Two-wire sensors: check the load, not just the wiring

A two-wire electronic switching sensor uses the same pair of conductors for its operating power and switching function. In common proximity sensing applications, the device sits in series with the load. It simplifies wiring, but it is not electrically identical to a mechanical contact. This guide covers electronic two-wire proximity switches; other sensors with two leads may use a different interface entirely.

The two specifications most likely to cause an unexpected result are OFF-state leakage current and ON-state voltage drop. There is also a minimum load current to check. A sensor can detect its target correctly while the connected PLC input still reads the wrong state.

Start by identifying the interface

A loop-powered pressure transmitter is not a substitute for a two-wire proximity switch. A NAMUR output should not be assumed compatible with an ordinary 24 V digital input, either. NAMUR describes an interface; hazardous-area suitability depends on the relevant device and system documentation.

Follow the complete series circuit

The power supply, sensor and load share one current path. The sensor obtains its operating energy through that path while changing its conducting state. There is no separate supply pair, but the circuit still needs a power source. Connecting a device intended for series operation straight across the supply without its required load may be an incorrect connection.

Choose between DC two-wire models, AC models and devices specifically rated for AC/DC operation. Their ratings are not interchangeable. Some DC products are polarity-dependent, some are protected against reverse polarity, and others are polarity-independent. Wire colours and the number of leads cannot replace the manufacturer's connection diagram.

Interactive example: compare two different loads

Use Start/Stop to control the animation and select either 2.2 kΩ or 22 kΩ. The illustrative circuit is a 24 V DC, normally open sensor with an assumed 2 V ON drop and 0.5 mA OFF current. These are teaching values, not specifications for a particular product.

The moving marks show the current path; their speed is not a calibrated current scale. For this resistive model, ON current is approximately (24 − 2) / R and OFF load voltage is approximately 0.0005 × R. Real sensor and PLC input characteristics may be nonlinear. Minimum operating current and input thresholds must still be checked.

Why an OFF sensor can leave voltage across the load

An electronic inductive or capacitive switch may need a small current to keep its sensing circuitry alive. That current also passes through the load. For an approximately resistive receiver, V_load,OFF ≈ I_leakage × R_load. Increasing the receiving impedance can therefore increase the OFF-state voltage.

In the example, 0.5 mA produces 1.1 V across 2.2 kΩ but 11 V across 22 kΩ. Neither figure alone proves what a PLC will read. Compare the sensor current with the input's specified OFF limits and ON thresholds. An optocoupler input cannot always be represented by one fixed resistor. Leakage may also produce a faint LED glow or prevent an electronic receiver from resetting.

High impedance does not automatically make every two-wire combination unusable. Low-leakage products and compatible inputs can work together. Equally, a relay or other coil load is not automatically immune: pickup, release and inductive-load requirements still matter. Leakage is not exclusive to two-wire products; three-wire electronic outputs can have an OFF-state current too.

A valid ON state needs enough voltage and current

When conducting, the sensor retains a voltage across its electronics. The load receives approximately V_supply − V_sensor,ON − V_wiring. A low supply setting or additional cable drop can leave the receiver below its operating threshold. Select the control-panel power supply as part of the complete circuit.

Check both minimum load current and maximum switching current. In the example, a 22 kΩ load would draw only 1 mA with 22 V across it. If the actual sensor requires a higher minimum current, the idealised ON calculation does not establish stable operation. The same principle applies to two-wire capacitive sensors.

How this differs from a three-wire sensor

A common three-wire DC sensor has two supply conductors and a separate output conductor. PNP and NPN describe the current-switching arrangement. A two-wire switch shares its supply and load path. If a manufacturer uses PNP/NPN or positive/negative switching terminology for a two-wire product, read its specific circuit diagram rather than transferring a three-wire wiring convention.

NO and NC describe a separate property. Under normal powered operation, a normally open output activates on detection; a normally closed output deactivates. DC NO products and DC NC products therefore behave differently. A fourth conductor is not a universal polarity or logic selector; it may provide a second output, a setting input or another product-specific function.

Dealing with leakage without creating another problem

First compare leakage current, ON drop and the permitted load-current range with the receiver specifications. Depending on the application, a lower-leakage model, a suitable interface module or a three-wire sensor may be appropriate. Where the manufacturer permits it, a bleeder resistor in parallel with the load provides another path for OFF-state current.

The resistor is not an arbitrary accessory. In a simplified DC model, it changes the receiving resistance to R_input ∥ R_bleeder, while its ON-state dissipation is approximately P = V_load² / R_bleeder. It also increases the current the sensor must carry when ON. Choose it using voltage, thermal and current limits. A value selected for a 24 V DC example is not a recommendation for an AC mains circuit.

Multiple sensors and commissioning checks

Putting two-wire switches in series can accumulate voltage drops and affect the power-up behaviour of downstream devices. In a parallel arrangement, OFF currents can add. Suitability depends on the manufacturer's limits, timing and load characteristics, rather than internal resistance alone. Separate PLC inputs usually make multiple conditions easier to inspect and diagnose.

Verify target distance, mounting, switching/counting frequency, IP protection and the fixed-cable or connector arrangement. If the sensor indicator changes but the PLC state does not, examine polarity, pin assignments, supply conditions and the receiver's OFF/ON requirements before condemning the sensor.

This illustration shows one possible internal arrangement. It does not establish that every two-wire sensor uses the same zener, MOSFET or power circuit.