Residual Voltage in Sensors: Meaning and PLC Input Checks
Residual voltage is the ON-state drop across a conducting sensor output. It leaves less voltage for the connected load. To judge compatibility, check the receiver's voltage and current requirements at the lowest supply, then check OFF-state leakage separately. A “24 V” label or illuminated sensor LED is not enough.
Identify the sensor output, the receiving input and their reference terminals before interpreting a voltage reading.
What does residual voltage mean?
An ideal closed switch would drop no voltage. A real semiconductor output has a conducting voltage drop, specified under particular load and operating conditions. OMRON calls this residual voltage. Datasheets may instead use ON-state voltage drop or specify output HIGH/LOW levels.
The practical question is how much voltage remains across the receiver. With a 24 V supply and a 2 V sensor drop, an otherwise ideal series load path has 22 V across the load. The output terminal's voltage relative to 0 V, however, depends on whether the sensor sources or sinks current.
| Parameter | When it applies | What it can affect |
|---|---|---|
| Residual voltage · V | Output ON and carrying load current. | Whether enough voltage reaches the load to operate. |
| Leakage current · mA or µA | Output OFF, with the specified circuit connected. | Whether the receiving input or load can reset. |
Here, “residual” does not mean voltage left after the supply is disconnected. An OFF-state reading is a different investigation. OMRON's output terminology separates these ON and OFF effects. If an input stays ON after the sensor output turns OFF, follow the leakage-current checks, rather than treating the reading as excessive ON-state drop.
PNP and NPN need different voltage reference points
A voltage always exists between two points. Pepperl+Fuchs' three-wire connection guide places the PNP load between output and L−, and the NPN load between L+ and output. Here, L+ is the positive supply and L− is the 0 V return, not automatically protective earth.
PNP: the ON output is near L+
For a conventional sourcing output, measure its ON-state drop from the sensor's L+ terminal to its output. Measure the receiving voltage from the input terminal to its designated return/common.
L+ · 24 V
Output · 22 V relative to 0 V
L− · 0 V
NPN: the ON output is near 0 V
For a conventional sinking output, measure its ON-state drop from output to the sensor's L− terminal. The load is above that output in the current path: a low output-to-0 V reading can be the expected ON state.
L+ · 24 V
Output · 2 V relative to 0 V
L− · 0 V
Both drawings assume an ON output, a common 24 V supply and no cable loss. They show voltage allocation, not terminal wiring, scale or a product rating. Use the exact pinout and PNP/NPN input-common arrangement for implementation.
Two-wire sensors share the power and load-current path
For an ordinary DC two-wire switching sensor, measure the ON drop across its two leads with the intended load connected. Its electronics obtain operating energy through that same series path. Pepperl+Fuchs explains why this creates both ON-state voltage loss and OFF-state current.
This is why a two-wire replacement needs more than matching voltage and NO/NC logic: its minimum load current and OFF leakage must suit the input. A push-pull output needs its specified HIGH/LOW limits instead; do not apply the open-collector examples without checking its circuit.
Check voltage, current and OFF reset together
There is no universal acceptable residual voltage. What matters is the remaining voltage and current at the actual receiving device, under the conditions for which the manufacturer guarantees operation.
Minimum load voltage = minimum supply − maximum sensor drop − remaining path losses
Use one consistent circuit boundary. If the supply is already specified at the field terminals, do not subtract upstream cable loss again. Include the relevant outgoing and return-path losses that remain inside your boundary.
Compare the result with the input's guaranteed ON region, not its nominal “24 V” label or one observed switching point. Being above an OFF limit does not, by itself, establish an ON state. Then check the resulting input current against both devices' limits: minimum-current requirements at the lowest supply, and maximum-current limits at the highest relevant supply.
An input characteristic may be nonlinear; use its documented current-versus-voltage data or circuit model rather than assuming a fixed resistance.
A documented two-wire example checks all three conditions
The OMRON E2E catalog, page 19, uses E2E-X7D1-N with a reference PLC input. This is a legacy catalog example, not a current-model recommendation or an xsz sensor rating.
| Check | Catalog calculation | Interpretation |
|---|---|---|
| ON voltage | 20.4 V supply − 3 V sensor drop = 17.4 V | Above the reference input's 14.4 V ON requirement. |
| ON current | (20.4 − 3 − 4) V ÷ 3 kΩ ≈ 4.5 mA | Within the example sensor's 3–100 mA control-output range. |
| OFF reset | 0.8 mA sensor leakage < 1.3 mA input OFF-current limit | Meets the example's OFF-current condition. |
The 4 V term belongs to the PLC's internal input model; it is not another sensor drop. It is used to calculate current, not subtracted again from 17.4 V when comparing the PLC terminal voltage with its ON requirement. The reference PLC values cannot be assumed for an unidentified module.
Why a voltage-only replacement check can be misleading
Illustrative scenario: a proposed two-wire sensor requires at least 3 mA while ON. The connected input receives 21 V and draws only 2 mA at that voltage. Assume those values satisfy the input's own ON conditions.
The PLC side appears satisfactory, but 2 mA is below the sensor's 3 mA minimum. The combination is outside the sensor's stated operating conditions. It does not become a valid match just because the measured voltage looks healthy.
Choose a compatible sensor/input combination or a manufacturer-approved interface. See minimum and maximum sensor load current before adding a load merely to make one trial work.
When comparing alternatives: obtain the maximum drop at the relevant current and temperature, not just a typical value. Check whether a supplied cable is included in the rating. Lower drop helps the voltage budget, but does not establish output polarity, current capacity, sensing performance or replacement suitability.
Series and parallel connections change different limits
Use these calculations only for a connection the manufacturer permits. OMRON's connection guidance distinguishes two-wire and three-wire arrangements and includes startup/current restrictions as well as steady-state limits.
Series: conducting drops consume the voltage budget
In a permitted series load path, add the applicable ON drops. Illustrative calculation: three 3 V drops leave 20.4 − 9 = 11.4 V before wiring losses. That does not meet a load requirement of 15 V.
Do not interpret this as permission to chain arbitrary sensors. A three-wire arrangement that switches another sensor's supply also raises operating-current and power-up behavior questions.
Parallel: OFF leakage can accumulate
If three permitted branches can each leak 0.8 mA into one input, their worst-case total is 2.4 mA. Compare the resulting OFF condition with the receiver's reset limits.
Outputs are not automatically safe to join. Keep separate PLC inputs and perform logic in the controller where appropriate, or use an expressly supported combining interface. A push-pull output especially must not be treated as a dry contact.
Measure the loaded circuit, then follow the missing voltage
For this check, use a stable condition that commands the electrical output ON. Target presence alone is not enough: a normally closed output may turn OFF when the target arrives. Keep the intended receiving load connected.
Plan the test safely. Prevent unintended machine motion and isolate hazardous energy before changing wiring or instrument connections. Any energized observations require qualified personnel, suitable rated equipment and the machine's approved procedure. This guide covers ordinary DC process-sensor interfaces, not mains, hazardous-area or personnel-protection validation.
- Identify the two reference points. Record the sensor suffix, output type, receiver terminals and input-group common. Use the PNP, NPN or two-wire measurement pair described above, not an assumed wire color.
- Record the supply and sensor drop under load. Note where the supply was measured and the actual output state. A cabinet reading and a field-device reading have different wiring losses inside their boundaries.
- Measure across the receiving input. If its voltage is lower than the supply minus sensor drop predicts, investigate the remaining cable, connector, interface and common-return path. Do not assign all missing voltage to the sensor.
- Check current and repeat the OFF-state test. Use the manufacturer's method and relevant input characteristic. Compare ON operation and OFF reset separately, including supply and temperature conditions that matter to the installation.
If a series current measurement is required, the meter must have a suitable fused range and resolution, and its burden can alter a low-current circuit. Follow its instructions: remove power before inserting it in the branch. Never put current-mode probes across the supply or sensor output. These precautions are illustrated in the Fluke 3000 FC manual's current-measurement section.
Why an unloaded meter reading can mislead
A high-impedance meter draws very little current. It can therefore display a voltage without showing whether the source can drive the intended input. Fluke's impedance explanation describes capacitively coupled “ghost” readings and warns that low-impedance instruments can disturb sensitive circuits.
For a sensor, OFF leakage, the connected input and reference wiring also matter. Do not assume every unexpected reading is harmless ghost voltage, or switch to LoZ as a universal sensor test. Use the specified load and measuring method. A steady multimeter value also cannot establish the shape or duration of a brief switching pulse.
What should change if the PLC does not follow the sensor?
The LED is a clue, not a measurement of receiver compatibility. Use observations that separate an output limitation from wiring loss, input configuration or a sensing problem.
| Observation | What it suggests checking | Useful next step |
|---|---|---|
| ON drop exceeds the model's limit at a permitted load | Measurement points, actual current, supply and temperature; possible output fault. | Confirm like-for-like test conditions before condemning the sensor. |
| Sensor drop is within limits, but receiver voltage is too low | Supply loss or additional drop outside the measured output stage. | Measure the remaining loaded path, including the common return. |
| Receiver meets steady-state electrical limits, but counts are missing | Input assignment, pulse duration, filtering and program capture. | Compare the terminal signal with the controller's actual input behavior. |
| Output is OFF, but the input will not reset | Leakage, joined branches, input reset limits or unintended current paths. | Run the separate OFF-state check; do not call it ON-state residual voltage. |
If the output itself changes as the target moves or vibrates, also check sensing distance, target and mounting stability. The proximity-sensor troubleshooting guide covers that broader investigation.
A resistor is not a general cure for excessive ON-state drop
A manufacturer-approved load resistor may address minimum-current or OFF-leakage behavior, but it changes current and heat dissipation. It does not restore missing supply voltage. Its effect on output drop, load rating and diagnostics must be evaluated; do not choose one from an unloaded meter reading.
The useful selection rule: first match the output and common arrangement; then check delivered ON voltage, the operating-current window and OFF reset. Before ordering, retain the sensor and input datasheets, the relevant circuit, and the supply/load conditions used in that comparison.
For an xsz sensor compatibility enquiry, share the full sensor and PLC input models, wiring diagram, minimum field supply and intended load. If troubleshooting, add the ON/OFF state and the exact two points used for each voltage reading.