What is a thyristor ? What are its uses ?

What is a thyristor ? What are its uses ?
Ans. :   A thyristor is a special kind of semi conductor device that uses internal feedback to produce  latching action.
Use : Used for controlling large amounts of load power in motors, heaters, lighting systems etc.
Explanation : Because of the unusual connection we have a +ve feedback also called regeneration. A change in current at any point in the loop is amplified and returned to the starting point with the same phase. For instance if the 02 base current increases, the 02 collector current increases. This force base current through 01. In turn this produces a large 01 collector current which drives the 02 base harder. This build up in currents will continue until both transistors are driven in saturation. In this case the latch acts like a closed switch.
 
The purpose of the gate is to enable the device to be switched from a non-conducting (forward blocking) mode into a low resistance, forward conducting state. Thus a small current applied to the gate is able to switch a much larger current (at a much higher voltage) applied between anode and cathode. Once the thyristor is conducting however, the gate current may be removed and the device will remain in a conducting state.
To turn the thyristor off, the current flowing between anode and cathode must be reduced below a certain critical "holding current" value, (near to zero); alternatively the anode and cathode may be reverse biased.
The thyristor is normally made to conduct by applying a gating pulse, while the main anode and cathode terminals are forward biased. When the device is reverse biased a gating pulse has no effect.
The main application for thyristors is in the switching of high power loads. They are the switching element in many domestic light dimmers and are also used as control elements in variable or regulated power supplies.
Figure 4 shows a typical characteristic curve for a thyristor. It can be seen that in the reverse biased region it behaves in a similar way to a diode. All current, apart from a small leakage current is blocked (reverse blocking region) until the reverse breakdown region is reached, at which point the insulation due to the depletion layers at the junctions breaks down.
In the forward biased mode, unlike a normal diode, no current apart from a small leakage current flows. This is called the forward blocking mode. If a gating pulse is applied however, the thyristor "fires" and the forward resistance of the device falls to a very low value, allowing very large (several amperes) currents to flow in the forward conducting mode. Thyristors can also be made to fire by applying a very large forward voltage between anode and cathode, but this is not desirable as the device is not then being used to CONTROL conduction.
On the other hand , if something causes the 02 base current to decrease, the 02 collector current will decrease. This reduces the 01 base current. In turn, there is less 01 collector current, which reduces the 01 base current even more. This regeneration continues until both transistors are driven into cut off. At this time the latches like a open switch. This latch will always stay in open or close position.
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