Re: [TCML] Fixing up an old coil -- tune-up suggestions

[Jason Goodman <goodman_jason@xxxxxxxxxxxxx>]

On Nov 13, 2007, at 10:58 PM, Barton B. Anderson wrote:
> Cool! A Tesla Coil lying around is the best kind (it's mostly  
> built). The primary is odd. You mentioned an inner diameter of 14",  
> outer of 30" and a height of 20" (is that the vertical height from  
> inner to outer?). If so that would put a little over 2.5" spacing av  
> between turns.

About that, maybe a bit less.

> In any event, this particular coil would need about .012uF with no  
> primary changes. That's approximately 12 bottle caps (beer sized  
> bottles).

Yeah, one of the problems I've been wrestling with is figuring out the  
capacitance of the bottles... something that experienced coilers are  
likely to know off hand, but which involves some work to actually  
measure.  We'll probably just throw the six-pack into a simple RC  
circuit with a known resistance, throw a square wave at it, and look  
at the charging time.  I don't believe we have a specially-made LRC  
meter, but it's easy enough to do by hand.

> Bottle caps are cool (salt, oil, and water, what could be better!),  
> but an MMC is just as cool and will outperform the bottle caps.  
> Lower losses are handy especially if the transformer you have is a  
> low 30mA size (btw, for a 12kv/30mA transformer, in the range of . 
> 01uF is ideal for a static gap.

The MMCs I've seen on coilers' sites look like they involve a lot of  
painstaking soldering.  I'll mention this possibility to my students,  
but I personally don't have time to assemble an array like this during  
the school term.  Got a link to an online mail-order place that can  
provide the right caps?

> Maybe, get the tuning right first with what you have. You can scope  
> this. Connect a variable sweep function generator to the bottom  
> secondary winding. Hang the o-scope probe a few feet away with a  
> small wire (about toroid level). Sweep the generator to find the  
> frequency (the scope waveform will remain flat with a few amplitude  
> rises, but when the frequency is found, the amplitude will go  
> noticeably high). It should be in the range of 290 kHz. This will at  
> least tell you what your secondary is resonating at with the all the  
> components in their position (toroid, primary, walls, ceiling,  
> etc.). Then it's just a matter of getting the primary and cap to  
> oscillate at the same frequency.

I did something similar but not so elegant.  I built a little voltage  
divider:
Frequency generator
 |
 R   (10 K)
 |
 + scope
 |
secondary winding+topload

and looked for the minimum amplitude on the scope.  I got 393 khz, not  
too far from your guess, given my sketchy description.

(I did this on my own, in advance of talking to the students: the  
lecture on LC resonant circuits is scheduled for tomorrow.)

I tried to measure the resonance point of the primary circuit, but  
this was a bit trickier.  I got multiple resonances in the 1 Mhz  
range.  Things were a bit sketchy, probably because I can't physically  
remove the secondary from within the primary coil.  But it does  
suggest that boosting the capacitance by a factor of 2 or more is the  
way to go.

I've played a bit with the JavaTC program, looks like it can give me a  
good estimate of the inductance of the primary coil.  If we measure  
the capacitance of the bottles using an RC setup, I should be able to  
estimate the resonant frequency quite well, and figure out how many  
bottles we need.

> Verify your transformer's output current before making any primary  
> changes.

I assume that by "verify", you mean "read the label".  Directly  
measuring the current by placing an ammeter across the HV leads seems  
like a good way to get into trouble.

> The transformer needs to have enough power to charge and discharge  
> the cap in a reasonable time. If a 12/30, near .01uF cap value is  
> good. The primary then just needs to be built to accommodate a  
> reasonable coupling between coils and the inductance required for  
> resonance. Whoever built the coil did a good job actually. The  
> coupling is good at 0.133. I don't like the particular primary  
> geometry, but for what it is, it's in the correct distance from the  
> coil for decent operation. So, maybe double up on your bottle caps  
> and you'll near the needed resonant frequency. It's mainly a tuning  
> problem, but with the caps you have, you would need more primary  
> turns to get there. Since you don't have those, adding more bottles  
> to the mix is required.

My students seem to have a never-ending supply of empty Corona bottles  
in their dorm room, so this should be no problem.

Mdemming wrote:
> Most physics departments have 10-15 Amp Variacs with fuse,power  
> switch,  and
> output socket built in. Use one of these to power the coil if  
> you're  not
> going to build these features in.

Yeah, we've got some of these.  Good suggestion.
_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxxxxxx
http://www.pupman.com/mailman/listinfo/tesla