Using 2-wire proximity sensors

Dear Control Engineering: Is it practical to replace a 3-wire proximity sensor with 2-wire type?

Panasonic Electric Works says it can be done. When using inductive proximity sensors for a control application, it is common to choose a 3-wire dc proximity sensor with a dedicated NPN (ground switching) or PNP (positive switching) control output and bring the output into a PLC input. Choosing between polarities means determining how the common is wired and selecting accordingly. If your machines mix NPN and PNP sensors, there’s an alternative to stocking both types of spares to prevent a line-down situation.

Panasonic suggests that instead of having the control output circuit separated from the power circuit, the 2-wire design puts everything in parallel, which consolidates circuitry into one loop. In a typical 3-wire PNP circuit, the output wire is specific in its polarity so that it will only function on with a 0 V common. With the 2-wire variation, the output operation is along the two power wires in the form of a voltage drop, thus making the sensor free to work with either polarity on the common.

With wiring as the diagram shows, a 2-wire sensor can replace NPN and PNP 3-wire models, just by following the flow of current. This greatly simplifies the usage and replacement of inductive proximity sensors across all applications, the company says.

Learn more and see other diagrams at Panasonic.

Also see the Control Engineering Sensor Channel.

-Edited by Mark T. Hoske, editor in chief, Control Engineering, www.controleng.com.