{"id":3407,"date":"2026-09-27T08:00:00","date_gmt":"2026-09-27T08:00:00","guid":{"rendered":"https:\/\/onpowbutton.com\/news\/indicator-light-leakage-current\/"},"modified":"2026-09-26T06:35:32","modified_gmt":"2026-09-26T06:35:32","slug":"indicator-light-leakage-current","status":"publish","type":"post","link":"https:\/\/onpowbutton.com\/pt\/news\/indicator-light-leakage-current\/","title":{"rendered":"Indicator Light Leakage Current: Why LEDs Glow When Off"},"content":{"rendered":"<p>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\u2019s suppression or diagnostic circuit, or a wiring issue. \u201cOff\u201d 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.<\/p>\n<p>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\u2019s <a href=\"https:\/\/onpowbutton.com\/ad16-series-product\/\">AD16 signal-lamp range<\/a> is one example of a panel indicator form; use the exact model\u2019s voltage and connection information when reviewing a real circuit.<\/p>\n<nav aria-label=\"Article contents\" class=\"b2b-toc\" style=\"background:#f5f8fa;padding:16px 20px;border-radius:8px;margin:24px 0\"><strong>Contents<\/strong><\/p>\n<ul>\n<li><a href=\"#what-off-state-leakage-current-means\">What off-state leakage current means<\/a><\/li>\n<li><a href=\"#common-causes-in-a-control-panel\">Common causes in a control panel<\/a><\/li>\n<li><a href=\"#a-safe-diagnostic-sequence\">A safe diagnostic sequence<\/a><\/li>\n<li><a href=\"#troubleshooting-table\">Troubleshooting table<\/a><\/li>\n<li><a href=\"#why-adding-a-resistor-is-not-an-automatic-fix\">Why adding a resistor is not an automatic fix<\/a><\/li>\n<li><a href=\"#prevent-the-issue-during-design-and-replacement\">Prevent the issue during design and replacement<\/a><\/li>\n<li><a href=\"#frequently-asked-questions\">Frequently asked questions<\/a><\/li>\n<li><a href=\"#educational-video\">Educational video<\/a><\/li>\n<li><a href=\"#related-onpow-products-and-guides\">Related ONPOW products and guides<\/a><\/li>\n<\/ul>\n<\/nav>\n<h2 id=\"what-off-state-leakage-current-means\">What off-state leakage current means<\/h2>\n<p>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\u2019s off-state behavior must be checked against the connected load, not assumed from the word \u201coff.\u201d<\/p>\n<p>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.<\/p>\n<p>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.<\/p>\n<h2 id=\"common-causes-in-a-control-panel\">Common causes in a control panel<\/h2>\n<p>Several mechanisms can look similar from the front of the panel:<\/p>\n<ul>\n<li><strong>Solid-state output leakage.<\/strong> A transistor or triac output can permit a small off-state current. Some suppression or diagnostic circuits also provide a path through the load.<\/li>\n<li><strong>Two-wire sensor current.<\/strong> 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\u2019s tolerance.<\/li>\n<li><strong>Coupling between adjacent conductors.<\/strong> 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.<\/li>\n<li><strong>Shared or incorrect wiring paths.<\/strong> 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.<\/li>\n<li><strong>A load mismatch.<\/strong> 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.<\/li>\n<\/ul>\n<p>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\u2019s off-state current and the indicator\u2019s circuit need to be considered as a pair.<\/p>\n<figure class=\"wp-block-image size-large\" style=\"margin:28px auto;text-align:center\"><img decoding=\"async\" alt=\"Red AD16-style cylindrical signal lamp mounted on an indoor industrial cabinet\" loading=\"lazy\" src=\"https:\/\/onpowbutton.com\/wp-content\/uploads\/2026\/09\/row78-body-image-1.png\" style=\"display:block;width:100%;max-width:900px;height:auto;margin:0 auto\" title=\"Panel signal lamp in an industrial control cabinet\"\/><figcaption class=\"wp-element-caption\">A faint glow should be diagnosed against the output and indicator specifications, not assumed to mean a product fault.<\/figcaption><\/figure>\n<h2 id=\"a-safe-diagnostic-sequence\">A safe diagnostic sequence<\/h2>\n<p>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.<\/p>\n<p>For live measurements, use personnel qualified for the circuit and instruments rated for the installation category and voltage. Follow the machine\u2019s 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\u2019s procedure.<\/p>\n<p>Then distinguish three observations:<\/p>\n<ol>\n<li><strong>The lamp emits visible light.<\/strong> This is a real optical observation, but it does not quantify current or identify its path.<\/li>\n<li><strong>A meter shows voltage across an open output.<\/strong> 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.<\/li>\n<li><strong>The receiving input or machine logic reads on.<\/strong> This indicates a functional interface problem that must be checked against the input thresholds and the output\u2019s off-state specification.<\/li>\n<\/ol>\n<p>Use a documented test method for the exact module and load. If the display indicates a false input, compare the module\u2019s specified off-state leakage and the input\u2019s 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.<\/p>\n<h2 id=\"troubleshooting-table\">Troubleshooting table<\/h2>\n<div style=\"overflow-x:auto;margin:20px 0\">\n<table style=\"width:100%;border-collapse:collapse;min-width:660px\">\n<thead>\n<tr>\n<th style=\"border:1px solid #d9e1e8;padding:9px 12px;background:#f5f8fa;text-align:left\">Observation<\/th>\n<th style=\"border:1px solid #d9e1e8;padding:9px 12px;background:#f5f8fa;text-align:left\">Areas to verify<\/th>\n<th style=\"border:1px solid #d9e1e8;padding:9px 12px;background:#f5f8fa;text-align:left\">Appropriate next step<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td style=\"border:1px solid #d9e1e8;padding:9px 12px;vertical-align:top\">A faint, steady glow while the output is commanded off<\/td>\n<td style=\"border:1px solid #d9e1e8;padding:9px 12px;vertical-align:top\">Output-module leakage, suppression path, indicator compatibility<\/td>\n<td style=\"border:1px solid #d9e1e8;padding:9px 12px;vertical-align:top\">Compare the exact output and indicator documentation; have a qualified technician trace the circuit<\/td>\n<\/tr>\n<tr>\n<td style=\"border:1px solid #d9e1e8;padding:9px 12px;vertical-align:top\">A brief flash at power-up or during channel switching<\/td>\n<td style=\"border:1px solid #d9e1e8;padding:9px 12px;vertical-align:top\">Diagnostics, startup state, shared wiring, transient behavior<\/td>\n<td style=\"border:1px solid #d9e1e8;padding:9px 12px;vertical-align:top\">Review startup logic and module diagnostics; reproduce under controlled commissioning conditions<\/td>\n<\/tr>\n<tr>\n<td style=\"border:1px solid #d9e1e8;padding:9px 12px;vertical-align:top\">A multimeter shows voltage but the input remains off<\/td>\n<td style=\"border:1px solid #d9e1e8;padding:9px 12px;vertical-align:top\">Meter impedance, coupling, output state, input threshold<\/td>\n<td style=\"border:1px solid #d9e1e8;padding:9px 12px;vertical-align:top\">Use the module maker\u2019s approved test method; do not treat voltage alone as proof of a usable source<\/td>\n<\/tr>\n<tr>\n<td style=\"border:1px solid #d9e1e8;padding:9px 12px;vertical-align:top\">The receiving PLC input reads on unexpectedly<\/td>\n<td style=\"border:1px solid #d9e1e8;padding:9px 12px;vertical-align:top\">Residual current versus input threshold, common wiring, two-wire sensor path<\/td>\n<td style=\"border:1px solid #d9e1e8;padding:9px 12px;vertical-align:top\">Check both devices\u2019 specifications and follow the manufacturer\u2019s interface guidance<\/td>\n<\/tr>\n<tr>\n<td style=\"border:1px solid #d9e1e8;padding:9px 12px;vertical-align:top\">The glow started after a replacement<\/td>\n<td style=\"border:1px solid #d9e1e8;padding:9px 12px;vertical-align:top\">New part voltage, terminal layout, output technology, wiring changes<\/td>\n<td style=\"border:1px solid #d9e1e8;padding:9px 12px;vertical-align:top\">Compare the installed part number and terminal diagram with the approved drawing<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>The table narrows the investigation. It is not a field wiring procedure and does not prescribe a universal correction. In particular, a \u201cbleeder\u201d component may create heat, consume power, affect output diagnostics, or violate the original design if it is not specified for that circuit.<\/p>\n<h2 id=\"why-adding-a-resistor-is-not-an-automatic-fix\">Why adding a resistor is not an automatic fix<\/h2>\n<p>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\u2019s electrical design. A component can also conceal a wiring fault or change the way a safety or diagnostic channel is monitored.<\/p>\n<p>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\u2019s instructions.<\/p>\n<figure class=\"wp-block-image size-large\" style=\"margin:28px auto;text-align:center\"><img decoding=\"async\" alt=\"AD16-style red signal lamp with its cylindrical rear body and nearby control wiring\" loading=\"lazy\" src=\"https:\/\/onpowbutton.com\/wp-content\/uploads\/2026\/09\/row78-body-image-2.png\" style=\"display:block;width:100%;max-width:900px;height:auto;margin:0 auto\" title=\"Signal lamp and control-output interface\"\/><figcaption class=\"wp-element-caption\">Check the complete indicator and output circuit, including the exact rear connection and off-state behavior.<\/figcaption><\/figure>\n<h2 id=\"prevent-the-issue-during-design-and-replacement\">Prevent the issue during design and replacement<\/h2>\n<p>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.<\/p>\n<p>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.<\/p>\n<p>For panel-light selection, see ONPOW\u2019s <a href=\"https:\/\/onpowbutton.com\/news\/industrial-indicator-light-colors\/\">indicator-light color guide<\/a> and <a href=\"https:\/\/onpowbutton.com\/news\/12v-vs-24v-led-indicator-lights\/\">12 V versus 24 V LED indicator comparison<\/a>. For the effect of ambient conditions on recognition, use the separate <a href=\"https:\/\/onpowbutton.com\/news\/panel-indicator-light-brightness\/\">panel indicator brightness guide<\/a>. These are related topics, but none substitutes for the schematic and documentation of the actual machine.<\/p>\n<h2 id=\"frequently-asked-questions\">Frequently asked questions<\/h2>\n<h3 id=\"does-a-faint-glow-mean-the-plc-output-is-on\">Does a faint glow mean the PLC output is on?<\/h3>\n<p>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.<\/p>\n<h3 id=\"can-a-high-impedance-multimeter-show-voltage-when-the-output-is-off\">Can a high-impedance multimeter show voltage when the output is off?<\/h3>\n<p>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\u2019 test procedures; do not infer that a circuit is safe from a single reading.<\/p>\n<h3 id=\"are-two-wire-sensors-more-likely-to-have-off-state-current\">Are two-wire sensors more likely to have off-state current?<\/h3>\n<p>Some two-wire sensors require residual current to keep their electronics operating. The receiving input must tolerate the sensor\u2019s specified current. Check both data sheets; do not apply a generic threshold to unrelated models.<\/p>\n<h3 id=\"should-i-install-a-resistor-across-the-pilot-light\">Should I install a resistor across the pilot light?<\/h3>\n<p>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.<\/p>\n<h3 id=\"is-a-dim-indicator-safe-to-ignore\">Is a dim indicator safe to ignore?<\/h3>\n<p>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.<\/p>\n<h2 id=\"educational-video\">Educational video<\/h2>\n<p>Tim Wilborne\u2019s 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.<\/p>\n<div style=\"position:relative;padding-bottom:56.25%;height:0;overflow:hidden;max-width:100%\"><iframe allow=\"accelerometer; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share\" allowfullscreen=\"\" loading=\"lazy\" src=\"https:\/\/www.youtube-nocookie.com\/embed\/b4tfs78maEA\" style=\"position:absolute;top:0;left:0;width:100%;height:100%;border:0\" title=\"PLC Output Leakage Current. Do your PLC outputs glow slightly? by Tim Wilborne\"><\/iframe><\/div>\n<p><a href=\"https:\/\/www.youtube.com\/watch?v=b4tfs78maEA\" rel=\"noopener\" target=\"_blank\">Watch the PLC output leakage-current explanation<\/a>.<\/p>\n<h2 id=\"references\">References<\/h2>\n<ul>\n<li><a href=\"https:\/\/literature.rockwellautomation.com\/idc\/groups\/literature\/documents\/at\/800-at001_-en-p.pdf\" rel=\"noopener\" target=\"_blank\">Rockwell Automation: Improving interface reliability by mitigating leakage current<\/a><\/li>\n<li><a href=\"https:\/\/www.ni.com\/en\/shop\/data-acquisition\/sensor-fundamentals\/choosing-the-right-industrial-digital-i-o-module-for-your-digita.html\" rel=\"noopener\" target=\"_blank\">National Instruments: Choosing the right industrial digital I\/O module for a digital output sensor<\/a><\/li>\n<li><a href=\"https:\/\/knowledge.ni.com\/KnowledgeArticleDetails?id=kA03q000000YI7ICAW&amp;l=en-US\" rel=\"noopener\" target=\"_blank\">National Instruments: Connecting two sinking or two sourcing I\/O signals together<\/a><\/li>\n<\/ul>\n<h2 id=\"related-onpow-products-and-guides\">Related ONPOW products and guides<\/h2>\n<ul>\n<li><a href=\"https:\/\/onpowbutton.com\/ad16-series-product\/\">AD16 series signal lamps<\/a><\/li>\n<li><a href=\"https:\/\/onpowbutton.com\/metal-signal-lamp\/\">Metal signal lamp product range<\/a><\/li>\n<li><a href=\"https:\/\/onpowbutton.com\/news\/panel-indicator-light-brightness\/\">Panel indicator light brightness guide<\/a><\/li>\n<li><a href=\"https:\/\/onpowbutton.com\/news\/industrial-indicator-light-colors\/\">Industrial indicator-light color meanings<\/a><\/li>\n<li><a href=\"https:\/\/onpowbutton.com\/news\/12v-vs-24v-led-indicator-lights\/\">12 V versus 24 V LED indicator lights<\/a><\/li>\n<\/ul>\n<p><script type=\"application\/ld+json\">{\"@context\":\"https:\/\/schema.org\",\"@type\":\"FAQPage\",\"mainEntity\":[{\"@type\":\"Question\",\"name\":\"Does a faint glow mean the PLC output is on?\",\"acceptedAnswer\":{\"@type\":\"Answer\",\"text\":\"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.\"}},{\"@type\":\"Question\",\"name\":\"Can a high-impedance multimeter show voltage when the output is off?\",\"acceptedAnswer\":{\"@type\":\"Answer\",\"text\":\"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\u2019 test procedures; do not infer that a circuit is safe from a single reading.\"}},{\"@type\":\"Question\",\"name\":\"Are two-wire sensors more likely to have off-state current?\",\"acceptedAnswer\":{\"@type\":\"Answer\",\"text\":\"Some two-wire sensors require residual current to keep their electronics operating. The receiving input must tolerate the sensor\u2019s specified current. 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Investigate it, and never use an indicator lamp as the sole proof that a machine is de-energized.\"}}]}<\/script><\/p>","protected":false},"excerpt":{"rendered":"<p>Learn why an industrial LED indicator may glow when off, how PLC leakage and wiring can contribute, and how to diagnose it safely.<\/p>","protected":false},"author":6,"featured_media":3384,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_uag_custom_page_level_css":"","footnotes":""},"categories":[1],"tags":[],"class_list":["post-3407","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-blog"],"uagb_featured_image_src":{"full":["https:\/\/onpowbutton.com\/wp-content\/uploads\/2026\/09\/row78-feature.webp",1672,941,false],"thumbnail":["https:\/\/onpowbutton.com\/wp-content\/uploads\/2026\/09\/row78-feature-150x150.webp",150,150,true],"medium":["https:\/\/onpowbutton.com\/wp-content\/uploads\/2026\/09\/row78-feature-300x169.webp",300,169,true],"medium_large":["https:\/\/onpowbutton.com\/wp-content\/uploads\/2026\/09\/row78-feature-768x432.webp",768,432,true],"large":["https:\/\/onpowbutton.com\/wp-content\/uploads\/2026\/09\/row78-feature-1024x576.webp",1024,576,true],"1536x1536":["https:\/\/onpowbutton.com\/wp-content\/uploads\/2026\/09\/row78-feature-1536x864.webp",1536,864,true],"2048x2048":["https:\/\/onpowbutton.com\/wp-content\/uploads\/2026\/09\/row78-feature.webp",1672,941,false],"trp-custom-language-flag":["https:\/\/onpowbutton.com\/wp-content\/uploads\/2026\/09\/row78-feature-18x10.webp",18,10,true]},"uagb_author_info":{"display_name":"Turbo","author_link":"https:\/\/onpowbutton.com\/pt\/author\/turbo\/"},"uagb_comment_info":0,"uagb_excerpt":"Learn why an industrial LED indicator may glow when off, how PLC leakage and wiring can contribute, and how to diagnose it safely.","_links":{"self":[{"href":"https:\/\/onpowbutton.com\/pt\/wp-json\/wp\/v2\/posts\/3407","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/onpowbutton.com\/pt\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/onpowbutton.com\/pt\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/onpowbutton.com\/pt\/wp-json\/wp\/v2\/users\/6"}],"replies":[{"embeddable":true,"href":"https:\/\/onpowbutton.com\/pt\/wp-json\/wp\/v2\/comments?post=3407"}],"version-history":[{"count":1,"href":"https:\/\/onpowbutton.com\/pt\/wp-json\/wp\/v2\/posts\/3407\/revisions"}],"predecessor-version":[{"id":3413,"href":"https:\/\/onpowbutton.com\/pt\/wp-json\/wp\/v2\/posts\/3407\/revisions\/3413"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/onpowbutton.com\/pt\/wp-json\/wp\/v2\/media\/3384"}],"wp:attachment":[{"href":"https:\/\/onpowbutton.com\/pt\/wp-json\/wp\/v2\/media?parent=3407"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/onpowbutton.com\/pt\/wp-json\/wp\/v2\/categories?post=3407"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/onpowbutton.com\/pt\/wp-json\/wp\/v2\/tags?post=3407"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}