{"id":1945,"date":"2026-02-21T11:50:14","date_gmt":"2026-02-21T06:20:14","guid":{"rendered":"https:\/\/study.madeeasy.in\/?p=1945"},"modified":"2026-02-21T11:53:56","modified_gmt":"2026-02-21T06:23:56","slug":"silicon-controlled-rectifier","status":"publish","type":"post","link":"https:\/\/www.madeeasy.in\/study\/ee\/power-electronic\/silicon-controlled-rectifier","title":{"rendered":"Silicon Controlled Rectifier (SCR)"},"content":{"rendered":"<ul>\n<li style=\"text-align: justify;\">SCR is the oldest and first member of the thyristor family.<\/li>\n<li style=\"text-align: justify;\">It is called SCR because, silicon is used for its construction and its operation as a rectifier (very low resistance in forward conduction and very high resistance in the reverse direction) can be controlled.<\/li>\n<li style=\"text-align: justify;\">It has three terminals {see fig. (a)} : (i) Anode (A); (ii) Cathode (K); (iii) Gate (G)<\/li>\n<\/ul>\n<p style=\"text-align: justify;\"><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-1947 size-full\" src=\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/circuit-symbol-scr.jpg\" alt=\"Circuit symbol of SCR\" width=\"502\" height=\"170\" srcset=\"https:\/\/www.madeeasy.in\/study\/wp-content\/uploads\/2024\/07\/circuit-symbol-scr.jpg 502w, https:\/\/www.madeeasy.in\/study\/wp-content\/uploads\/2024\/07\/circuit-symbol-scr-300x102.jpg 300w\" sizes=\"auto, (max-width: 502px) 100vw, 502px\" \/><\/p>\n<ul style=\"text-align: justify;\">\n<li>The structure of SCR {see fig. (b)}<br \/>\nThe terminal connected to outer \u2018p\u2019 region is called Anode (A).<br \/>\nThe terminal connected to outer \u2018n\u2019 region is called Cathode (K).<br \/>\nThe terminal connected to inner \u2018p\u2019 region is called the Gate (G).<\/li>\n<li>SCR is a unidirectional device. It blocks the current flow from cathode to anode.<\/li>\n<\/ul>\n<div id=\"ez-toc-container\" class=\"ez-toc-v2_0_79_1 ez-toc-wrap-left counter-hierarchy ez-toc-counter ez-toc-light-blue ez-toc-container-direction\">\n<div class=\"ez-toc-title-container\">\n<p class=\"ez-toc-title\" style=\"cursor:inherit\">Table of Contents<\/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=\"Toggle Table of Content\"><span class=\"ez-toc-js-icon-con\"><span class=\"\"><span class=\"eztoc-hide\" style=\"display:none;\">Toggle<\/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.madeeasy.in\/study\/ee\/power-electronic\/silicon-controlled-rectifier\/#Static-V-I-Characteristics-of-SCR\" >Static V-I Characteristics of SCR<\/a><ul class='ez-toc-list-level-3' ><li class='ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-2\" href=\"https:\/\/www.madeeasy.in\/study\/ee\/power-electronic\/silicon-controlled-rectifier\/#1-Forward-Blocking-Mode\" >1. Forward Blocking Mode<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-3\" href=\"https:\/\/www.madeeasy.in\/study\/ee\/power-electronic\/silicon-controlled-rectifier\/#Forward-Conduction-Mode\" >Forward Conduction Mode<\/a><\/li><\/ul><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-4\" href=\"https:\/\/www.madeeasy.in\/study\/ee\/power-electronic\/silicon-controlled-rectifier\/#Protection-of-SCR\" >Protection of SCR<\/a><ul class='ez-toc-list-level-3' ><li class='ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-5\" href=\"https:\/\/www.madeeasy.in\/study\/ee\/power-electronic\/silicon-controlled-rectifier\/#didt-Protection\" >di\/dt Protection<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-6\" href=\"https:\/\/www.madeeasy.in\/study\/ee\/power-electronic\/silicon-controlled-rectifier\/#dvdt-Protection\" >dv\/dt Protection<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-7\" href=\"https:\/\/www.madeeasy.in\/study\/ee\/power-electronic\/silicon-controlled-rectifier\/#Design-of-Snubber-Circuit\" >Design of Snubber Circuit<\/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.madeeasy.in\/study\/ee\/power-electronic\/silicon-controlled-rectifier\/#Over-Voltage-Protection\" >Over Voltage Protection<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-9\" href=\"https:\/\/www.madeeasy.in\/study\/ee\/power-electronic\/silicon-controlled-rectifier\/#Over-Current-Protection\" >Over Current Protection<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-10\" href=\"https:\/\/www.madeeasy.in\/study\/ee\/power-electronic\/silicon-controlled-rectifier\/#Gate-Protection\" >Gate Protection<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-11\" href=\"https:\/\/www.madeeasy.in\/study\/ee\/power-electronic\/silicon-controlled-rectifier\/#Thermal-Protection\" >Thermal Protection<\/a><\/li><\/ul><\/li><li class='ez-toc-page-1 ez-toc-heading-level-2'><a class=\"ez-toc-link ez-toc-heading-12\" href=\"https:\/\/www.madeeasy.in\/study\/ee\/power-electronic\/silicon-controlled-rectifier\/#LASCR-Light-Activated-Thyristor\" >LASCR (Light Activated Thyristor)<\/a><ul class='ez-toc-list-level-3' ><li class='ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-13\" href=\"https:\/\/www.madeeasy.in\/study\/ee\/power-electronic\/silicon-controlled-rectifier\/#The-DIAC-Bidirectional-Thyristor-Diode\" >The DIAC (Bidirectional Thyristor Diode)<\/a><\/li><li class='ez-toc-page-1 ez-toc-heading-level-3'><a class=\"ez-toc-link ez-toc-heading-14\" href=\"https:\/\/www.madeeasy.in\/study\/ee\/power-electronic\/silicon-controlled-rectifier\/#TRIAC\" >TRIAC<\/a><\/li><\/ul><\/li><\/ul><\/nav><\/div>\n<h2><span class=\"ez-toc-section\" id=\"Static-V-I-Characteristics-of-SCR\"><\/span>Static V-I Characteristics of SCR<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<ul style=\"text-align: justify;\">\n<li>An elementary circuit diagram for obtaining static V-I characteristics of SCR is shown in figure,<\/li>\n<\/ul>\n<p style=\"text-align: justify;\"><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-1951 size-full\" src=\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/characteristics-of-scr.jpg\" alt=\"Characteristics of SCR\" width=\"344\" height=\"159\" srcset=\"https:\/\/www.madeeasy.in\/study\/wp-content\/uploads\/2024\/07\/characteristics-of-scr.jpg 344w, https:\/\/www.madeeasy.in\/study\/wp-content\/uploads\/2024\/07\/characteristics-of-scr-300x139.jpg 300w\" sizes=\"auto, (max-width: 344px) 100vw, 344px\" \/><\/p>\n<ul style=\"text-align: justify;\">\n<li>The Anode and Cathode are connected to main source through the load.<\/li>\n<li>The Gate and Cathode are fed from another source \u2018E<sub>g<\/sub>\u2019.<\/li>\n<li>The static V-I characteristics of SCR are shown below.<\/li>\n<\/ul>\n<p style=\"text-align: justify;\"><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-1953 size-full\" src=\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/anode-voltage-e1721208204194.jpg\" alt=\"Anode Voltage\" width=\"464\" height=\"375\" srcset=\"https:\/\/www.madeeasy.in\/study\/wp-content\/uploads\/2024\/07\/anode-voltage-e1721208204194.jpg 464w, https:\/\/www.madeeasy.in\/study\/wp-content\/uploads\/2024\/07\/anode-voltage-e1721208204194-300x242.jpg 300w\" sizes=\"auto, (max-width: 464px) 100vw, 464px\" \/><\/p>\n<p style=\"text-align: justify;\">V<sub>a<\/sub> = Anode voltage; I<sub>a<\/sub> = Anode current<br \/>\nV<sub>BO<\/sub> = Forward breakover voltage<br \/>\nV<sub>BR<\/sub> = Reverse breakdown voltage<br \/>\nI<sub>g<\/sub> = Gate current<br \/>\nI<sub>L<\/sub> = Latching current<br \/>\nI<sub>H<\/sub> = Holding current<\/p>\n<ul style=\"text-align: justify;\">\n<li>The three basic modes of operation of SCR are :<br \/>\n(i) Forward blocking mode; (ii) Forward conduction mode; (iii) Reverse blocking mode<\/li>\n<\/ul>\n<h3 style=\"text-align: justify;\"><span class=\"ez-toc-section\" id=\"1-Forward-Blocking-Mode\"><\/span>1. Forward Blocking Mode<span class=\"ez-toc-section-end\"><\/span><\/h3>\n<ul style=\"text-align: justify;\">\n<li>When anode is at a higher potential than cathode, with gate circuit open, thyristor is said to be forward biased.<\/li>\n<li>It is seen from the figure that J<sub>1<\/sub>, J<sub>3<\/sub> are forward bias but junction J<sub>2<\/sub> is reverse bias.<\/li>\n<li>In this mode, a small current, called forward leakage current flows from anode to cathode.<\/li>\n<li>OM in the VI characteristics represents the forward blocking mode of SCR.<\/li>\n<li>SCR is treated as an open switch in the forward blocking mode.<\/li>\n<\/ul>\n<h3 style=\"text-align: justify;\"><span class=\"ez-toc-section\" id=\"Forward-Conduction-Mode\"><\/span>Forward Conduction Mode<span class=\"ez-toc-section-end\"><\/span><\/h3>\n<ul style=\"text-align: justify;\">\n<li style=\"text-align: justify;\">When anode to cathode forward voltage is increased with gate circuit open, reverse biased junction J<sub>2<\/sub> will have an avalanche breakdown at a voltage called forward breakover voltage V<sub>BO<\/sub>.<\/li>\n<li style=\"text-align: justify;\">After this breakdown, thyristor gets turned ON with point \u2018M\u2019 at once shifting to \u2018N\u2019. Here NK represents the forward conduction mode.<\/li>\n<li style=\"text-align: justify;\">A thyristor can be brought from forward blocking mode to forward conducting mode by turning it on by applying<br \/>\n(i) a positive Gate pulse between gate and cathode (or)<br \/>\n(ii) a forward breakover voltage (VBO) across anode and cathode<\/li>\n<li style=\"text-align: justify;\">Voltage drop across the SCR \u2018V<sub>T<\/sub>\u2019 increases slightly with an increase in anode current. It can be seen from NK<\/li>\n<\/ul>\n<h2 style=\"text-align: justify;\"><span class=\"ez-toc-section\" id=\"Protection-of-SCR\"><\/span>Protection of SCR<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<h3 style=\"text-align: justify;\"><span class=\"ez-toc-section\" id=\"didt-Protection\"><\/span>di\/dt Protection<span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p style=\"text-align: justify;\">A thyristor requires a minimum time to spread the current conduction uniformly throughout the junctions. If the rate of rise of anode current is very fast compared with the spreading velocity of a turn ON process, a localized \u201chot spot\u201d heating may occur due to high current density and the device may fail, as a result of excessive temperature.<\/p>\n<p style=\"text-align: justify;\">The practical devices must be connected against high di\/dt. Let us consider the circuit in figure. Under steady state <img loading=\"lazy\" decoding=\"async\" class=\"alignright wp-image-1960 size-full\" src=\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/dt-protection.jpg\" alt=\"dt Protection\" width=\"242\" height=\"155\" \/> operation, D<sub>m<\/sub> conducts when thyristor T<sub>1<\/sub> is off. If T<sub>1<\/sub>is fired when D<sub>m<\/sub> is still conducting, di\/dt can be very high and limited only by the stray inductance of the circuit.<\/p>\n<p style=\"text-align: justify;\">The di\/dt is limited by adding a series inductor Ls , as shown in figure. The forward di\/dt is <strong>di\/dt = Vs\/Ls<\/strong><\/p>\n<h3 style=\"text-align: justify;\"><span class=\"ez-toc-section\" id=\"dvdt-Protection\"><\/span><strong>dv\/dt Protection<\/strong><span class=\"ez-toc-section-end\"><\/span><\/h3>\n<ul style=\"text-align: justify;\">\n<li>If the rate of rise of suddenly applied voltage across thyristor is high the device may get turned-on. Such phenomena of turning on a thyristor, called dv\/dt turn-on must be avoided as it leads to false operation.<\/li>\n<li>False turn-on of a SCR by large dv\/dt, even with out application of gate signal can be prevented by using a Snubber circuit in parallel with the device.<\/li>\n<\/ul>\n<h3 style=\"text-align: justify;\"><span class=\"ez-toc-section\" id=\"Design-of-Snubber-Circuit\"><\/span>Design of Snubber Circuit<span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p style=\"text-align: justify;\"><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-1962 size-full\" src=\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/snubber-circuit.jpg\" alt=\"Snubber Circuit\" width=\"292\" height=\"183\" \/><\/p>\n<p style=\"text-align: justify;\">A snubber circuit consists of a series combination of resistance R<sub>s<\/sub> and capacitance C<sub>s<\/sub> in parallel with the thyristor as shown in figure.<\/p>\n<p style=\"text-align: justify;\"><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-1964 size-full\" src=\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/snubber.jpg\" alt=\"Snubber\" width=\"263\" height=\"166\" \/><\/p>\n<p style=\"text-align: justify;\">When switch \u2018s\u2019 is closed, a sudden voltage is appearing across the circuit. Capacitor C<sub>s<\/sub> behaves like a short circuit, therefore voltage across SCR is zero. With the passage of time, voltage across C<sub>s<\/sub> built up at a slow rate such that the dv\/dt across \u2018C<sub>s<\/sub> \u2019 and therefore across SCR is less than the specified maximum dv\/dt rating of the device.<\/p>\n<p style=\"text-align: justify;\">In order to limit the magnitude of discharge current, a resistance R<sub>s<\/sub> is inserted in series with \u2018C<sub>s<\/sub>\u2019<\/p>\n<h3 style=\"text-align: justify;\"><span class=\"ez-toc-section\" id=\"Over-Voltage-Protection\"><\/span>Over Voltage Protection<span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p style=\"text-align: justify;\">A SCR may be subjected to internal or external over voltages.<\/p>\n<h4 style=\"text-align: justify;\">Internal Over Voltages<\/h4>\n<p style=\"text-align: justify;\">Due to the presence of the series inductance L<sub>s<\/sub> of the SCR circuit, large transient voltages <img loading=\"lazy\" decoding=\"async\" class=\"size-full wp-image-1965 alignright\" src=\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/internal-over-voltages.jpg\" alt=\"Internal Over Voltages\" width=\"75\" height=\"49\" \/>is produced and this internal voltage may be several times the breakover voltage of the SCR, the thyristor may be<br \/>\ndestroyed permanently.<\/p>\n<h4 style=\"text-align: justify;\">External Over Voltages<\/h4>\n<ul style=\"text-align: justify;\">\n<li>Over voltages are caused due to the lightning strokes and switching surges.<\/li>\n<li>For reliable operation, the over voltages must be suppressed by adapting voltage clamping (v.c) device.<\/li>\n<\/ul>\n<p style=\"text-align: justify;\"><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-1967 size-full\" src=\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/varistor.jpg\" alt=\"Varistor\" width=\"311\" height=\"150\" srcset=\"https:\/\/www.madeeasy.in\/study\/wp-content\/uploads\/2024\/07\/varistor.jpg 311w, https:\/\/www.madeeasy.in\/study\/wp-content\/uploads\/2024\/07\/varistor-300x145.jpg 300w\" sizes=\"auto, (max-width: 311px) 100vw, 311px\" \/><\/p>\n<ul style=\"text-align: justify;\">\n<li>A voltage clamping (V.C) device is a non-linear resistor connected across SCR as shown in figure. The V.C. device has falling resistance characteristics with increasing voltage. Under normal conditions the device has a high resistance and draws only a small leakage current. When a voltage surge appears, the V.C. device operates in low resistance region and produces a short-circuit across SCR. After the surge is dissipated it returns to normal high resistance.<\/li>\n<\/ul>\n<p style=\"text-align: justify;\"><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-1969 size-full\" src=\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/external-over-voltages.jpg\" alt=\"External Over Voltages\" width=\"319\" height=\"166\" srcset=\"https:\/\/www.madeeasy.in\/study\/wp-content\/uploads\/2024\/07\/external-over-voltages.jpg 319w, https:\/\/www.madeeasy.in\/study\/wp-content\/uploads\/2024\/07\/external-over-voltages-300x156.jpg 300w\" sizes=\"auto, (max-width: 319px) 100vw, 319px\" \/><\/p>\n<h3 style=\"text-align: justify;\"><span class=\"ez-toc-section\" id=\"Over-Current-Protection\"><\/span>Over Current Protection<span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p style=\"text-align: justify;\">If a SCR is subjected to over current due to faults, short circuits (or) surge currents, its junction temperature may exceed the rated value and the device may be damaged. There is a need for the over current protection of SCR.<\/p>\n<ul style=\"text-align: justify;\">\n<li>Fast acting current limiting fuse (FACLF)<\/li>\n<li>Circuit breakers are used<\/li>\n<\/ul>\n<h3 style=\"text-align: justify;\"><span class=\"ez-toc-section\" id=\"Gate-Protection\"><\/span>Gate Protection<span class=\"ez-toc-section-end\"><\/span><\/h3>\n<ul style=\"text-align: justify;\">\n<li>Over voltages and over currents across the gate circuit can cause false triggering of the SCR.<\/li>\n<li>Protection against over voltages is achieved by connecting a zener diode across the gate circuit.<\/li>\n<li>Protection against the over current is achieved by connecting a resistor \u2018R2\u2019 in series with gate circuit.<\/li>\n<li>A capacitor and a resistor are also connected across gate to cathode to by pass the noise signals.<\/li>\n<\/ul>\n<p style=\"text-align: justify;\"><strong>Circuit Components Showing the Thyristor Protection<\/strong><\/p>\n<p style=\"text-align: justify;\"><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-1971 size-full\" src=\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/thyristor-protection.jpg\" alt=\"Thyristor Protection\" width=\"396\" height=\"204\" srcset=\"https:\/\/www.madeeasy.in\/study\/wp-content\/uploads\/2024\/07\/thyristor-protection.jpg 396w, https:\/\/www.madeeasy.in\/study\/wp-content\/uploads\/2024\/07\/thyristor-protection-300x155.jpg 300w\" sizes=\"auto, (max-width: 396px) 100vw, 396px\" \/><\/p>\n<h3 style=\"text-align: justify;\"><span class=\"ez-toc-section\" id=\"Thermal-Protection\"><\/span>Thermal Protection<span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p style=\"text-align: justify;\">If temperature increases, insulation is weakened, resulting in leakage currents which interference with the operation. Heat sinks which include aluminium discs are used along with ventilating ducts, increased surface area, coolants are used.<\/p>\n<h2 style=\"text-align: justify;\"><span class=\"ez-toc-section\" id=\"LASCR-Light-Activated-Thyristor\"><\/span>LASCR (Light Activated Thyristor)<span class=\"ez-toc-section-end\"><\/span><\/h2>\n<ul style=\"text-align: justify;\">\n<li>LASCR is turned on by direct radiation of silicon with light.<\/li>\n<li>The triggering current produced because of the electron-hole pairs which are created due to radiation under the influence of electric field.<\/li>\n<li>It is used in high-voltage and high-current applications, e.g., in HVDC transmission.<\/li>\n<li>The typical di\/dt rating is 250 A\/\u00b5s and dv\/dt rating could be as high as 2000 V\/\u00b5s.<\/li>\n<\/ul>\n<p style=\"text-align: justify;\"><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-1972 size-full\" src=\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/circuit-symbol.jpg\" alt=\" Circuit symbol\" width=\"446\" height=\"243\" srcset=\"https:\/\/www.madeeasy.in\/study\/wp-content\/uploads\/2024\/07\/circuit-symbol.jpg 446w, https:\/\/www.madeeasy.in\/study\/wp-content\/uploads\/2024\/07\/circuit-symbol-300x163.jpg 300w\" sizes=\"auto, (max-width: 446px) 100vw, 446px\" \/><\/p>\n<h3 style=\"text-align: justify;\"><span class=\"ez-toc-section\" id=\"The-DIAC-Bidirectional-Thyristor-Diode\"><\/span>The DIAC (Bidirectional Thyristor Diode)<span class=\"ez-toc-section-end\"><\/span><\/h3>\n<ul>\n<li style=\"text-align: justify;\">DIAC stands for Diode for Alternating Current.<\/li>\n<li style=\"text-align: justify;\">It is a device which has two electrodes and which conducts electrical current only after its breakover voltage (V<sub>BO<\/sub>).<\/li>\n<li style=\"text-align: justify;\">The main application of DIAC is in TRIAC triggering circuit, lamp dimmer circuit, heat control circuit etc.<\/li>\n<li style=\"text-align: justify;\">If voltage V<sub>12<\/sub>, with terminal 1 positive with respect to terminal 2, exceeds breakover voltage V<sub>B01<\/sub>, then structure <sub>pnpn<\/sub> conducts.<\/li>\n<li style=\"text-align: justify;\">In case terminals 2 is positive with respect to terminal 1 and when V<sub>21<\/sub> exceeds break over voltage\u00a0 V<sub>B02<\/sub>, structure <sub>pnpn<\/sub>\u2032 conducts.<\/li>\n<li style=\"text-align: justify;\">I-V characteristics of a DIAC.<\/li>\n<\/ul>\n<p><strong>NOTE:<\/strong> The term DIAC is obtained from capital letters, Diode that can work on AC. A DIAC is some times called a gate less TRIAC.<\/p>\n<h3><span class=\"ez-toc-section\" id=\"TRIAC\"><\/span>TRIAC<span class=\"ez-toc-section-end\"><\/span><\/h3>\n<p>A TRIAC is a bidirectional thyristor with three terminals. It is used extensively for the control of power in ac circuits.<br \/>\nTRIAC is the word derived by combining the capital letters from the word TRIode and AC.<br \/>\nIts three terminals are usually designated as MT<sub>1<\/sub> (Main terminal 1), MT<sub>2<\/sub> and the gate.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-1982 size-full\" src=\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/triac.jpg\" alt=\"TRIAC\" width=\"496\" height=\"447\" srcset=\"https:\/\/www.madeeasy.in\/study\/wp-content\/uploads\/2024\/07\/triac.jpg 496w, https:\/\/www.madeeasy.in\/study\/wp-content\/uploads\/2024\/07\/triac-300x270.jpg 300w\" sizes=\"auto, (max-width: 496px) 100vw, 496px\" \/><\/p>\n<p>The TRIAC can however be turned on in each half cycle of the applied voltage by applying a positive (or) negative voltage to the gate with respect to terminal MT<sub>1<\/sub>.<\/p>\n<p><strong>The turn-on process of a triac can be explained as under:<\/strong><\/p>\n<p><strong>MT<sub>2<\/sub> positive and Gate current is also positive:<\/strong> When Gate current has injected sufficient charge into P<sub>2<\/sub> layer, reverse biased junction N<sub>1<\/sub>P<sub>2<\/sub> breaks down.<\/p>\n<p>As a result, triac starts conducting through P<sub>1<\/sub>, N<sub>1<\/sub>, P<sub>2<\/sub>, N<sub>2<\/sub> layers.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-1984 size-full\" src=\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/gate-current.jpg\" alt=\" Gate current\" width=\"163\" height=\"228\" \/><\/p>\n<p><strong>MT<sub>2<\/sub> is positive but Gate is negative:<\/strong> Gate current flows through P<sub>2<\/sub> N<sub>3<\/sub> junction.<br \/>\nReverse biased junction N1P<sub>2<\/sub> is forward biased by injecting sufficient charge into P<sub>2<\/sub>.<br \/>\nAs a result, TRIAC starts conducting through P<sub>1<\/sub>N<sub>1<\/sub>P<sub>2<\/sub>N<sub>3<\/sub><\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-1985 size-full\" src=\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/gate-negative.jpg\" alt=\"Gate is negative:\" width=\"158\" height=\"222\" \/><\/p>\n<p><strong>MT<sub>2<\/sub> is negative but Gate current is positive:<\/strong> Gate current I<sub>g<\/sub> forward biases P<sub>2<\/sub>N<sub>2<\/sub> junction. Layer N<sub>2<\/sub><br \/>\ninjects electrons into P<sub>2<\/sub> layer. As a reset, reverse biased junction N<sub>1<\/sub>P<sub>1<\/sub> breaks down.<br \/>\nThe path P<sub>2<\/sub>N<sub>1<\/sub>P<sub>1<\/sub>N<sub>4<\/sub> is completely turned-on.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-1987 size-full\" src=\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/gate-current-1.jpg\" alt=\" Gate current\" width=\"143\" height=\"228\" \/><\/p>\n<p><strong>Both MT<sub>2<\/sub> and Gate current are negative:<\/strong><br \/>\nThe gate current I<sub>g<\/sub> flows from P<sub>2<\/sub> to N<sub>3<\/sub>.<br \/>\nReverse biased junction N<sub>1<\/sub>P<sub>1<\/sub> is broken, and finally, the path P<sub>2<\/sub> N<sub>1<\/sub> P<sub>1<\/sub> N<sub>4<\/sub> is turned on.<\/p>\n<p><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter wp-image-1989 size-full\" src=\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/gate-current-negative.jpg\" alt=\"Gate Current Negative\" width=\"159\" height=\"240\" \/><\/p>\n<p style=\"text-align: center;\"><a class=\"btn btn-danger\" role=\"button\" href=\"https:\/\/study.madeeasy.in\/ee\/power-electronic\/power-electronic-applications\/\" target=\"_blank\" rel=\"noopener\">&lt;&lt; Previous<\/a> | <a class=\"btn btn-success\" role=\"button\" href=\"https:\/\/study.madeeasy.in\/ee\/power-electronic\/zcs-and-zvs-resonant\/\" target=\"_blank\" rel=\"noopener\"> Next &gt;&gt;<\/a><br \/>\n<strong> Must Read: <\/strong> <a href=\"https:\/\/study.madeeasy.in\/subjects\/what-is-power-electronics\/\" target=\"_blank\" rel=\"noopener\"><strong>What is Power Electronics?<\/strong><\/a><\/p>\n<p>&nbsp;<\/p>\n","protected":false},"excerpt":{"rendered":"<p>SCR is the oldest and first member of the thyristor family. It is called SCR because, silicon is used for<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[511,5],"tags":[545,546,543,544,542],"class_list":["post-1945","post","type-post","status-publish","format-standard","hentry","category-power-electronic","category-ee","tag-characteristics-of-scr","tag-design-of-snubber-circuit","tag-external-over-voltages","tag-internal-over-voltages","tag-varistor"],"_links":{"self":[{"href":"https:\/\/www.madeeasy.in\/study\/wp-json\/wp\/v2\/posts\/1945","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.madeeasy.in\/study\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.madeeasy.in\/study\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.madeeasy.in\/study\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.madeeasy.in\/study\/wp-json\/wp\/v2\/comments?post=1945"}],"version-history":[{"count":0,"href":"https:\/\/www.madeeasy.in\/study\/wp-json\/wp\/v2\/posts\/1945\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.madeeasy.in\/study\/wp-json\/wp\/v2\/media?parent=1945"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.madeeasy.in\/study\/wp-json\/wp\/v2\/categories?post=1945"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.madeeasy.in\/study\/wp-json\/wp\/v2\/tags?post=1945"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}