why does this SCR ever turn off after the first cycle?

[gello21]

this oscillates why?

Maybe i am overlooking the obvious, but why does this SCR ever turn off after the first cycle? The capacitor will charge and cause an inrush to get the pulse but
with the 470 ohm resistor in the cathode, why isn't it happy just providing
(ignoring the 10 ohm resistor at stasis) V+(20V) - Vled(1.6V) - V Scr(1.6V max at 10A) /470 ohms which is 35mA
well above the holding current of the SCR which is 5mA.
Is there some effect i am missing (spiking?) or am i just that rusty at
thyristor circuits.

I have not built the circuit, so can not state unequivocally that it does work.
I am quite interested in it as a low component
count, high amp, high supply voltage pulser.

**broken link removed**

thanks

gello

 

[aProgrammer]

this oscillates why?

the 470 ohm resitance discharge the capacitor when the scr is off,

and the scr charge the capacitor when swiched on, but the scr cannot stay on
because when the capacitor is charged,
the scr current will be to low (below holding).
so the scr goto off state...

 

[gello21]

this oscillates why?

that is what i would have thought, but 470 ohms is too small. the curves for the device show that the voltage drop a-K is only 1.6V at 10A and even less at lower currents. so the capacitor will charge for a time constant or so then the resistor passes all the current. at 470 ohms the current is 35 mA. the holding current for the device is 5mA max. so there should be an inrush of current then a steady state of 35mA or so through the resistor. now at 4700 ohms i would think it might work depending on the variation of the device spec which might work for a one off but not for production. unless of course i am calculating steady state current incorrectly. Any other ideas?