Understanding thyristors (?) (was:AW: My new VCO)

[Rene Schmitz]

At 01:47 03.06.99 +0200, I wrote:

>How I think it is triggered: the "Gate" of the thyristor, the point where
>D1 and Q1s base meet, is held at 5volts (well almost there is a drop at Q3
>and R6), the current sink sinks towards 0V. Initially the cap is
>discharged, both plates at 10V. When the cap reaches (5volts - Ube) of Q1
>it starts to conduct. (A thyristor can be triggered from the anode too.)
>Then the current thru Q1 turns on Q2 which in turn acts to raise the gate
>potential, further increasing conduction in Q1 and so on... The two
>transistors are very soon fully saturated, and now the cap can get
>discharged very fast.
>When the cap is discharged only small currents can flow and the thyristor
>resets. I *guess* D1 is there to prevent the thyristor to keep conducting
>on the current that the expo convertor sinks, by altering the turn off
>current.

Forgot to mention, that the leakage of Q2 cannot forward bias the BE
junction of Q1, since the emitter of Q1 is at a higher potential than the
base. With that diode back biased the current is forced to flow out of the
gate, or if the gate is open it cannot flow! 

             ,   : (uzs159 at uni-bonn.de)
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