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MY TWO CENTS

The following list is a compilation of thoughts and feelings about various aspects of coiling that I have gathered or experienced in the past couple of years.  Each item is current, and if anyone ask, that's what I'd say I believe today.  Of course, I'll be the first to admit that they may not be right...may even be totally wrong, but if I was to rebuild a coil today, these are the items I would take as advice.



- Spark Gap

To date the best performance I have seen out of my coil has been from a RQ Style gap that I slapped together early on, just to get the coil working.  It consists of 10-or-so 1/2 inch diameter copper pipes 2 inches long spaced .020" apart inside a 6 inch diameter x 6 inch long PVC pipe with a small "personal" fan from Wal-Mart doing the blowing.  I thought the construction and materials were pretty feeble at best, but attempts to use various other designs have only resulted in lower arc length...even after making tuning changes.  We'll see what designs the future holds...Working on a gap that will be infinitely adjustable between say .000 to .250 inches and can be adjusted while the coil is running (from a safe distance of course).



- Secondary Form Drying

I have read of people going to extraordinary effort to sand, bake, seal, coat, and otherwise finish the primary form or using exotic material like Acrylic, Teflon, etc.  I used plain old PVC, the moisture absorption coefficient is only .03% (which is essentially as low as teflon at .01% )  I suppose you could bake, sand and coat, but almost anything you lay on top will likely absorb more water than the PVC.  If you found some scrap PVC at the bottom of a river or something, bake it, otherwise, put your time and effort into other areas of coil performance.



- Capacitor

I can not think of any other area of coil performance that has become more revolutionized in the past few years.  The MMC is king!  You MUST use polyPROPYLENE/metal foil caps.  Any other cap will die.  If you decide to go the long route with homemade caps, they will die.  If you inherited a capacitor grade film plant from your uncle, go homemade.  If you want a cheap cap to test the coil, go with salt water and bottles (the lower cost, make these caps economical for trial runs).   If you are considering homemade LDPE, go MMC.  The homemade LDPE will be a poorer, more expensive cap in the long run, and it will still die!

BTW - Bleed resistors on the MMC are a must! otherwise, you may die.
 
 



- NST Choice

My first coil attempt was made with a "typical" sign shop NST 12,000V @ 60mA rated.  I punched out some shunts to get 120 mA closed circuit and ran it through a variac at 140V out which should equate to 14,000V out of the NST.  I can't seem to kill the thing...protection circuitry is nonexistent and the safety gap has never fired (it's way too wide).  For my time and effort, I'd junk any "protection circuitry" and go with a safety gap and properly grounded system.  (See also Grounding)

Additionally, I have now built up several hours of run time on my 9,000 volt / 150mA transformer.  This one has been "hot-rodded" to make a closed circuit output of 490mA.  That's over 325% of the nameplate rating!!  I have heard of others that have increased the power by 1.5 - 2.5x the nameplate.  Of course I don't expect infinite life, and if you cherish your NST I would recommend against this practice, but if you have some spares laying around, give it a shot!



- Grounding

Some people seem to go to extraordinary lengths to ground the system. (i.e.. "I ve got eight 500 gallon tanks buried 30 feet down encased in salt solution with three high pressure water injection wells to keep everything wet!"  Since my coil's birth, I have used one - 4 foot long copper clad steel rod driven into the ground about 15 feet from the coil.  The rod is connected to the coil with a strip of aluminum made from heavy duty aluminum foil folded length-wise 4x. (i.e. fold in half, then in half again)  If the soil is dry, I take a few minutes to run some water near where the rod is, otherwise that is it.  Running without a ground does seem to destroy components, but at least for my power level and soil conditions, the 4 foot rod seems to be enough.



- Toroid

Not too much mystery here (is there??)  A bigger toroid will produce longer, yet fewer arcs while a smaller toroid will produce more, yet shorter arcs.  Smoothness also affects arc length additionally, arcs have been noted to "dance" around very smooth toroids whereas they tend to concentrate on imperfections on rough toroids. Aluminum dryer ducting is available in a wide variety of sizes and is easy to work with.  Using plastic ducting with aluminum foil strips and spray adhesive may be a (barely) slightly cheaper, yet more time consuming way to go.



- Killing equipment

The following is a list of items that have died from use if my tesla coil and a small obituary.

1.  Home made capacitor #1 - OK, this was back in the early days...I selected some nice black polyethylene for my cap (I had it laying around)  After the cap was made, it quickly failed at only about 6,000 volts (should have been around 30,000)  The moral of the story...Don't use any colored plastic for cap construction.  (BTW - MMC's are really the best way to go now)

2.  Homemade capacitor #2 - Used some good 60 mil polyethylene, but ran the spark gap too wide.  An arc actually hit the capacitor and blasted through one of the plastic sheets.  (A MMC would have been really nice!)

3.  Garage door opener - The garage door opener had been hit several times, but I also had a ground strap hooked to it.  (The door opener also has a tendency to open automatically if left plugged in, so I unplug it.)  One day I was demonstrating the coil and forgot to unplug the opener.  As the door automatically opened, it ripped the ground strap loose.  I closed the door  and unplugged the opener, but didn't notice the missing ground.  The opener and door took several good hits.  When the demo was over, the opener wouldn't work at all.  Moral - Now I don't let the arcs hit the opener or the door, so far no more trouble even though the opener is only about 6 feet from the coil at most.

4.  Computer Modem - I ran the coil, then later tried to log on to the net...no luck.  Seems the modem and the card slot in the computer are both dead.  Coincidence...I think not.  I only discovered this at the higher power levels, but I think I will unhook the phone line from now on!

5.  Secondary Coil - I fried this on Halloween Night 2000.  At any other time, I would have seen the first flash over arc and shut the coil down.  However, "The show must go on" so I cut the power down and took off most of the toroids, but by the end of the night, still had 20-30 good burn marks on the secondary.  Moral - More power means more primary-to-secondary distance for the same coupling.  Over coupled coils are bad!
 

While this list may seem long, everything but the computer modem can be explained by direct contact with the coil or its output.  I think I will continue to take some modest precautions and hope that the coil doesn't kill too much more!  I will also continue to knock on wood that my full auto camera doesn't get zapped, or my video camera, or any other number of house electronics (tv, stereo, microwave, etc) that never get unplugged.