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Re: [TCML] New Coil Questions

----- Original Message ----- From: "Tyler Pauly" <rpggod714@xxxxxxxxx>
To: <Tesla@xxxxxxxxxx>
Sent: Wednesday, May 27, 2009 3:47 PM
Subject: [TCML] New Coil Questions

Hi Tyler,

Welcome back! ;^) I am certainly no authority on this
subject but do have some practical experience with
pig driven coil systems, so I'll give it a shot:

Hello all,

I've been out of the tesla coil "circuit" for a while now but I'm looking to get back in. I've had a 10kVA pig sitting in my garage for a while and I want to put it to use. Some questions....

1) Would it be okay to go over rating on the pig? I assumed as much, but figured it wouldn't hurt to ask. I'm thinking somewhere in the range of 12-13kVA if I do.

From my experience with pole pigs, they are very brute beast
and can easily handle overrunning of their kVA ratings, if the
duty cycle is kept within reason. "I" would say that you could
run a 10 kVA pig at 13 kVA max all day long w/out any ill-
effects. Remember, their nameplate rating means at 100% duty
cylce (24/7) and in the most extreme of environmental conditions
(like in the full sun, at noon, in July, in Phoenix, AZ) ;^), so I
don't think your running yours for a few minutes at a time in
your garage to drive your coil is gonna hurt it at all.

2) Is the most straightforward route for the tank capacitor an MMC, or are there other (easy-to-find) capacitors that would work as well?

For pig sized coils, it gets to where the MMC route is not longer
practical, as it will require literally hundreds of them to get the
adequate voltage standoff in combination with a sufficient capa-
citance! As Scot suggested, I'd try to hit up fellow list members
for extra Maxwell pulse caps that they could part with for a rea-
sonable fee. Caps like these do occasionally show up on eBay
but seem to be getting harder and harder to find here in recent
years. Of course, if money is no object, you could always just
order (or have one custom made) from one of those commercial
pulse capacitor manufacturers.

> 3) I have an equation that tells me the capacitive reactance of my xmfr. Do I want my tank capacitor to have that value, or would the resonant effects be a bad thing? What value should I shoot for?

Resonance impedance matching of the transformer with the primary
capacitor in Tesla coil circuits is a somewhat obsolete technology
these days, as it has been found that running near resonance with
NSTs usually causes an early death for the NST. And the required
capacitance to run res. with a pole pig would prove to be impracti-
cally large. Besides, the external ballasting required for a pole pig
would completely throw all of those calculations out the window
anyway. ;^) "I" like around .1 uFd for my primary cap for running
with an asynch rotary spark gap with a 10 kVA, 14,400 pig.

4) Where have most of you guys gotten your variacs? Talking about the 0-280V 28A ones. I can't seem to find any, and I would like 2. I assume they'll be expensive once I find them...

Sometimes they can be found used on ebay for a reasonable price.
However, as has already been mentioned, they are heavy beast and
are therefore costly to ship. You could also see if there are list mem-
bers that would be willing to part with one of these, too.

5) Are the CDE 942 caps still the best choice in MMC building? Who are some suppliers? I can only find/think of Digikey.

CDE 942 series snubber caps are great for MMCs for small to
medium coil systems i.e. NST or OBIT driven, but as already
pointed out in response #3, they are impractical for larger pig
driven systems. Commercial pulse caps (Maxwell/General Ato-
mics, NWA, Plastic Capacitors, ect.) pulse caps are generally
a better choice for larger coils. You will also have to be sure
that your capacitor has a sufficient internal construction and
low loss dielectric system to withstand the very rapid pulsing
of a typical spark gap driven coil circuit. If your cap has noti-
cable external warmth after just a few seconds of running your
coil, it has a high loss dielectric system that will likely render it
unfit for coil use. "I" personally use a Plastic Capacitors BNZ
2500-104 (.1 uFd, 25 kVAC rating) for my coil.

6) I know the coil's secondary will be into the 3:1 realm, but how big should it be? I'm mainly asking for 12kVA usage with the pig, as that's what I hope to run it with.

Yes, aspect ratios in the range of 3:1 to 5:1 are generally the
best for secondary coils. "I" use a 12.5" OD x 54" long gray
PVC duct pipe wound with 49"of #19 magnet wire. I think
my secondary came out to about 1250 turns.

7) Can I wire an arc welder in series with my outlet leads to current limit? I would think so, but again, it doesn't hurt to ask.

Yes, a 225 amp stick welder with its 240 input leads in
series with the input leads to the pig can be used as a ballast.
Many people that go this route simply short the output welding
leads and use the amp selector switch for input current control.
However, as Scot has also already mentioned here, most arc
welders are made as cheaply as can be gotten away with and
typically have like a 20% duty cycle at their maximum setting.
I finally ended up making a homemade ballast from the core
of a defunct x-ray transformer. You would have to make sure
that your core has sufficient cross sectional area to handle the
power that you're looking to control, too. Others can comment
on formulae for this.

8) My pig only has 1 HV bushing, so should I just split the wiring I attach to it to get 2 leads off of it to make the primary circuit? Sorry for the bad wording, I hope I get my question across.

I also use a "single eared" pig to drive my current big coil with
out any problems. I isolate the case "return" of the pig as RF
ground and do not allow it to be attached to mains ground so
that the nasty RF kickbacks don't end up on the metal
surfaces of my control panel and bite me. ;^0 "I" use
that high voltage rated x-ray cable to run the HV from my
pig to my tank circuit and hook the single output bushing of
the pig to the inner conductors of the cable and the outer tank
to the outer braiding of the cable. This cable is basically coaxi-
al cable on steroids and is generally rated at around 75 kVDC.

Thank you for bearing with me, if you got through all that. And thanks again if you answer any of them! :P

No problem and good luck with your coil project and keep
us informed with your progress.

Also, Bart Anderson's JavaTC is a great computer program
tool for Tesla coil design, and it's free to download/access
from Bart's site!

You can check out a couple of pics of my coil in action at:


David Rieben
Tesla mailing list