SCR Thyristor

SCR Thyristor (Silicon Controlled Rectifier) is a unidirectional semiconductor machine fabricated from silicon. This gadget is the solid-state equivalent to thyratron and hence it is also known as thyristor or thyroid transistor. In fact, SCR is a commerce name given to the thyristor by General Electric Company. Principally, SCR is a three-terminal, four-layer semiconductor machine consisting of alternate layers of p-type and n-type materials.

SCR Thyristor
Therefore it has three pn junctions J1, J2, and J3. The machine has terminals Anode(A), Cathode(Ok), and Gate(G). The Gate terminal(G) is hooked up to the p-layer nearer to the Cathode(Ok) terminal.


The Following Working Modes of SCR Thyristor

1. Reverse Blocking Mode of SCR

On this mode, the SCR is reverse biased by connecting its anode terminal (A) to the destructive finish and the cathode terminal (K) to the constructive finish of the battery. This results in the reverse biasing of the junctions J1 and J3, which in flip prohibits the stream of the present through the machine, despite the fact that the junction J2 stays in ahead biased condition.

In this state, the SCR behaves as a typical diode. In this reverse-biased situation, solely reverse saturation present flows through the machine as within the case of the reverse-biased diode which is proven in the attribute curve by a blue line. The machine also reveals the reverse breakdown phenomenon past a reverse secure voltage limit just like a diode.


2. Forward Blocking Mode of SCR

Right here a positive bias is utilized to the SCR by connecting anode terminal (A) to the constructive and cathode terminal (K) to the destructive terminal of the battery. Beneath this situation, junction J1 and J3 get ahead biased whereas junction J2 will get reverse biased.

Right here also present cannot pass through the thyristor except the tiny present flowing as saturation current as shown by the blue curve in the traits curve below.


3. Forward Conduction Mode of SCR

The SCR will be made to conduct both

(i) By rising the positive voltage utilized at anode terminal (A) passed the Break Over Voltage, VB or

(ii) By making use of positive voltage on the gate terminal (G) 

In the first case, the increase in the applied bias causes the initially reverse-biased junction J2 to interrupt down at the level similar to ahead Break Over Voltage, VB. This leads to the sudden enhance in the current flowing through the SCR as proven by the pink curve in the characteristic curve, though the gate terminal of the SCR stays unbiased.

Here it's seen that on applying a constructive voltage on the gate terminal, transistor Q2 switches ON and its collector present flows into the base of transistor Q1. This causes Q1 to show ON which in flip leads to the stream of its collector present into the base of Q2.



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