haklesup wrote: I think you're right about SW2, it must be to reset the SCR. A series resistor with the Anode of Q3 would limit any firing current which would be undesireable. On the line segnment just below the image of T1 maybe.
The HV generator includes a voltage doubler, with the output
across C4 (1000pF), but the capacitor bank and accelerator coil
(L1) are in parallel. Why have C4? 1000pF // 600uF = 600.001uF
A resistor in series between C4/D2 and R3 would limit the
available current from the HV generator to less than the holding
current of the SCR (once fired). It would slow the charging
of the capacitor bank, too. (Weapon firing rate). Another
method would be to stall the HV generator when the weapon
fires, so the SCR resets once the capacitor bank is depleted.
haklesup wrote: I'm not sure why the need to slow the gate voltage, dv/dt false triggering is ususlly associated with the voltage on the Anode.
Which can't rise quickly due to the realatively high
output impedance of the HV power supply...
haklesup wrote: D8 may act as a snubber for back EMF generated in the accelerator coil. D9 likewise seems to protect the capacitor bank from reverse bias due to the same back EMF.
Electrical energy in the accelerator coil that is not converted to
kinetic energy to move the projectile will cause a back-EMF
spike, reverse biasing the capacitor bank, so D9 prevents it.
The accelerator coil is mounted in a metal frame, effectively
producing a "shorted turn" secondary at each end of the
coil. This would dissipate energy coupled to the metal by
induction, robbing the energy transmitted to the projectile.
Cutting slots in the metal at each end of the coil would
prevent it. I wonder if the weapon's performance improves
(measureably) by doing so?
D8 can only conduct if the output of the HV generator were
reversed (go negative). I doubt any effect would be seen if
D8 is removed. (Unless due to poor layout the ground
bounce from the firing circuit reaches the low voltage stages,
as you noted).
haklesup wrote: I was incorrectly thinking bipolar supply on the 3V comment. I suppose the Zener would only turn on at 14V if the comparitor failed (pin 3 to 6 short) or ground bounced when the device is fired. This may be the ultimate reason for C7, R13, to keep the VGK constant with an AC transient on ground.
Why use zeners as signal diodes? Why
have two mutually exclusive LEDs, one would do.
I have doubts about the design...
haklesup wrote: Several sections of this circuit beg to be simulated
Or, there is old fashioned "black magic" in this one!
The author has a video on the linked website. The prototype
electronics were constructed on a solderless breadboard.
The finished weapon bearly nicks an empty soda can at
one metre when fired. A toy?