[TCML] Toroid size

[bartb]

Hi Jim,

It's not that the cap is too big. It's the fact that the cap size will 
resonate with the transformer and cause the output voltage to go "very 
high" which is termed "resonant rise". These high voltages can overcome 
the caps standoff voltage or the transformers insulation causing a short 
in the high voltage windings. Thus, we tend to increase the cap size 
until we are out of resonance to keep the voltage down around the 
transformers rated output voltage. If the transformer and cap can handle 
this voltage, then fine. But if it's an NST, it will usually fail.

I can't say why your getting only 2 foot arcs without some more info. 
Spark lengths for power input to the coil is very dependent on the 
losses between input to output. The biggest area for losses is the spark 
gap. What type of spark gap do you have running?

Bart

Jim Calvin wrote:
> Hi Bart,
> 	I picked that value because I figured the transformers could drive
> that load (Z = 1/2*pi*60* C).  Is this too much capacitance?  And if this is
> the right size toroid why am getting only 2 foot arcs at my break out point.
> I figures I should get around 45-47 inches.  I used an oscilloscope/ signal
> generator to determine the secondary impedance and I tuned my primary to
> this frequency.
>
> -----Original Message-----
> From: tesla-bounces at pupman.com [mailto:tesla-bounces at pupman.com] On Behalf
> Of bartb
> Sent: Wednesday, March 05, 2008 11:30 PM
> To: Tesla Coil Mailing List
> Subject: Re: [TCML] Toroid size
>
> Hi Jim,
>
> I'm thinking 6" x 30" would be about right. This gives a decent smaller 
> diameter but not too big. The 30" outer size is due to the coil size.
>
> You are apparently running a resonant cap size with the transformer. Why?
>
> Take care,
> Bart
>
> Jim Calvin wrote:
>   
>> I have just finished a building a medium sized tesla coil.  It uses 9KVA @
>> 120ma.  It has an 8" x 33" secondary and uses a dual tungsten spark gap
>>     
> with
>   
>> a fan.  It has a primary tank capacitance of .0346uF made from an MMC cap.
>>
>>  
>>
>> I tried running it with my 30 inch toroid that I use on my big coil
>>     
> (7.8KVA,
>   
>> 7 feet tall).  I know this toroid is too big for this coil but it is the
>> only one I had available, besides my 12 inch toroid which I believe is too
>> small.  With no breakout point I got nothing.  When I put a breakout point
>>     
> I
>   
>> got about 2 foot arcs.  Now I expect to get about double that with the
>>     
> right
>   
>> size toroid.  So my question is: what is the proper size toroid for a coil
>> of this size and power? (18 inch, 20 inch . etc). 
>>
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>>
>>
>>   
>>     
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