Re: Zero Crossing Circuit

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Barton B. Anderson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <tesla123-at-pacbell-dot-net>

Hi Jon,

Thanks for clarifying the schematic. This circuit looks pretty good. I
think I'll have to
give it a go. How much delay did you say you could achieve with this?

Thanks again,
--
Barton B. Anderson
http://www.classictesla-dot-com

Tesla list wrote:

> Original poster: "Jon Tebbs by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<jgtebbs-at-eos-dot-net>
>
> Hi Barton,
>
> I've inserted my comments below:
>
> Tesla list wrote:
> >
> > Original poster: "Barton B. Anderson by way of Terry Fritz
> <twftesla-at-qwest-dot-net>" <tesla123-at-pacbell-dot-net>
> >
> > Hi - whoever posted the schematic at the following url:
> > <http://www.macinhouse-dot-com/tesla/stsg_zc_driver.gif>http://www.macinhouse.c
> > om/tesla/stsg_zc_driver.gif
> >
> > My humble appologies. I was going through mail, brought up the schematic,
> > deleted the email, then looked at the schematic and wanted to reply. Sorry
> > about that.
> >
> > Can you verify the following for me? (I'm curious if I'm getting the
> circuit):
> >
> > It looks like the zero-crossing is taking place at Q1 and Q2. These
> transistors
> > are used to drive the input of pin 2 of the 555 at zero-crossing.
>
> This is correct. Diodes D1 and D2 along with R1 & R2 maintain Q1 base in
> conduction until the zero crossing occurs. Q1 and Q2 buffer the rising
> and falling edges to produce the trigger pulse to the 555.
>
> > (Couldn't a
> > comparitor be used here to replace Q1, Q2, and the 555?).
>
> Yes. In the interest of keeping high impedance devices out of an
> otherwise harsh RF environment, I chose to forgo the use of a
> comparator. In an even simpler approach, one could conceivably drive the
> trigger pin 2 of the 555 from the D1 D2 network, with the addition of a
> 12 volt zener to clamp the half waves from exceeding Vcc. The rise and
> fall times of this method would be rather slow and there would be
> additional delay introduced due to the 1/3 Vcc threshold that would add
> to the minimum delay obtainable.
>
> > Anyway, the detected
> > zero crossing input is clocking the 555 output, of which is driving the
> base of
> > Q3, whose 50k pot is adjusting the voltage ratio for the base of Q3.
>
> Point of confusion: There is no connection between R6, R7 and R8. This
> is a cross-over, R6 connects to R8 forming the variable time constant
> with C3. The pin 3 of the 555 connects only to R7. This provides drive
> to the Q3 emitter follower which then provides drive current gain for Q4.
>
> > It appears this is a circuit where a delay is used (variable R6, R8, and
> C3 for
> > when Q3 turns on) which defines when Q4 dumps the voltage across C4 and R10
> > through the primary of the HEI.
>
> Q4 is in conduction from the time of each zero crossing until the preset
> delay. Q4 is then switched off, just like in an automotive ignition
> system. C4 is just a snubber and D5 the usual damper. Think of this as
> an Horizontal Output stage. Same principle, the H.O.T. maintains current
> flow through the flyback transformer primary and when it is switched
> off, the flyback does it's thing.
>
> > I guess this is what I'm getting at.
> > Is this circuit detecting zero-crossing, then delaying "from zero" when
> the gap
> > will fire?
>
> This is correct.
>
> > Thanks for any help with this
> > (I'm not the greatest at circuit theory),
> > --
> > Barton B. Anderson
> > <http://www.classictesla-dot-com>http://www.classictesla-dot-com
> >
>
> Thanks for the opportunity to clarify the finer points. It is good to
> see the interest in this.
>
> Cheers,
> --
> Jon G. Tebbs
> <jgtebbs-at-eos-dot-net>