{"id":20782,"date":"2026-11-04T10:14:17","date_gmt":"2026-11-04T02:14:17","guid":{"rendered":"https:\/\/www.wehopower.com\/news\/two-wire-sensor-off-plc-input-still-on-leakage\/"},"modified":"2026-11-04T10:14:17","modified_gmt":"2026-11-04T02:14:17","slug":"two-wire-sensor-off-plc-input-still-on-leakage","status":"publish","type":"news","link":"https:\/\/www.wehopower.com\/ko\/news\/two-wire-sensor-off-plc-input-still-on-leakage\/","title":{"rendered":"Two-Wire Sensor Off but PLC Input Still On: Check Leakage and Thresholds"},"content":{"rendered":"<div class=\"weho-geo-content\">\n<p><strong>Direct answer:<\/strong> A two-wire sensor can draw a small current even when it is OFF. If that current keeps the PLC input above its guaranteed OFF condition, the input may remain ON. Compare the sensor\u2019s maximum OFF-state leakage with the actual input\u2019s documented OFF-current and voltage limits, then separately check ON-state voltage and current. A sensor indicator alone does not prove what the PLC input receives.<\/p>\n<figure><a href=\"https:\/\/www.wehopower.com\/ko\/product\/ndr-dinrail-240w-ac-to-dc-single-output-switching-power-supply\/\" aria-label=\"View WEHO NDR-240-24 product details\"><img fetchpriority=\"high\" decoding=\"async\" src=\"https:\/\/www.wehopower.com\/wp-content\/uploads\/2026\/11\/product-scene-approved-2026-10-28.png\" alt=\"Two-Wire Sensor Off but PLC Input Still On: Check Leakage and Thresholds  title=\" width=\"1672\" height=\"941\" two-wire sensor off but plc input still on: check leakage and thresholds title=\"\"><\/a><figcaption>Application illustration; not a customer installation photo. Scene created with AI assistance.<\/figcaption><\/figure>\n<div id=\"ez-toc-container\" class=\"ez-toc-v2_0_86 counter-hierarchy ez-toc-counter ez-toc-grey ez-toc-container-direction\">\n<div class=\"ez-toc-title-container\">\n<p class=\"ez-toc-title\" style=\"cursor:inherit\">\ubaa9\ucc28<\/p>\n<span class=\"ez-toc-title-toggle\"><a href=\"#\" class=\"ez-toc-pull-right ez-toc-btn ez-toc-btn-xs ez-toc-btn-default ez-toc-toggle\" aria-label=\"\ubaa9\ucc28 \ud1a0\uae00\"><span class=\"ez-toc-js-icon-con\"><span class=\"\"><span class=\"eztoc-hide\" style=\"display:none;\">\ube44\ub140\uc7a5<\/span><span class=\"ez-toc-icon-toggle-span\"><svg style=\"fill: #999;color:#999\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" class=\"list-377408\" width=\"20px\" height=\"20px\" viewbox=\"0 0 24 24\" fill=\"none\"><path d=\"M6 6H4v2h2V6zm14 0H8v2h12V6zM4 11h2v2H4v-2zm16 0H8v2h12v-2zM4 16h2v2H4v-2zm16 0H8v2h12v-2z\" fill=\"currentColor\"><\/path><\/svg><svg style=\"fill: #999;color:#999\" class=\"arrow-unsorted-368013\" xmlns=\"http:\/\/www.w3.org\/2000\/svg\" width=\"10px\" height=\"10px\" viewbox=\"0 0 24 24\" version=\"1.2\" baseprofile=\"tiny\"><path d=\"M18.2 9.3l-6.2-6.3-6.2 6.3c-.2.2-.3.4-.3.7s.1.5.3.7c.2.2.4.3.7.3h11c.3 0 .5-.1.7-.3.2-.2.3-.5.3-.7s-.1-.5-.3-.7zM5.8 14.7l6.2 6.3 6.2-6.3c.2-.2.3-.5.3-.7s-.1-.5-.3-.7c-.2-.2-.4-.3-.7-.3h-11c-.3 0-.5.1-.7.3-.2.2-.3.5-.3.7s.1.5.3.7z\"\/><\/svg><\/span><\/span><\/span><\/a><\/span><\/div>\n<nav><ul class='ez-toc-list ez-toc-list-level-1' ><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-1\" href=\"https:\/\/www.wehopower.com\/ko\/news\/two-wire-sensor-off-plc-input-still-on-leakage\/#Why_can_the_sensor_be_OFF_while_the_PLC_is_ON\" >Why can the sensor be OFF while the PLC is ON?<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-2\" href=\"https:\/\/www.wehopower.com\/ko\/news\/two-wire-sensor-off-plc-input-still-on-leakage\/#Compare_the_documented_worst_cases\" >Compare the documented worst cases<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-3\" href=\"https:\/\/www.wehopower.com\/ko\/news\/two-wire-sensor-off-plc-input-still-on-leakage\/#What_about_several_sensors\" >What about several sensors?<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-4\" href=\"https:\/\/www.wehopower.com\/ko\/news\/two-wire-sensor-off-plc-input-still-on-leakage\/#A_practical_fault-isolation_sequence\" >A practical fault-isolation sequence<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-5\" href=\"https:\/\/www.wehopower.com\/ko\/news\/two-wire-sensor-off-plc-input-still-on-leakage\/#Where_the_power_supply_fits\" >Where the power supply fits<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-6\" href=\"https:\/\/www.wehopower.com\/ko\/news\/two-wire-sensor-off-plc-input-still-on-leakage\/#Frequently_asked_questions\" >Frequently asked questions<\/a><ul class='ez-toc-list-level-3' ><li class='ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-7\" href=\"https:\/\/www.wehopower.com\/ko\/news\/two-wire-sensor-off-plc-input-still-on-leakage\/#Does_a_lower_leakage_value_automatically_solve_every_problem\" >Does a lower leakage value automatically solve every problem?<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-8\" href=\"https:\/\/www.wehopower.com\/ko\/news\/two-wire-sensor-off-plc-input-still-on-leakage\/#Can_I_use_an_indicator_LED_instead_of_measuring_the_input\" >Can I use an indicator LED instead of measuring the input?<\/a><\/li><\/ul><\/li><\/ul><\/nav><\/div>\n<h2><span class=\"ez-toc-section\" id=\"Why_can_the_sensor_be_OFF_while_the_PLC_is_ON\"><\/span>Why can the sensor be OFF while the PLC is ON?<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>A two-wire electronic sensor shares its load circuit with its own operating electronics. Its OFF state is not necessarily an open circuit. Rockwell\u2019s <a href=\"https:\/\/literature.rockwellautomation.com\/idc\/groups\/literature\/documents\/wd\/gi-wd002_-en-p.pdf\" target=\"_blank\" rel=\"noopener\">sensor interface guide<\/a> identifies OFF-state leakage as a reason an input may stay energized. The actual result depends on the receiving module\u2019s input circuit, voltage, threshold, leakage conditions and wiring\u2014not simply the sensor\u2019s nominal supply voltage.<\/p>\n<h2><span class=\"ez-toc-section\" id=\"Compare_the_documented_worst_cases\"><\/span>Compare the documented worst cases<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<div class=\"table-scroll\">\n<table>\n<thead>\n<tr>\n<th>Check<\/th>\n<th>Hypothetical values<\/th>\n<th>Decision<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>OFF current<\/td>\n<td>Sensor maximum leakage 1.4 mA; PLC guaranteed OFF at or below 0.8 mA<\/td>\n<td>1.4 &gt; 0.8 mA: OFF is not guaranteed<\/td>\n<\/tr>\n<tr>\n<td>Alternative OFF current<\/td>\n<td>Another sensor maximum 0.3 mA; same 0.8 mA input OFF condition<\/td>\n<td>0.3 &lt; 0.8 mA: current screen passes; check voltage and conditions too<\/td>\n<\/tr>\n<tr>\n<td>ON input voltage<\/td>\n<td>24 V supply \u2212 3 V sensor drop \u2212 2 V wiring drop = 19 V; input requires 15 V minimum<\/td>\n<td>19 V \u2265 15 V: this voltage screen passes only under the stated conditions<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/div>\n<p>These numbers are illustrative, not WEHO product ratings. Check that each specification is valid at the actual operating voltage, temperature, polarity and load. A PLC can specify both OFF voltage and current; neither comparison alone substitutes for the module\u2019s complete input specification. ON also requires the sensor\u2019s minimum and maximum load current and the PLC input current to be compatible.<\/p>\n<figure><a href=\"https:\/\/www.wehopower.com\/wp-content\/uploads\/2026\/11\/technical-2.svg\"><picture><source media=\"(max-width: 560px)\" srcset=\"https:\/\/www.wehopower.com\/wp-content\/uploads\/2026\/11\/technical-mobile-2.svg\"><img decoding=\"async\" src=\"https:\/\/www.wehopower.com\/wp-content\/uploads\/2026\/11\/technical-2.svg\" width=\"1200\" height=\"550\" alt=\"Two-Wire Sensor Off but PLC Input Still On: Check Leakage and Thresholds  title=\" two-wire sensor off but plc input still on: check leakage and thresholds title=\"\"><\/picture><\/a><figcaption>Engineering checks use illustrative values only. <a href=\"https:\/\/www.wehopower.com\/wp-content\/uploads\/2026\/11\/technical-2.svg\">View full-size diagram<\/a>.<\/figcaption><\/figure>\n<h2><span class=\"ez-toc-section\" id=\"What_about_several_sensors\"><\/span>What about several sensors?<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>When two-wire sensors are paralleled, their OFF-state leakage can add at the common input. In series, available operating voltage and each sensor\u2019s start-up behavior need review; the switching sequence can also affect the result. Omron discusses these model-dependent issues in its <a href=\"https:\/\/www.ia.omron.com\/support\/faq\/answer\/41\/faq00311\/\" target=\"_blank\" rel=\"noopener\">sensor series\/parallel FAQ<\/a>. Do not transfer its permitted quantities or specific circuit conditions to another manufacturer\u2019s sensor.<\/p>\n<h2><span class=\"ez-toc-section\" id=\"A_practical_fault-isolation_sequence\"><\/span>A practical fault-isolation sequence<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<ol>\n<li>Identify the exact sensor output type, rated supply, wire configuration and the PLC input module, common reference and input polarity.<\/li>\n<li>Check the supplier\u2019s worst-case leakage and ON-state residual voltage at the relevant operating conditions, along with minimum and maximum load current. Confirm that the actual PLC input and any approved interface provide the sensor\u2019s specified minimum ON-state load current.<\/li>\n<li>Read the PLC module\u2019s guaranteed ON and OFF ranges and input current, including any filtering or diagnostics.<\/li>\n<li>Have a qualified person measure input voltage and current at the receiving terminals in the approved OFF and ON conditions, using the correct instrument and procedure. Record supply and load conditions.<\/li>\n<li>If margins are insufficient, ask the module and sensor suppliers about a compatible interface or an approved load arrangement. Any proposed resistor must be engineered for both states, its dissipation and temperature, component ratings, and device instructions; there is no universal resistor value.<\/li>\n<li>Repeat the check after cable, sensor quantity, input module or supply changes. Document any remaining undefined threshold region instead of treating a single successful test as a guarantee.<\/li>\n<\/ol>\n<h2><span class=\"ez-toc-section\" id=\"Where_the_power_supply_fits\"><\/span>Where the power supply fits<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<p>\uadf8\ub9cc\ud07c <a href=\"https:\/\/www.wehopower.com\/ko\/product\/ndr-dinrail-240w-ac-to-dc-single-output-switching-power-supply\/\">WEHO NDR DIN-rail power-supply range<\/a> is relevant to a cabinet\u2019s supply selection. The pictured NDR-240-24 is a power supply, not the two-wire sensor or PLC input being diagnosed. Its presence does not establish sensor-to-input compatibility. Obtain the actual sensor and PLC data sheets as well as the selected supply\u2019s operating documentation.<\/p>\n<figure><a href=\"https:\/\/www.wehopower.com\/ko\/product\/ndr-dinrail-240w-ac-to-dc-single-output-switching-power-supply\/\"><img decoding=\"async\" src=\"https:\/\/www.wehopower.com\/wp-content\/uploads\/2026\/11\/product-original-1.jpg\" alt=\"Two-Wire Sensor Off but PLC Input Still On: Check Leakage and Thresholds  title=\" width=\"1000\" height=\"1000\" two-wire sensor off but plc input still on: check leakage and thresholds title=\"\"><\/a><figcaption>Original WEHO NDR-240-24 product photograph. It does not establish sensor or PLC compatibility.<\/figcaption><\/figure>\n<h2><span class=\"ez-toc-section\" id=\"Frequently_asked_questions\"><\/span>Frequently asked questions<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<h3><span class=\"ez-toc-section\" id=\"Does_a_lower_leakage_value_automatically_solve_every_problem\"><\/span>Does a lower leakage value automatically solve every problem?<span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p>No. It can improve the OFF margin, but the ON voltage, minimum load, input current, wiring polarity and actual environmental conditions still need verification.<\/p>\n<h3><span class=\"ez-toc-section\" id=\"Can_I_use_an_indicator_LED_instead_of_measuring_the_input\"><\/span>Can I use an indicator LED instead of measuring the input?<span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p>No. The sensor indicator and the voltage\/current at the PLC terminal represent different points in the circuit. Check both with appropriate, documented test methods.<\/p>\n<p class=\"article-cta\">Send WEHO the exact power-supply requirement and, for the interface review, the sensor and PLC input part numbers, wiring arrangement, supply tolerance and symptom. Request the applicable product documents before selecting components. <a href=\"https:\/\/www.wehopower.com\/ko\/contact\/\">WEHO\uc5d0 \ubb38\uc758\ud558\uc138\uc694<\/a>.<\/p>\n<\/div>","protected":false},"excerpt":{"rendered":"<p>Diagnose a two-wire sensor that leaves a PLC input on by checking off-state leakage, input thresholds and on-state voltage.<\/p>","protected":false},"author":2,"featured_media":20753,"parent":0,"menu_order":35,"comment_status":"","ping_status":"","template":"","format":"standard","meta":{"_acf_changed":false,"_joinchat":[],"footnotes":""},"news_category":[284],"class_list":["post-20782","news","type-news","status-publish","format-standard","has-post-thumbnail","hentry","news_category-product-articles","entry"],"acf":[],"post_mailing_queue_ids":[],"_links":{"self":[{"href":"https:\/\/www.wehopower.com\/ko\/wp-json\/wp\/v2\/news\/20782","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.wehopower.com\/ko\/wp-json\/wp\/v2\/news"}],"about":[{"href":"https:\/\/www.wehopower.com\/ko\/wp-json\/wp\/v2\/types\/news"}],"author":[{"embeddable":true,"href":"https:\/\/www.wehopower.com\/ko\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.wehopower.com\/ko\/wp-json\/wp\/v2\/comments?post=20782"}],"version-history":[{"count":0,"href":"https:\/\/www.wehopower.com\/ko\/wp-json\/wp\/v2\/news\/20782\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.wehopower.com\/ko\/wp-json\/wp\/v2\/media\/20753"}],"wp:attachment":[{"href":"https:\/\/www.wehopower.com\/ko\/wp-json\/wp\/v2\/media?parent=20782"}],"wp:term":[{"taxonomy":"news_category","embeddable":true,"href":"https:\/\/www.wehopower.com\/ko\/wp-json\/wp\/v2\/news_category?post=20782"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}