What is a Thyristor | Types of thyristors

What is a Thyristor | Types of thyristors

In numerous ways, the silicon controlled therapy, SCR or just thyristor as it's further generally known, is analogous in construction to a transistor. 

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It's a multilayer semiconductor device, hence the name"silicon"from its name. It requires a gate signal to turn it"ON", the" controlled" part of the name and once the"ON"behaves like a therapy diode, which is the" therapy" part of the name. In fact, the circuit law for the thyristor indicates that this device operates like a controlled therapy diode. 

Thyristor symbol 

Still, unlike a junction diode which is a two- subcaste semiconductor device (PN), or the generally used bipolar transistor which is a three- subcaste switching device (PNP, or NPN), a thyristor is a four- subcaste semiconductor device (PNPN) that contains on three PN junctions in series, represented by the symbol as shown. 

A thyristor, like a diode, is a unidirectional device, i.e. it'll conduct current in only one direction, but unlike a diode, a thyristor can be made to operate either as an open circuit switch or as a therapy diode depending on how the gate of the thyristor is operated. In other words, the thyristor can only operate in switching mode and can not be used for modification. 

Silicon Controlled Rectifier, SCR, is one of numerous power semiconductor bias along with Triacs (Triode AC's), Diacs (Diode AC's) and UJT (Unijunction Transistor) which are all able of operating like veritably fast solid- state AC switches To control large voltages of interspersing current. and currents. So for electronics scholars this makes these solid state bias veritably handy for controlling AC motors and lights and for controlling phase. 

A thyristor is a three-terminal device called “ anode”, “ cathode” and “ gate” and consists of three PN junctions that can be switched “ ON” and “ OFF” at a veritably fast rate, or they can be switched “ ON” for variable ages of time during half cycles To deliver a specified quantum of power to theload.The operation of the thyristor can be better explained by assuming that it consists of two transistors connected in series as a brace of reciprocal regenerative switches as shown. 

The two transistor original circuits show that the collector current of the NPN TR2 transistor directly feeds into the base of the PNP TR1 transistor, while the collector current TR1 feeds into the base of TR2. These two connected transistors are dependent on each other for conduction as each transistor gets the base-emitter current from the collector-emitter current of the other. So until one of the transistors is given some base current, nothing can be indeed if there's an anode to cathode voltage. 

 When the anode outstation of the thyristor is negative with respect to the cathode, the center N-P junction is forward prejudiced, but the PN external junctions are prejudiced and bear like a normal diode. Thus, the thyristor prevents the inflow of rear current until the breakdown voltage point of the two external junctions is exceeded at the high voltage position, and the thyristor conducts without applying the gate signal. 

This is an important negative property of the thyristor, as the thyristor can be inadvertently turned on by conduction by rear overvoltage as well as overheating or a rapid-fire rise in the dv/ dt voltage similar as shaft. 

Still, the external PN junctions are now forward prejudiced but the center N – P junction rear prejudiced, If the anode outstation is positive with respect to the cathode. So the forward current is alsoblocked.However, the performing collector current flows into the base of the TR1 transistor, If a positive current is fitted into the base of the NPN TR2 transistor. This in turn causes the collector current to inflow in the PNP transistor, TR1 adding the base current for TR2 and so on. 

Controlled modified silicon 

The two transistors force each other veritably snappily to souse as they're connected in a regenerative feedback circle that can not be stopped. Once it's switched on in conduction, the current flowing through the device between anode and cathode is only limited by the resistance of the external circuit as the forward resistance of the device when connected can be veritably low at lower than 1 degree hence the voltage drops across it and the power loss is also low. 

Also we can see that the thyristor blocks current in both directions of the AC source in its “ OFF” state and can be turned “ ON” and made to work like a normal therapy diode by applying positive current to the base of the transistor, TR2 which is called for the silicon control therapy terminal” the gate".