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

[Mddeming@xxxxxxx]

Hi Dr. R, Jason,


I fully agree with the MMC being much more efficient, but these two  quotes 
show you may be headed for different goals. If the intent is  
renovation/modernization, then MMC is definitely the way to go. If restoration  is the intent 
then stick with the bottle caps.  
 
" I would also recommend 
replacing the very lossy glass bottles  (20-25% loss in dielectric heating) 
with more efficient and modern MMC  capacitors."



> "Thanks for any help you can give.  Please keep in mind that  we're not 
> interested in redesigning and rebuilding this from the  ground up: we  just 
> want to get it working to original design  spec."


Other comments interspersed below:

> Hi, all, I'm new to this list.  I'm a physics professor at  a small 
> liberal arts college.  About 5 years ago, before I was  hired, one of  our 
> physics students built a Tesla coil for his  senior project.  It's  been 
> gathering dust in our machine  shop ever since.  Some of my  current 
> Electricity and  Magnetism students and I are working on  putting it back 
> into  working order.
>
> We haven't been able to find the original  construction plans which  the 
> student followed to build it; if  you recognize the design, a link  to the 
> plans would be  appreciated.



Many Tesla coils are built without a formal plan or blueprint. There  are 
only a half-dozen parts to a basic coil.
 

> I've got several questions about getting this thing tuned up  and  working 
> well, so let me describe it first.  Here are  the vital  statistics of the 
> coil as we found it.  I'm  writing this from home,  so I'm going by memory 
>  here....
>
> 125 v->12.5 kV transformer to power primary  circuit, toaster-sized  white 
> box (neon sign transformer?)
 
If the white toaster has well-rounded top corners and edges, it is  probably 
an Actown NST. In my experience, these are somewhat more fragile than  
Franceformer brand, which have sharply rectangular edges. 

> 6 brine-filled glass bottle capacitors made from root beer  bottles
 
You may need more of these on parallel.
 
> Spark gap: single gap made from two trailer hitch balls mounted on a  
> wooden frame
 
Brass balls of the same size would dissipate heat and quench better.  Blowing 
air across the gap would also increase performance. If the mounting  bolts 
screw directly into the wooden frame, this could be a source of power  loss and 
danger. At these voltages, wood becomes conductive, especially if  there is 
any humidity present. This can lead to carbon arcing and provide a  bypass 
alternative to the gap.  A piece of plastic pipe as a collar  between  the bolts 
and the wood should improve both safety and  performance.
 

> Primary circuit wiring: Heavy gauge wire (4-gauge?  6?)  with pretty  thick 
> insulation, type unknown.   Alligator-clip connectors to primary  coil.
> Primary coil:  Inverted conical helix made from 1/4" copper pipe,  about 
> 7-8  turns, smallest diameter 14 inches, largest diameter 30  inches, 
>  height about 20 inches, mounted on triangular plywood supports.
>  Secondary coil: 14-16 gauge wire on white PVC pipe, 5"ish diameter,  36"  
> length
> Top load: 30" toroid made from 4" diameter metal  ventilation ducting, 
> mounted on foil-covered plywood  disk.
>
> There are no safety or protection circuits built into  the system as  we 
> found it -- no safety spark gap across the  transformer, no filters  or 
> switches on the mains side of  things, no grounding rail above the 
> primary, all stuff I've read  about.  I've made it very clear to the 
> students that the primary  circuit is absolutely lethal... I'm not  sure 
> they took me  seriously at first, but they sure did once they saw  and 
> heard  the spark gap firing.
>
 
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.
 

> Repairs:
> The bottle caps had been sitting filled for  five years.  Exciting 
> chemistry had taken place between the  brine, the oil, and the metal  bolts 
> used as electrodes, leaving  an unholy sludgy mess inside.  We  just threw 
> these out,  replacing them with a six-pack of Corona (heh)  bottles, built 
>  more or less according to http://www.pupman.com/ 
>  listarchives/1995/december/msg00040.html
>

> We wired the thing with the supply transformer bridging across  the 
> capacitor  (http://en.wikipedia.org/wiki/Image:Tesla_coil_4.svg  ) --  I  
> now understand that it's better for the transformer to put the  
> transformer bridging across the spark gap (http://en.wikipedia.org/  
> wiki/Image:Tesla_coil_3.svg).
 
The spark gap across the transformer will greatly improve the longevity  of 
your transformer, adding a safety gap would be a big plus. I would not open  
the main gap more than 6 mm.
 
  The "ground" end of the secondary was 
> attached to a  copper pipe providing compressed air throughout the 
>  building.
>
 
This could be dangerous by raising the potential of the building's air  
system to several hundred volts above ground potential. =:0

> We placed a grounded pipe (galvanized 1/2" steel electrical  conduit)  near 
> the toroid to draw a spark.
>
 
Having the base of the coil and the rod grounded to the same place will  
reduce ground loops, eddy currents, and shock potential.
 

> Testing:
> We first ran a test of the power supply without  the primary coil in  the 
> circuit, shorting the alligator clips  together.  With the spark  gap too 
> wide, we heard a hum and  a quiet snappy crackle.  We could  see some 
> corona  discharge from the aluminum foil surrounding the  bottles, but they 
 
> didn't seem to be heating up etc.  With the spark  gap set  at 5-7 mm, we 
> got a nice loud, bright spark, and the students   were impressed.
>
> When we hooked up the primary coil, a  fluorescent tube held in the  hand 
> lit up from about 6 feet  away.  We got corona discharge from the  end of 
> the  grounded pipe to the toroid when the end was about 8-10  inches away,  
> and occasional dim sparks when the end came within 6-8   inches.  When the 
> pipe end was raised so the shaft of the pipe  rather  than the end was 
> closest to the toroid, we could draw a  steady  crackle of bright sparks 
> 5-6" long.
>
>  Tuning was done by adjusting the position of alligator clips on the 
>  primary using trial-and-error, just fiddling to try to get the  longest  
> spark.  Best we could do was 6-8 inches of spark.
>
 
Inside connection attached permanently to the beginning of the primary  coil 
would be an improvement as every temporary connection is a potential  power 
loss.
>
> So, not too bad, but I feel like this coil is  probably capable of  more. 
> Questions for you:
>
>  1)  What should we expect from this coil?
 
Double or triple the output spark size.
 

> 2)  If we should expect more, is the problem likely one of  LC tuning,  or 
> should we be looking for power loss  elsewhere?
 
Tuning is probably the biggest problem.
 

> 3)  Is it worth going to the extra effort of measuring the  resonance  of 
> secondary and primary using an oscillator and  oscilloscope and  tuning 
> that way, or is that just lipstick on a  pig at this point?
 
I would get an LCR meter and determine actual values, and use a  scope.
 

> 4)  Is a little corona discharge from the bottle capacitors  a  problem? 
> If so, how can we correct it?  (Trim sharp  edges from the  aluminum foil?)
 
It's always a loss, but looks cool. Eliminating sharp edges would  help.
 

> 5) Do the two circuits mentioned earlier perform equally well,  with  the 
> only difference being protection for the transformer,  or does one  give a 
> better zap?
>
 
Commented earlier.
 
Hope this helps,
 
Matt D.






************************************** See what's new at http://www.aol.com
_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxxxxxx
http://www.pupman.com/mailman/listinfo/tesla