{"id":6731,"date":"2022-08-20T07:33:28","date_gmt":"2022-08-20T07:33:28","guid":{"rendered":"https:\/\/mdr.foobrdigital.com\/?p=6731"},"modified":"2022-08-20T07:33:28","modified_gmt":"2022-08-20T07:33:28","slug":"two-transistor-model-of-scr-or-thyristor","status":"publish","type":"post","link":"https:\/\/mudassirbackup.infinitycodestudio.com\/index.php\/2022\/08\/20\/two-transistor-model-of-scr-or-thyristor\/","title":{"rendered":"Two Transistor Model of SCR or Thyristor"},"content":{"rendered":"\n
\"\"<\/figure>\n\n\n\n

Basic operating principle of SCR, can easily be understood by the two transistor model of SCR<\/strong>, as it is a combination of p and n layers.<\/p>\n\n\n\n

\"silicon<\/figure>\n\n\n\n

This is a pnpn thyristor. If we bisect it through the dotted line then we will get two transistors i.e. one pnp transistor with J1<\/sub> and J2<\/sub> junctions and another is with J2<\/sub> and J3<\/sub> junctions as shown in figure below.<\/p>\n\n\n\n

\"two<\/figure>\n\n\n\n

The relation between the collector current and emitter current is shown below<\/p>\n\n\n\n

\"two<\/figure>\n\n\n\n

Here, IC<\/sub> is collector current, IE<\/sub> is emitter current, ICBO<\/sub> is forward leakage current, \u03b1 is common base forward current gain and relationship between IC<\/sub> and IB<\/sub> is<\/p>\n\n\n\n

\"\"\/<\/figure>\n\n\n\n

Where, IB<\/sub> is base current and \u03b2 is common emitter forward current gain.
Let\u2019s for transistor T1<\/sub> this relation holds<\/p>\n\n\n\n

\"\"\/<\/figure>\n\n\n\n

And that for transistor T2<\/sub><\/p>\n\n\n\n

\"\"\/<\/figure>\n\n\n\n

Now, by the analysis of two transistors model we can get anode current,<\/p>\n\n\n\n

\"\"\/<\/figure>\n\n\n\n

From equation (i) and (ii), we get,<\/p>\n\n\n\n

\"\"\/<\/figure>\n\n\n\n

If applied gate current is Ig<\/sub> then cathode current will be the summation of anode current and gate current i.e.<\/p>\n\n\n\n

\"\"\/<\/figure>\n\n\n\n

By substituting this valyue of Ik<\/sub> in (iii) we get,<\/p>\n\n\n\n

\"\"\/<\/figure>\n\n\n\n

From this relation we can assure that with increasing the value of<\/p>\n\n\n\n

\"\"\/<\/figure>\n\n\n\n

towards unity, corresponding anode current will increase. Now the question is how<\/p>\n\n\n\n

\"\"\/<\/figure>\n\n\n\n

increasing? Here is the explanation using two transistor model of SCR<\/strong>.
At the first stage when we apply a gate current Ig<\/sub>, it acts as base current of T2<\/sub> transistor i.e. IB2<\/sub> = Ig<\/sub> and emitter current of the T2<\/sub> transistor IE2<\/sub> = Ik<\/sub>. Hence establishment of the emitter current gives rise \u03b12<\/sub> as<\/p>\n\n\n\n

\"\"\/<\/figure>\n\n\n\n

Presence of base current will generate collector current as<\/p>\n\n\n\n

\"\"\/<\/figure>\n\n\n\n

This IC2<\/sub> is nothing but base current IB1<\/sub> of transistor T1<\/sub>, which will cause the flow of collector current,<\/p>\n\n\n\n

\"\"\/<\/figure>\n\n\n\n

IC1<\/sub> and IB1<\/sub> lead to increase IC1<\/sub> as<\/p>\n\n\n\n

\"\"\/<\/figure>\n\n\n\n

and hence, \u03b11<\/sub> increases. Now, new base current of T2<\/sub> is<\/p>\n\n\n\n

\"\"\/<\/figure>\n\n\n\n

, which will lead to increase emitter current<\/p>\n\n\n\n

\"\"\/<\/figure>\n\n\n\n

and as a result \u03b12<\/sub> also increases and this further increases<\/p>\n\n\n\n

\"\"\/<\/figure>\n\n\n\n

.
As<\/p>\n\n\n\n

\"\"\/<\/figure>\n\n\n\n

, \u03b11<\/sub> again increases. This continuous positive feedback effect increases<\/p>\n\n\n\n

\"\"\/<\/figure>\n\n\n\n

towards unity and anode current tends to flow at a very large value. The value current then can only be controlled by external resistance of the circuit.<\/p>\n","protected":false},"excerpt":{"rendered":"

Basic operating principle of SCR, can easily be understood by the two transistor model of SCR, as it is a combination of p and n layers. This is a pnpn thyristor. If we bisect it through the dotted line then we will get two transistors i.e. one pnp transistor with J1 and J2 junctions and […]<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[439],"tags":[],"_links":{"self":[{"href":"https:\/\/mudassirbackup.infinitycodestudio.com\/index.php\/wp-json\/wp\/v2\/posts\/6731"}],"collection":[{"href":"https:\/\/mudassirbackup.infinitycodestudio.com\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/mudassirbackup.infinitycodestudio.com\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/mudassirbackup.infinitycodestudio.com\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/mudassirbackup.infinitycodestudio.com\/index.php\/wp-json\/wp\/v2\/comments?post=6731"}],"version-history":[{"count":0,"href":"https:\/\/mudassirbackup.infinitycodestudio.com\/index.php\/wp-json\/wp\/v2\/posts\/6731\/revisions"}],"wp:attachment":[{"href":"https:\/\/mudassirbackup.infinitycodestudio.com\/index.php\/wp-json\/wp\/v2\/media?parent=6731"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/mudassirbackup.infinitycodestudio.com\/index.php\/wp-json\/wp\/v2\/categories?post=6731"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/mudassirbackup.infinitycodestudio.com\/index.php\/wp-json\/wp\/v2\/tags?post=6731"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}