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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Two transistor analogy of scr is a silicon based semiconductor device, which is used in electrical circuits for switching operation. Combining the two transsitor currents I C1 and I C2 yields This is a pnpn thyristor. The current gain a 1 varies with emitter current I E1 which is equal to I A ; and a 2 varies transistot emitter current I E2 which is equal to I k. Two transistor analogy of scr continuous positive feedback effect increases towards unity and anode current tends to flow at a very large value.

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.

The current through capacitor C j2 can be expressed as. An increase of I A which is an increase of I E1 would increase a 1 as shown in figure 4.

The two-transistor model is shown in figure 4. The content is copyrighted to www. That is why the name of thyristor consists of first four letters of thyratron tube and last five letters of transistor.

But we cannot turn it OFF by twl signal. 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.


SCR-Two Transistor analogy

It must be noted that a large current through the junction capacitors may cause damage to the device. Now the question is how increasing? 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.

The capacitance of the pn junctions are shown in figure teo. If the gate current I Two transistor analogy of scr is increased from zero to some positive value, this will increase the anode sdr I A as shown by equation 6. A typical variation of current two transistor analogy of scr a with emitter current I E is shown in figure 4.

The device has ideal states, i. Although there are many different members two transistor analogy of scr available in thyristor family, but silicon controlled rectifiers are so widely used that as if thyristor and SCR become synonymous.

Similarly, the collector current for transistor Q 2 is I C2 where SCRwhose full form is silicon controlled rectifieris also a well known member of thyristor family.

Silicon Controlled Rectifier SCR | Two Transistor Model | Operating Principle

It trqnsistor block both forward and reverse voltage but can conduct only in one direction. Here is the explanation using two transistor model of Two transistor analogy of scr. The value current then can only be controlled by external resistance of the circuit. 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.

Connects Facebook Tdansistor Videos. One being pnp and the other npn.


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

This model is used to demonstrate the regenerative or latching action due to positive feedback in the thyristor. A thyristor can be considered as two complementary transistors.

If a thyristor is in the blocking state two transistor analogy of scr a rapidly rising voltage is applied to the device, high currents would flow through the junction capacitors. That means we have control upon its turn ON, once it goes to conduction mode, we lose control over it. Related pages Concept of Power Electronics.

What is Thyristor or SCR? We can turn it ON by sending a gate current signal between second P layer and two transistor analogy of scr. The characteristic of thyristor consists of the characteristic of thyratron tube and characteristic of transistor. The emitter current of transistor Q 1 is the anode current I A of the thyristor and collector current I C1 is given by. Similarly, the collector current for transistor Q 2 is I C2 where.

Under transient conditions, the capacitances of the pn junctions influence the characteristics of the thyristor. At the first stage when we apply a gate current I git acts as base current of T 2 transistor sc.