firing angle

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abdulazeem001

Joined: 07 Apr 2006
Posts: 7

11 May 2006 14:43 firing angle

HI. CAN THE FRIRING ANGLE OF THYRISTOR BE CONTROLLED FROM 0 TO 360 DEGREES.
IF YES, HOW?

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IanP

Joined: 05 Oct 2004
Posts: 6490
Helped: 1542
Location: West Coast

12 May 2006 3:09 Re: firing angle

With one SCR there is no point of controlling its gate beyond 180° as it is "dead" in that period of time - between 180° and 360°, that is ..

With two SCRs or one triac the control occures simetrically - first pulse is "shot" between 0° and 180°, the second pulse is generated (the same dalay from the zero-crossing) between 180° and 360°, so to a certain degree one can call it 0°-360° control ..

Regards,
IanP

vicky

Joined: 11 Aug 2004
Posts: 240
Helped: 19

12 May 2006 18:29 Re: firing angle

The thyristor is a four-layer semiconducting device, with each layer consisting of an alternately N-type or P-type material, for example P-N-P-N. The main terminals, labeled anode and cathode, are across the full four layers, and the control terminal, called the gate, is attached to one of the middle layers. The operation of a thyristor can be understood in terms of a pair of tightly coupled transistors, arranged to cause the self-latching action.

[edit]
Function of the gate terminal
The thyristor has three p-n junctions (serially named J1, J2, J3 from the anode).

Layer Diagram of Thyristor
When the anode is at a positive potential VAK with respect to the cathode with no voltage applied at the gate, junctions J1 and J3 are forward biased, while junction J2 is reverse biased. As J2 is reverse biased, no conduction takes place (Off state). Now if VAK is increased beyond the breakdown voltage VBO of the thyristor, avalanche breakdown of J2 takes place and the thyristor starts conducting (On state).
If a positive potential VG is applied at the gate terminal with respect to the cathode, the breakdown of the junction J2 occurs at a lower value of VAK. By selecting an appropriate value of VG, the thyristor can be switched into the on state immediately.
It must be noted that VG need not be applied after the avalanche breakdown has occurred. Hence VG can be a voltage pulse, such as the voltage output from an UJT relaxation oscillator.
These gate pulses are characterized in terms of gate trigger voltage (VGT) and gate trigger current (IGT). Gate trigger current varies inversely with gate pulse width in such a way that it is evident that there is a minimum gate charge required to trigger the thyristor.

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