Sensor Leakage Current Explained for 2-Wire and 3-Wire Sensors

What does leakage current mean on a sensor datasheet?

Leakage current is the current that can still flow through a solid-state switching path when the output is OFF. OFF therefore does not mean a perfect open circuit. A small specified current can be normal; the problem starts when the connected input or load cannot reliably reset with that current present.

This guide covers ordinary electronic proximity and photoelectric switches connected to discrete control inputs. Here, OFF means the electrical output state—not necessarily “no target.” A normally closed sensor can be electrically ON when no object is detected.

Do not confuse output leakage with current leaking to a metal housing or protective earth, moisture across a connector, or the intentional signal in a 4–20 mA loop. Those are different conditions requiring different checks.

Why are 2-wire and 3-wire leakage currents different?

The difference is where the electronics get their power. Wire count helps identify that arrangement, but the actual output circuit remains the authority.

2-wire: operating power and load current share a path

A conventional 2-wire electronic switch is connected in series with its load. Even while OFF, its electronics need energy, so a limited current continues through that shared loop. Replacing a mechanical contact with a 2-wire sensor can therefore change the input’s behavior without changing the number of conductors.

For a concrete example, OMRON lists 0.8 mA maximum leakage for the E2E-X3D1-M1GJ 0.3M DC 2-wire sensor. That is a specification for this exact model, not a universal value for every 2-wire device.

3-wire: distinguish the supply branch from the output branch

A 3-wire sensor has dedicated supply and return conductors plus a switched output. Its normal electronics current belongs to the supply budget; its specified output OFF-state current belongs to the PLC reset check. A separate output does not guarantee zero leakage.

Pepperl+Fuchs publishes the distinction explicitly for its NEN20-18GM50-E2-V1 3-wire PNP sensor: no-load supply current is at most 11 mA, while output OFF-state current is at most 10 µA (0.010 mA). Comparing 11 mA with a PLC’s OFF-current limit would compare the wrong branches. These values do not transfer to another model or suffix.

How do you check whether the PLC input will turn OFF?

Start with the sensor’s maximum leakage and the input module’s guaranteed state-0 region. Use the manufacturer’s voltage and current criteria together with the documented input circuit. A typical leakage value or a general “24 V input” label is not enough.

Check OFF, ON and timing separately

For the ON-state voltage check, the basic budget is supply voltage minus sensor drop minus wiring loss. Use values applicable to the actual load and circuit. Low leakage cannot compensate for insufficient ON voltage or a load below the sensor’s operating requirement.

Likewise, a published input impedance is not automatically a constant resistance near the OFF threshold. Input electronics can contain diodes, protection networks and current conditioning. Use a manufacturer-approved calculation or input current–voltage data before treating I × R as an actual PLC terminal voltage.

Outside guaranteed OFF does not mean guaranteed ON. An input can enter an unspecified region or remain in its previous state. Use the exact thresholds and hysteresis behavior; do not turn a failed OFF-margin check into a predicted output state.

What changes when several sensors share one input?

Check the complete circuit, not just one sensor. Series and parallel arrangements change different parts of the electrical budget and are permissible only where the manufacturers support the exact connection.

Parallel branches can add their OFF currents

Where an approved topology feeds several OFF-state leakage paths into one input, screen with the sum of their maximum currents. Two hypothetical 0.8 mA branches give 0.8 + 0.8 = 1.6 mA. Against the example input’s <1 mA OFF criterion, the combined worst-case current no longer supports guaranteed reset—even if each branch appeared satisfactory alone.

This calculation is an illustration, not permission to parallel these particular products. Branch interaction and pulses can impose further restrictions. Separate PLC inputs with the required logic in the controller also preserve individual sensor status for troubleshooting.

Series connections reduce the available ON voltage

Series electronic switches can accumulate ON-state voltage drops. They can also power one another unevenly and introduce startup pulses. Combining their states in wiring is therefore not equivalent to joining ideal mechanical contacts. A leakage-current check alone cannot validate a series chain.

Should you add a bleeder resistor or change the interface?

A bleeder resistor can help in a manufacturer-approved circuit, but it is not a universal repair. Connected across the input/load, it provides another current path so less leakage is available to hold that input above its reset region.

The trade-off appears when the sensor turns ON: the resistor still draws current and dissipates heat. The design must check resistance tolerance, maximum supply voltage, resistor rating and thermal derating, total output current, mounting, and failure behavior. A value copied from a different PLC manual does not establish those conditions.

Before adding hardware, consider the available options:

• A compatible input: a module explicitly supporting the selected 2-wire sensor’s leakage and load requirements.
• A different sensor output: for example, a suitably specified 3-wire output if another conductor is available and the PNP/NPN arrangement matches.
• An approved interface or bleeder: designed and documented for the actual sensor/load combination.

How can you tell leakage from a wiring fault or a pulse?

First confirm which signal is wrong: the sensor indication, the module’s physical input, or the value used by the PLC program. A forced input, latched variable, inverted logic or incorrect address can imitate a sensor that never turns OFF.

Check the installed load and the correct reference

For a voltage check, use the input terminal and its documented common—not an assumed chassis or earth reference. Keep the intended input/load in the circuit. An open-circuit measurement on a 2-wire sensor changes the circuit you are trying to assess.

A high-impedance voltmeter can show a voltage sustained by very little current. Capacitive coupling can also produce a misleading reading. Neither proves that the input has enough current to be ON, and neither proves a conductor is safe to touch. A low-impedance meter mode can disturb sensitive control electronics; use it only where the circuit and instrument instructions permit it.

Isolate power and prevent unexpected motion before changing wiring. Energized checks should be performed by qualified personnel under the site’s electrical procedure. If a current measurement is needed, it requires the correct series connection, fused input and range: never place a meter in current mode directly across the supply. Meter burden and resolution also matter when measuring small leakage currents.

Separate a steady level from a short event

A longer input filter can reject some short events, but it cannot make excessive steady leakage disappear. It can also suppress legitimate short pulses. Change filtering only after identifying the waveform and the machine’s required response.

What should you confirm before accepting a replacement?

Match the replacement to the input module, not just the old sensor’s voltage, housing and detection distance. Ask for the exact order code and output diagram, maximum OFF-state current, ON-state voltage drop, load-current range, and startup/output-pulse information. Identify the PLC module and channel configuration in the same review.

Then document the result with the intended cable and load: OFF and ON behavior, startup and shutdown, and the relevant extremes of supply, temperature and machine operation. A test demonstrates the conditions actually exercised; it does not replace missing ratings for conditions that were never tested.

The acceptance rule is simple: use the right current specification, keep both states inside the supported sensor–input operating window, and retain enough evidence to repeat the decision when a model, cable, input module or configuration changes.