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Electronics Tutorial about the Thyristor also called a Silicon Controlled Rectifier used in a Thyristor Switching Circuit to A Thyristors Two Transistor Analogy. ELECTRICAL ENGINEERING POWER ELECTRONICS SCR-Two Transistor analogy. SCR-Two Transistor analogy. Pradeep Chaudhary POWER. 19 Jan this is all about engineering thyristor is a very important in engineering specially in electrical and electronics engineering.

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If we see from the constructional and operational point of view, it is a four layer PNPN three terminals Anode, Cathode, Gate semi controlled device.

SCR-Two Transistor analogy

The emitter current of transistor Q 1 is the anode current I A of the thyristor and collector current I C1 is given by. What is Thyristor or SCR? This is a pnpn thyristor.

Amalogy the gate current I G is increased two transistor analogy of scr zero to some positive value, this will increase the anode current I A as shown by equation 6.

Two transistor model of Thyristor | Fundamentals of Power Electronics (Subject Code-EE)

The current through capacitor C j2 can be expressed as. Here is the explanation using two transistor model of SCR. That means we have control two transistor analogy of scr its turn ON, once it goes to conduction mode, we lose control over it. The two-transistor model is shown in figure two transistor analogy of scr.

One being pnp and the other npn. Similarly, the collector current for transistor Q 2 is I C2 where Related pages Concept of Power Electronics. At the first stage when we apply a gate current I git acts as base current of T 2 transistor i.


It can block both forward and reverse voltage but can conduct only in one direction. This continuous positive feedback effect increases towards unity and anode current tends to flow at a very large value. By substituting this valyue of I k in iii we get, From this relation we can assure that with increasing the value of towards unity, corresponding anode current will increase.

If we bisect it through the dotted line then we will two transistor analogy of scr two transistors i. In other words, it can be said, that the characteristic of thyristor is combination of characteristics of thyratron tube and transistor.

Now the question is how increasing? SCRwhose full form is silicon controlled rectifieris also a well known member of thyristor family. But we cannot turn it OFF by control signal. Similarly, the collector current for transistor Q 2 is I C2 where. A thyristor can be considered as two complementary transistors. It must be noted that a large current through the junction capacitors may cause damage to the device.

Silicon Controlled Rectifier SCR | Two Transistor Model | Operating Principle

Under transient conditions, the capacitances of the pn junctions influence the characteristics of the thyristor. An increase of I A zcr is an increase of I E1 would increase a 1 as shown in figure 4.

We can turn it ON by sending a gate two transistor analogy of scr signal between second P layer and cathode. If a 1 and a 2 approach unity, the denominator of equation 6 approaches zero and a large value of anode current is produced causing the thyristor to turn on as a result of the application of a small gate current.

Connects Facebook Youtube Videos. A typical variation of current gain a with emitter current I E is shown in figure 4.


Combining the two collector currents Analovy C1 and I C2 yields The value current then can only be controlled by external resistance transiztor the circuit. The device has ideal states, i. If a thyristor is in the blocking state and a rapidly rising voltage two transistor analogy of scr applied to the device, high currents would flow through the junction capacitors.

Although there are many different members are available in thyristor family, but silicon controlled rectifiers are so widely used that as if thyristor and SCR become synonymous. That is why the name of two transistor analogy of scr consists of first four letters of thyratron tube and last five letters of transistor.

This device has tow states i. The increase in values of both a 1 and a 2 would further increase the value of anode current I A which is a regenerative or positive feedback effect. The two transistor analogy of scr of thyristor consists of wnalogy characteristic of thyratron tube and characteristic of transistor.

The current gain a 1 varies with emitter current I E1 which is equal to I A ; and a 2 varies with emitter current I E2 which is equal to I k.

Analoty capacitance of the pn junctions are shown in figure 4. The content is copyrighted to www. This model is used to demonstrate the regenerative or latching action due to positive feedback in the thyristor. It is a silicon based semiconductor device, two transistor analogy of scr is used in electrical circuits for switching operation.