Re: Toroid selection

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

Original poster: "G by way of Terry Fritz <twftesla-at-qwest-dot-net>" <bog-at-cinci.rr-dot-com>

Yes, John, I have limited space and was trying to avoid having 
several sizes of toroids. Ideally, I'd like to use the same primary, 
secondary, and topload with both the nst and mot power supplies. I 
estimated the toroid size by plugging in the mot coil numbers into 
QTC and trying toploads till I got a good match for the primary cap. 
After giving this some more thought, it may be easier to have one 
toroid per PSU. Here are some of the specs I have come up with using 
the QTC excel worksheet:

~MOT COIL (two xfmr/doubler)~
secondary: 8.5 X 26 winding, 26 awg, 1625 turns;
freq. 109 kHz unloaded, 49 kHz with 52 pf topload;
primary cap: geek mmc, 3 strings X 11 caps, 0.41 uf;
primary: 1/4" copper, 1/4" spacing, tapped at 16 turns for 251 uh;
gap: asrg- rt. angle grinder, spdt configuration, 833 bps max

~15/50 COIL~
secondary: same
primary cap: MMC set to 0.27 uf
primary: same, tapped at 17 turns for 283 uh
topload: 35 pf for 57 kHz
gap: 120 bps srsg

I like the numbers I'm seeing with the 8" secondary. Currently I have 
a 6" secondary. Frequency has come down a lot, and I still have many 
turns to limit gap losses too!

ps- I happened across my 8" secondary in a construction scrap pile. 
The builders had pieces of 14" dia. pvc laying there too, too!! Curse 
my small house...



>Gregory,
>
>Yes, you can calc the largest toroid size that will breakout
>for your coil.  But the calc is a ballpark figure.  Many factors
>go into the calc, such as:  MMC value, break-rate, secondary
>size, type of spark gap, safety gap spacing and design,
>input voltage (normal, or 16% over).
>
>If I understand you correctly, you're saying that you need
>a 55pF toroid to place the coil into proper tune.  But you can
>easily retune it by changing the tap point on the primary, for
>a larger or smaller toroid, so I don't see
>any problem with just building the coil, and trying the 55pF
>toroid, and if it doesn't breakout, simply retuning the coil for
>the new toroid size.  Or are you saying that you want to
>avoid the job of building various toroids?  How did you
>determine that you need a 55pF toroid?
>
>John Freau

>Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" 
><FutureT-at-aol-dot-com>
>
>In a message dated 8/10/01 6:37:47 PM Eastern Daylight Time, tesla-at-pupman-dot-com
>writes:
>
>>  Original poster: "G by way of Terry Fritz <twftesla-at-qwest-dot-net>"
><bog-at-cinci.rr.
>>  com>
>> 
>>   Hello all,
>> 
>>   Is there a way to determine maximum toroid size in the design stage?
>>   I have designed an MMC and secondary for a MOT coil, but to use them
>>   with a de-potted 15/50 NST, I would need a 55 pF toroid. While I
>>   won't have a problem building a toroid in this size, I would like to
>>   know if it will breakout before I buy the MMC caps and are committed
>>   to a frequency range.
>> 
>>   Thanks!!
>>   Gregory