any wear and tear. Thus they demonstrate a long working life and at the same a high level of reliability.

A distinction is drawn between the following types of proximity switches:

Reed switches
 
Capacitive proximity switchess
 
Inductive proximity switches
 
Optical proximity switches
 

 

Reed Switches

reed-switches-pneumatic.jpg

Reed switches are proximity switches which are activated magnetically. They are used, for example, in electropneumatic controls to determine the position of the cylinder pistons (initial and/or end position). Reed switches consist of two contact reeds which are enclosed in a miniature glass-tube filled with inert gas. If a magnet approaches the switch, contact between the two reeds is closed and electrical current can flow. The switch functions in this case like an NO contactor. NC like operation is possible by "biasing" the contact reeds with small magnets, so that they are closed in the rest state. Then they are opened by the considerably larger magnets used for switching such models.

Reed switches exhibit very long working lives as well as rapid switching time (around 0.2 ms). They are maintenance-free, but may not be deployed in a region with powerful magnetic fields, as otherwise this may lead to unwanted triggers.

The subsequent animation (left-hand partial image) shows how a reed switch operating as an NO contactor detects the piston position. If a piston with a permanent magnet attached approaches the switch, which is embedded in the cylinder housing, the contacts close and thus the circuit closes too. The symbol to the right is the relevant circuit symbol. (Note: in contrast to the simplified depiction found in the animation, real reed switches have three terminals, of which two serve to supply voltage and the third one supplies a HIGH pulse when the switch is closed).

 

reed-switch-animation.gif

 

Sensors built in the form of reed switches are typically connected to a load via a relay. This has the following reasons:

High-power loads could destroy the sensor due to high current levels
This arrangement allows signals to be used for multiple purposes and vice versa

The following circuit diagram shows an example of how to switch a signal lamp H via a reed switch used in conjunction with a relay K.

 

reed-switch-circuit-relay.jpg

 

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