The silicon controlled rectifier SCR, is one of several power semiconductor devices along with Triacs (Triode AC’s), Diacs A Thyristors Two Transistor Analogy. Two Transistor model of Thyristor: The regenerative (latching) action of thyristor due to positive feedback can be explained using the two transistor model of the. SCR-Two Transistor analogy. Pradeep Chaudhary POWER Chopper drives-Two quadrant chopper drives ยท Electric drives-Three phase DC drives.

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The characteristic of thyristor consists of the characteristic of thyratron tube and characteristic of transistor. The capacitance of the pn junctions are shown in figure 4.

If we bisect it through the dotted line then we will get two transistors i. This device has tow states i. It is a silicon based semiconductor device, which is used in electrical circuits for switching operation.

Combining the two collector currents I C1 and I C2 yields 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. But we cannot turn it OFF by control signal. The value current then can only be controlled by external resistance of the circuit. This is a pnpn thyristor.

Although there are many different members are available aanlogy thyristor family, but silicon controlled rectifiers are so widely used that as if thyristor and SCR become synonymous. Now the question is how increasing? In other words, it can be said, that the characteristic of thyristor is combination of characteristics of thyratron tube and tdansistor. A thyristor can be considered as two complementary transistors.


If the gate current I G is increased from zero to some positive value, this will increase the anode current I A as shown by equation 6.

SCRwhose full form is silicon controlled rectifieris also a snalogy known member of thyristor family. An increase of I A which is an increase of I E1 would increase a 1 as shown in figure 4. Here is the explanation using two transistor model of Analogu.

That is why the name of thyristor consists of first four letters of thyratron tube and last five letters of transistor. The device has ideal 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.

Silicon Controlled Rectifier SCR | Two Transistor Model | Operating Principle

Connects Facebook Youtube Videos. The emitter analohy of transistor Q 1 is the anode current I A of the thyristor and collector current I C1 is given by. If a thyristor transisyor in the blocking state and a rapidly rising voltage is applied to the device, high currents would flow through the junction capacitors.

At the first stage when we apply a gate current I git acts as base current of T 2 transistor i. 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 analogu I k. A typical variation of current gain a with emitter current I E is shown in figure 4. The current through capacitor C j2 can be expressed as.


SCR-Two Transistor analogy

We can turn oof ON by sending a gate current signal between second P layer and cathode. The two-transistor model is shown in figure 4. This continuous positive feedback effect increases towards unity and anode current tends to flow at a very large value. 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.

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It can block both forward and analovy voltage but can conduct only in one direction. Under transient conditions, the capacitances of the pn junctions influence the characteristics of the thyristor.

Related pages Concept of Power Electronics. This model is used to demonstrate the regenerative or latching action due to positive feedback in the thyristor.

Similarly, the collector analogj for transistor Q 2 is I C2 where It must be noted that a large current through the junction tqo may cause damage to the device. Similarly, the collector current for transistor Q 2 is I C2 where. 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.

That means we have control upon its turn ON, once it goes to conduction mode, we lose control over it. One being pnp and the other npn.