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

[Rene Schmitz]

Hi 
>Does this make sense ? And is it different from "normal" thyristor action ?
>(What prevents the leakage currents in a normal tyristor from forming
>a positive feedback without external gate voltage, btw. ?)
>And what is the purpose of the diode ?
>
>I don't know if this shed some light. If not, I hope it will "trigger"
>someone
>else to put it right or explain it further. (I admit when I did my first
>experiments
>with these circuits, I was looking for a reverse breakdown to trigger it
>(;->) )
>
>JH.

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.

Bye
 René




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