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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 J_{1} and J_{2} junctions and another is with J_{2} and J_{3} junctions as shown in figure below.
The relation between the collector current and emitter current is shown below
Here, I_{C} is collector current, I_{E} is emitter current, I_{CBO }is forward leakage current, α is common base forward current gain and relationship between I_{C} and I_{B} is
I_{C }= βI_{B}
Where, I_{B} is base current and β is common emitter forward current gain.
Let’s for transistor T_{1} this relation holds
And that for transistor T_{2}
Now, by the analysis of two transistors model we can get anode current,
From equation (i) and (ii), we get,
If applied gate current is Ig then cathode current will be the summation of anode current and gate current i.e.
By substituting this valyue of Ik in (iii) we get,
From this relation we can assure that with increasing the value of
α_{1} + α_{2}
towards unity, corresponding anode current will increase. Now the question is how
α_{1} + α_{2}
increasing? Here is the explanation using two transistor model of SCR.
At the first stage when we apply a gate current I_{g}, it acts as base current of T_{2} transistor i.e. I_{B}_{2} = I_{g} and emitter current of the T_{2} transistor I_{E2} = I_{k}. Hence establishment of the emitter current gives rise α_{2} as
Presence of base current will generate collector current as
This IC_{2} is nothing but base current IB_{1} of transistor T_{1}, which will cause the flow of collector current,
IC_{1} and IB_{1} lead to increase IC_{1} as
and hence, α_{1} increases. Now, new base current of T_{2} is,
which will lead to increase emitter current
and as a result α_{2} also increases and this further increases.
As, α_{1} again increases. This continuous positive feedback effect increases
(α_{1 }+ α_{2})
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.
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