An indicator light that remains faintly lit after the control is switched off can be caused by a small off-state current, capacitive coupling between conductors, an output device’s suppression or diagnostic circuit, or a wiring issue. “Off” at a PLC command does not always mean that absolutely no current can flow through the indicator. The actual cause depends on the output type, wiring, lamp input, and other loads on the circuit.
For an industrial panel, treat a dim glow as a symptom to diagnose, not as proof that a particular product is defective or that the machine is safely de-energized. Check the circuit drawing and the output-module specifications first. A visible lamp alone is not a safety device. Do not add a resistor, capacitor, or other bypass part without approval from the equipment manufacturer or a qualified controls engineer. ONPOW’s AD16 signal-lamp range is one example of a panel indicator form; use the exact model’s voltage and connection information when reviewing a real circuit.
What off-state leakage current means
Leakage current is a small current that can still flow when a switching device is commanded off. It may be part of the way a solid-state output is built, or come from components used for suppression, monitoring, or input sensing. National Instruments describes off-state leakage in two-wire sensors and explains why it can affect some digital input modules. That is a useful reminder that a device’s off-state behavior must be checked against the connected load, not assumed from the word “off.”
An LED indicator can be sensitive to a small current because its light output does not require the same current as many older incandescent lamps. A high-impedance input or indicator may therefore show a faint glow or a measurable voltage when a mechanical contact would appear open. The result can vary with the indicator circuit, output technology, and whether other devices share the wiring path.
Do not confuse this with a normal, fully energized indication. A steady bright lamp, an intermittent flash, and a barely visible glow can point to different conditions. Record what happens and when: after an output command changes, during startup, when another channel switches, or when a nearby load is energized.
Common causes in a control panel
Several mechanisms can look similar from the front of the panel:
- Solid-state output leakage. A transistor or triac output can permit a small off-state current. Some suppression or diagnostic circuits also provide a path through the load.
- Two-wire sensor current. Some two-wire sensors draw operating current through the line even when they are not active. NI notes that this residual current can be incompatible with a digital input module if it exceeds that module’s tolerance.
- Coupling between adjacent conductors. Long parallel cable runs can couple a small voltage into an otherwise high-impedance circuit. This is not the same thing as proving that the output is actively driving the load.
- Shared or incorrect wiring paths. A common connection, wrong supply version, cross-connection, or indicator wired to a different point than the drawing shows can keep the lamp partially energized.
- A load mismatch. The indicator and output may each be within their own ratings yet behave unexpectedly together, particularly when a module expects a minimum load or has an off-state leakage specification.
Rockwell Automation documents a specific example in which leakage from a solid-state PLC output leaves a pilot light dimly illuminated. That does not mean every faint glow has the same cause. It shows why the module’s off-state current and the indicator’s circuit need to be considered as a pair.
A safe diagnostic sequence
Start with the machine schematic and identify the indicator supply, output type, common, and any parallel devices. Confirm that the fitted indicator matches the specified voltage and wiring arrangement. If the panel drawing and actual terminals do not agree, stop and resolve that discrepancy before changing components.
For live measurements, use personnel qualified for the circuit and instruments rated for the installation category and voltage. Follow the machine’s lockout and electrical-safety procedures. Do not expose energized terminals or remove guards merely to obtain a photograph or voltage reading. When the circuit must be inspected for continuity or wiring changes, isolate it and verify the required safe condition using the site’s procedure.
Then distinguish three observations:
- The lamp emits visible light. This is a real optical observation, but it does not quantify current or identify its path.
- A meter shows voltage across an open output. A high-impedance meter can display coupled or residual voltage. That reading by itself does not show how much current is available to the load.
- The receiving input or machine logic reads on. This indicates a functional interface problem that must be checked against the input thresholds and the output’s off-state specification.
Use a documented test method for the exact module and load. If the display indicates a false input, compare the module’s specified off-state leakage and the input’s on/off thresholds rather than relying on a generic internet value. National Instruments explains that a receiving module can misread a two-wire sensor if residual current exceeds what its input is designed to tolerate. That is a system compatibility problem, not a universal threshold for every PLC or indicator.
Troubleshooting table
| Observation | Areas to verify | Appropriate next step |
|---|---|---|
| A faint, steady glow while the output is commanded off | Output-module leakage, suppression path, indicator compatibility | Compare the exact output and indicator documentation; have a qualified technician trace the circuit |
| A brief flash at power-up or during channel switching | Diagnostics, startup state, shared wiring, transient behavior | Review startup logic and module diagnostics; reproduce under controlled commissioning conditions |
| A multimeter shows voltage but the input remains off | Meter impedance, coupling, output state, input threshold | Use the module maker’s approved test method; do not treat voltage alone as proof of a usable source |
| The receiving PLC input reads on unexpectedly | Residual current versus input threshold, common wiring, two-wire sensor path | Check both devices’ specifications and follow the manufacturer’s interface guidance |
| The glow started after a replacement | New part voltage, terminal layout, output technology, wiring changes | Compare the installed part number and terminal diagram with the approved drawing |
The table narrows the investigation. It is not a field wiring procedure and does not prescribe a universal correction. In particular, a “bleeder” component may create heat, consume power, affect output diagnostics, or violate the original design if it is not specified for that circuit.
Why adding a resistor is not an automatic fix
Online discussions often recommend placing a resistor or capacitor across a lamp to consume residual current. That may be a manufacturer-approved solution for some documented combinations, but it is not a safe generic instruction. The required value depends on supply voltage, output behavior, component power and temperature ratings, enclosure conditions, and the equipment’s electrical design. A component can also conceal a wiring fault or change the way a safety or diagnostic channel is monitored.
Ask the output-module manufacturer whether a compatible load or interface module is approved. If a design change is considered, have a qualified engineer calculate the complete circuit and update its drawing, thermal review, and validation record. If the output is part of a safety function, preserve the validated architecture and use the safety-equipment manufacturer’s instructions.
Prevent the issue during design and replacement
Include the indicator model, output type, voltage, terminal arrangement, and off-state behavior in the control-panel specification. When replacing a relay output with a solid-state output, recheck the lamp circuit rather than assuming the old load will behave identically. The same applies when changing from an incandescent indicator to an LED model.
During commissioning, test the commanded off state at startup, after a normal stop, and when adjacent outputs switch. If the lamp has a remote or shared supply, include those conditions too. Record the actual observation, module model, indicator part number, and the manufacturer-approved resolution. This turns a recurring nuisance into a traceable compatibility check.
For panel-light selection, see ONPOW’s indicator-light color guide and 12 V versus 24 V LED indicator comparison. For the effect of ambient conditions on recognition, use the separate panel indicator brightness guide. These are related topics, but none substitutes for the schematic and documentation of the actual machine.
Frequently asked questions
Does a faint glow mean the PLC output is on?
Not necessarily. A small leakage or coupled current may pass through an LED while the logic command is off. Check the module status, receiving input, and circuit documentation. The lamp by itself cannot confirm the state of the PLC or the electrical safety of the circuit.
Can a high-impedance multimeter show voltage when the output is off?
Yes. A high-impedance meter can detect residual or coupled voltage that may not be able to operate a normal load. Follow the instrument and equipment manufacturers’ test procedures; do not infer that a circuit is safe from a single reading.
Are two-wire sensors more likely to have off-state current?
Some two-wire sensors require residual current to keep their electronics operating. The receiving input must tolerate the sensor’s specified current. Check both data sheets; do not apply a generic threshold to unrelated models.
Should I install a resistor across the pilot light?
Only if the output or machine manufacturer approves a specific interface solution for that exact circuit and a qualified engineer verifies its ratings and effects. Do not copy a resistor value from another panel.
Is a dim indicator safe to ignore?
No. A dim glow may be harmless in one documented circuit and a sign of a wiring or interface fault in another. Investigate it, and never use an indicator lamp as the sole proof that a machine is de-energized.
Educational video
Tim Wilborne’s educational video discusses PLC output leakage and the slight glow it can produce. Treat it as background only: the exact off-state current and compatible load must come from the documentation for the installed output module and indicator.
Watch the PLC output leakage-current explanation.
References
- Rockwell Automation: Improving interface reliability by mitigating leakage current
- National Instruments: Choosing the right industrial digital I/O module for a digital output sensor
- National Instruments: Connecting two sinking or two sourcing I/O signals together
