Re: Help! My Coil Blew Up

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

Original poster: Sean Taylor <sstaylor-at-uiuc.edu> 

On Thu, 05 Feb 2004 18:04:03 -0700, Tesla list <tesla-at-pupman-dot-com> wrote:

>Original poster: Finn Hammer <f-h-at-c.dk> Tesla list wrote:
>Steve,
>I`m fond of the antenna feedback circuit, but I`m not going to flame you 
>for pointing out that it is flawed.
>It is still the best we`ve got right now.

What about the capacitor in the ground path idea that Ken Herrick uses?
It gives a reliable signal, and a 90 degree phase shift is built in so when 
the voltage across the cap is at a max, the current is at zero - or more 
easily detected is the zero voltage crossing - where current is a 
maximum.  Ken can go into more detail since he's had a lot of experience 
doing this but I believe a capacitor in the ground gives a much more 
reliable signal for sensing when to switch and keep the system in resonance.


>However, when I swich from 494 to comparator, it is obvious that the 
>antenna does not drive the coil at it`s res. freq.
>
>It appears to me, that it would be good to ad an adjustable phase shift 
>option between the antenna and the comparator but I don`t know how to do that.

There are several methods for adjusting phase shift, but I suppose you want 
something with a constant phase shift over a wide range of 
frequencies.  All you need to construct is an all-pass filter with a dual 
variable resistor (I think that'll work - just need to keep the poles and 
zeros reciprocals).  But more likely would be something with a constant 
time delay to compensate for switching time.  Of course, a time advance 
can't be made, so a delay of a full cycle is the only option, but the 
obvious problem is that the length of a full cycle changes with Fres, which 
is exactly what we're trying to track changes of.  So, a PLL is the best 
thing I can think of to actually work well, but of course that is getting 
way to complicated.  Maybe some DSP on the primary current waveform to try 
to keep it as clean as possible by changing the switching time? :-)

Okay, enough "good" ideas for one night . . .

Sean