Re: 4 Wonderful MMC/Gap questions

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

Original poster: "Terry Fritz" <twftesla-at-uswest-dot-net>

Hi Matt,

At 09:09 AM 4/22/2001 -0400, you wrote:
>Hi,
>I have been on and off gradually building my coil, and am finally at the 
>Capacitor point, and have a couple of questions that I'm hoping people can 
>help me with.  The construction has gone suprisingly well (for me) up to 
>this point, and I'd like to have to solder caps/resistors only once (vision 
>failing and what not).
>I realize that these are pedestrian questions, but would very much 
>appreciate some assistance.
>
>My coil's specs:
>12 kv 60 ma NST
>4.24" x 23.25" secondary (22 ga wire.... pvc form, polyurethane, etc)
>Primary is 1/4" cu tubing 15 turns 6.5" I.D., 20.5" o.d. 1/4 inch spacing 
>(inclined 15 deg.)
>Toroid is 17" dia., (4" dia. ducting)
>Filter is modeled after Terry's (complete with MOVs even)
>Gap (un-built, but will be static  -see below )
>
>
>Question 1.
>(probably a most common one):
>I am wondering what size cap I should construct.
>I have used Ed Sonderman's spreadsheet and E-tesla, and I think I am doing 
>things correctly, but am hoping for some validation.
>The program gave me a cap size of 8.33 nf
>I have read that larger may be better, but I have no plans to build a rotary 
>gap.  So..... I was wondering if I should target 8.33 nf or can I go up in 
>size a bit (and if so, what would be a practical limit).
>(any benefit to a LTR size cap without a rotary gap?)

"I" would go for the static LTR size which is 1.5 x the resonant size.
This is safer for the NST and such.

For a 12/60 NST the resonant size is 13.263nF.  So an LTR size is 20nF.
Your peak voltage is around 17kV.

>
>Question 2.
>I am using the same caps that Terry F. uses (Panasonic 1600V .056 uf) for my 
>mmc, and originally settled on 4 (or 5) strings of 26 caps but that seems 
>like a heck of a long string....... I have enough caps for that, and would 
>rather have a bullet proof mmc, but ---> is there a point where too many 
>caps per string degrade performance or create some kind of problem?

13 caps per string will handle the voltage very easily (I use 10 :-))
Using MMCcalc...

MMC Calculator  Ver. 2.2   9/12/2000  Terry Fritz
Transformer voltage =  12000 
Transformer current =  .06
Firing voltage =  16970.52 
Fo =  200000 
Break rate =  120 
Thermal dissipation constant =  40 
Individual cap value =  5.6E-08 
 
Strings  Caps/Str  Capacitance  Voltage   Temp C    Cost              I Arms
  4       11         20.36        17600     4.08   79.20  :-))  :-))  7.94 
--------------------------------------------------------------------
  5       14         20.00        22400     2.52  126.00  :-))  :-))  7.8 
  6       17         19.76        27200     1.71  183.60  :-))  :-))  7.71 
  7       19         20.63        30400     1.37  239.40  :-))  :-))  8.05 
  7       20         19.60        32000     1.23  252.00  :-))  :-))  7.64 

So you have say 8 amps RMS.  Three strings minimum are needed.

Sooo.  You need at least 13 caps per string and at least 3 strings for a
good solid Static LTR MMC.  The value should be 18-20nF.

4 x 11 would be fine.  5 x 14 would be super fine.  4 x 12 may give a
little better match with a filter.  So "I" would go with 4 x 12 caps.  5 x
15 would be super tough but I am not sure the extra $50 in caps is worth it.

>
>ok,... a couple more since I'm here:
>Question 3.
>Are 4 (or 5) strings adequate for the specs given?

Yes, the currents around 8 amps max and each string handles 3.5 amps.  

>
>Question 4. (non cap question)
>The gap..... I sure am having trouble finding info on gap spacing.
>Any recommendations for a good NST safe total gap spacing for a 12kv NST?
>(also I would welcome suggestions on the optimum number of segments for a 
>static gap too)

I have and adjustable one at:

http://hot-streamer-dot-com/TeslaCoils/Misc/terrygap.jpg

>
>Sorry for the glut of ?'s and the relative lack of depth therein ....  
>Obviously I am green to H.V. and coiling.....  But am looking forward to the 
>"first light" feeling.
>I welcome any replies (thanks in advance).
>I'm prepared to face those little caplets with a look of false confidence 
>(inspired by other's suggestions) so they don't sense my weakness.
>

If you like resonant size caps, here is the data:

MMC Calculator  Ver. 2.2   9/12/2000  Terry Fritz
Transformer voltage =  12000 
Transformer current =  .06 
Firing voltage =  16970.52 
Fo =  200000 
Break rate =  200 
Thermal dissipation constant =  40 
Individual cap value =  5.6E-08 
 
Strings  Caps/Str  Capacitance  Voltage   Temp C    Cost              I Arms
  1        6          9.33         9600    22.85   10.80  :-((  :-((  4.7 
  2       12          9.33        19200     5.71   43.20  :-))  :-)   4.7 
--------------------------------------------------------------------
  2       13          8.62        20800     4.87   46.80  :-))  :-))  4.34 
  3       18          9.33        28800     2.54   97.20  :-))  :-))  4.7 
  3       19          8.84        30400     2.28  102.60  :-))  :-))  4.45 

Hope all this is clear 8-))  Ask more if you need...  See the post "Re: Res
Caps" today too.

Cheers,

	Terry


>Matt.
>
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