Reed versus PNP sensors on cylinders

How do reed and PNP sensors work?

Both sensor types detect the ring magnet in the cylinder piston and report its end position. The reed sensor closes a magnetically actuated contact: simple, near potential-free and suitable for DC or AC, but mechanically wearing and prone to bounce. The electronic PNP sensor uses a Hall or magnetoresistive chip, switches without wear and quickly, but needs a DC supply and a third wire for the signal.

Reed or PNP: which fits my application?

For simple, slow queries at a low cycle rate the reed sensor is cheap and sufficient. As soon as high cycle counts, short cycle times or a PLC with defined PNP inputs come into play, the electronic sensor is the more robust choice. Also check the residual current in the off state so that sensitive inputs are not falsely read as high.

• Check the controller's wire count: 2-wire for reed, 3-wire for PNP.
• Match the sensor's switching capacity to the PLC input load.
• On short strokes, watch the switching distance and repeat accuracy.
• An LED status display on the sensor eases commissioning and fault finding.

How do I mount and wire the sensors?

The sensor clamps into the slot of the cylinder profile and is aligned over the ring magnet at the end position. Depending on the cylinder there are different slot forms; the sensor must fit the slot or it will not seat securely. When wiring, mind the polarity and the assignment of positive, negative and signal wire, and bring the moving piston slowly to the end position until the LED switches reliably.

Frequently asked questions