Re: Another capacitive transformer TC

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

Original poster: "Antonio Carlos M. de Queiroz by way of Terry Fritz <twftesla-at-qwest-dot-net>" <acmq-at-compuland-dot-com.br>

Tesla list wrote:

> Original poster: "Jolyon Vater Cox by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <jolyon-at-vatercox.freeserve.co.uk>

> With regard to increasing the capacitance of the timing
> capacitor C3 from 1.8nF is there any suggested
> value/construction/tolerance?
> maybe increase C3 by 10 to 18nF or (next best) 22nF?
> Or would 10nF be acceptable?

The oscillation frequency of a 555 oscillator, in the configuration
with 2 resistors, Ra between Vcc and pin 7 and Rb between pins
6 and 7 is f=1.44/((Ra+2*Rb)*C). The output periods are:
High: t1=0.695*(Ra+Rb)*C
Low:  t2=0.695*Rb*C
What is wanted is an almost square wave (Rb>>Ra) where each period
is long enough for the dissipation of the transient.
This time must be somewhat greater than Q/fo, where Q is the effective
Q of the coil system and fo is the average oscillation frequency.
So, a good value for the capacitor in the oscillator is:
C >> Q/(fo*0.695*Rb)
For my system, for example: Q=100, fo=300000, Rb=22000
C >> 21.8 nF
I am using 220 nF, and Ra=1000 Ohms.

> How critical is capacitor construction/tolerance for the
> timing cap?
> I understand electrolytic not advisable for timing
> plastic film is possibly OK and
> polystyerene or silver mica is better.

Nothing is critical in this case. You just need the fast transitions
to excite the system.
 
> My visualisation of the output circuit is this:-
>  Vcc------+
>           | c
>           /   TR1
> Pin 3-+--|    BC337
>       |   \ e
>       +---|------>   O/P Up to 1nF              |
>           +------>   O/P Greater than1nF
>           |
>           100R
>           |
>           +------> low volt/ low Z
>           |
>           1R
>           |
>      0V --+------>GND
> 
> Am I correct?

Yes.

Antonio Carlos M. de Queiroz