# Thyristor ON !?!

Hullo.
I was asked this question during my Lab viva.
" There is a circuit with SCR, without the gating circuit of the SCR
connected ( Assume it to be isolated so that the firing of the SCR wont occur anyway due to it. ) The present Voltage source (V1) is a 100V rms sine supply. Presently, the SCR doesnt condect, i.e the SCR is OFF. The V1 is disconnected, another Voltage source (V2), 100 V rms, sine o/p is connected. The SCR now conducts !!! How ??? "
The answers given by me were 1. The new Vsource must have given a step i/p and hence the Voltage breakdown ( dv/dt) of the SCR must have occured. ( He shook his head, as in NO ) 2. The temperature must have changed during the process of changin of Vsource. ( Again shook his head.) 3. The internal resistance of the V2 must be less so that it gives somewht higher o/p to cause breakdown of SCR.. ( Again the wrong answer he said. )
So, WHat else can it be ?????
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What it *is* is a stupid question, with an almost unlimited number of potential ( ;) answers.
What's the frequency of the second voltage source? Is it superimposed on another voltage? Is the circuit next to a radar dish and the transmitter has started up? etc, etc, etc.
Engineers use the application(s) and environment(s) to construct paradigms by which to produce solutions to problems. Give them an infinite number of both and they will come up with an infinite number of solutions. Was the teacher ex-army?
-- Sue
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0. The frequency part is a likely answer as freq. of none of the sources is mentioned... 1. Filter all the "unlimited answers" via a practical mindset in a normal laboratory without any radar dish etc. things ( although a nice piece to think on. ) 2. Will be very much thankful to you if you can detail your answer instead of the myriad possibilities. 3. Nope, the teacher wasnt any ex-army guy.
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You have now (partially) defined the environment. Which you hadn't done in the original question. Now define the application. There are still a myriad of possible answers.
Including the obvious one that should always be considered first in any experiment. Instrumentation error.
The next obvious is that the new source has frequency components such as to excede the dv/dt rating of the thyristor. This may be because the fundamental frequency has changed, or because it is no longer a simple sinusoid (eg has harmonics..square wave, perhaps?), or because it has superimposed noise, etc.
The next obvious is that energy is being injected into the junctions by another route. eg the thyristor has a transparent case and a high level of incident radiation (visible or invisible) is being directed on it.
The next obvious..
Sorry, the list goes on and on and on. I repeat, it was a stupid question to ask of intelligent students who understand the underlying principles of electronic devices.
-- Sue
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Maybe he wants to hear you say that an SCR with a floating gate behaves as a Shockley diode.
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Mmmm.... plz elaborate on tht one....
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In article

I would have given a similar answer. I will add, LOOK IT UP!
For more fun, misspell it as Schottky.
Bill