Hello 'e' ,
Well, when i mentioned a limiting res on the gate, i was
actually referring to the diagram that i had drawn.
I had not looked at yours very closely, i assumed that it
was workable in all its major points.
I've increased my settings to 800 x 600 just so as i can
get most of your diagram on my screen without panning.
My monitor (Dell Vi 428 EBP) does not take kindly to my
video driver at that density, and reduces visible area
by about a quarter of an inch all round. Most unfriendly.
Especially on this little monitor!
However, ive had a good look at your diagram now.
Yes, i can see your intention for the transistor Q1.
It seems that you want its output to be at a very low
potential, to keep both SCRs switched OFF, until the 555
tells it otherwise.
Most SCRs are pretty fast, i would have to look up the
details to know just how fast, but i think they would
give a 2N2222 a run for its money.
See where i'm going here? ... that 2222 has to get its
instruction to conduct, and keep its collector voltage
nice and low, from the 555,
before those SCRs can
react, cos once they've started ... well, you know
If you do it the other way round, that is with nothing
conducting until the gate voltage is reached, i think
thats much better.
Heres a small extract from something i found at:
https://www.fairchildsemi.com/an/AB/AB-17.pdf
**********
analysis points out the one problem with the
circuit of Figure 2, namely, the voltage at
which crowbarring occurs is very dependent on
the characteristics of the SCR gate threshold:
the SCR is guaranteed to be off with 200mV on
its gate, but not guaranteed to be on with
less than 1.5V. The 1.3V spread means that
designing the SCR to trip on at a minimum of
3.0V means that it may not turn on until 4.3V.
**********
It points out that SCR trigger thresholds can vary quite
a lot, more than i thought actually. I have always taken
it to be between one and two volts somewhere, and at very
little current.
What i can say is that the trigger is always the same on
the device, although it may vary to another device.
I use crowbar clampdown on my own home made supplies, and
have also used gate repeating caps on some. This is an
arrangement where the resistor is just a little too high
for 'hold-on' current within the supply, and the SCR drops
out of conduction, allowing the zener to re-instate the
stabilised voltage. On one such supply i made, i used a
small LS and it clicks like a buzzy-bee if i short the
supply into over-current until i remove the short and then
the voltage rises back to its stabilised setting.
The same article mentions 2 microseconds for the SCR to
operate, well that sounds awful slow to me, if its that
slow, then maybe the 2222 would actually have time to
respond to the 555 and go into conduction
before the
SCRs have a chance to engage !
Which of course is what that circuit requires.
But i still prefer to have the SCRs just waiting for their
gates to come up, with no other bits active. The R-C work
on the gate is quite normal, as you say a second is a bit
longer than usual, thats why i would put a limiting res
on the gate, because if the cap has to be bigger than say
5 or 6 mfd, that might be a bit much for the gate, so i
would put a series res just so it wouldn't damage the gate.
The circuit you have drawn looks perfectly workable, so
long as Q1 doesn't get a brief 'lo' from the 555 just as
its starting up.
Thanks for the interest, i don't get to chat much about
electronics. No-one round here where i live has any
interest in it. Well not that i've seen anyway.
I'm going to reduce the resolution on my monitor now cos
i'm running 98se and i worry about crashes. This resolution
uses more RAM, and i only have 64 meg in there.
Cheers, John
(Oh, and thanks for the shorter line lengths!!)