Junction of SCR (J2) and junction of Diode

1.) When the SCR is in the forward conduction mode and positive gate signal is applied to the p region of junction j2 then why is it that after a threshhold value is reached the gate signal can be switched off?

2.) Won't the J2 become reverse biased again?

3.) In a transistor (BJT) a constant 0.7 volts is applied to the emitter and base junction when the external voltage is applied to collector and a steady flow is established can we not turn of the emitter-base voltage?

4.) Are not both the situations identical?

"Thyristor, the name is derived from a combination of two words THYRatron and transISTOR."

5.) What happens when we apply a gate signal? Do electrons get injected into the junction J2 and breakdown the junction i.e. push the other electrons (repel) forward as they are excited by another voltage?



---------- Post added at 09:13 ---------- Previous post was at 07:55 ----------

I researched around a bit and found out that the SCR accomplishes this (gate turn off) by regenerative action i.e even when the gate is turned off SCR conducts so how does it actually accomplish this... Please don't give me the 2 transistor analogy, I want to know the physicists viewpoint on this....

Please don't give me the 2 transistor analogy, I want to know the physicists viewpoint on this....

Click to expand...

Sounds like, "I can't understand how a transistor works, now I want to understand SCR". That's helpless, because the charge carrier injection mechanism is effectively the same. After understanding it in case of a transistor, you shouldn't have problems with SCR explanation.

I want to know why SCR conducts even though the gate signal is turned off. It sounds like you don't know anything about electronics yourself otherwise you would have given me an answer explaining me the exact operation of an SCR rather than quoting me in your answer...How does the regeneration mechanism work???

The reason is internal feedback, which can be perfectly visualized with the two-transistor model (also from a physicists viewpoint).

In an scr when anode and cathode terminals are forward biased and gate signal is applied the conduction starts.therefore when gate signal is removed it will not stop the conduction because the current is now flowing through the scr this make all the junctions in the scr to remain in forward biased condition..remember that if you give ac voltage across the anode and cathode terminals the gate signal must be continuously applied or else the conduction stops due to natural commutation.

After you remove the gate signal only the +ve anode w.r.t cathode remains hence electrons are flowing one way and holes are flowing the other way, now if that is the case then current must necessarily flow in common emitter mode of a transistor with collector-emitter voltage applied and base-emitter voltage removed(i.e 0.7 v)!!!

Why is this happening only in SCR?

What is this regenerative action that is taking place only in the SCR??

you must refer to the two transistor analogy.

vignesh ssvv said:

you must refer to the two transistor analogy.

Click to expand...

a small addition to vignesh


https://www.allaboutcircuits.com/vol_3/chpt_7/5.html
the scr is make actually as two transistors this link will show u why once its on it stays on and no need for gate volts.