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asimov13647

Wed May 13 2009, 10:59PM

Registered Member #2048 Joined: Wed Mar 25 2009, 08:58PM
Location:
Posts: 8

I have acquired almost all of the items I need to begin assembling my second Tesla coil. I have a 15kv 30ma NST; a 12-inch OD PVC tube; 3/8-inch copper tubing for the primary. I plan to wind the secondary with 1800 turns of 24 awg magnet wire. Of all the components I have tried to find, cornering the caps has been the hardest. The poly caps recommended on the various websites are difficult to locate. Sometimes I find a cap with a given capacity and voltage to be in the $3-$4 range and then find another cap manufactured by the same company with a slighty different rating for 50 cent - $1. What gives? Is there that much variation in the retailers? Or is there a quality difference in the cap? And why don't coilers make caps from plate glass anymore? I have yet to find a discussion about plate glass caps. I have not yet decided what type of spark gap I will use. I may fabricate several and swap them out and observe differences. Suggestions and criticism are welcome.

Z28Fistergod

Thu May 14 2009, 12:19PM

Registered Member #2040 Joined: Fri Mar 20 2009, 10:13PM
Location: Fairfax VA
Posts: 180

Just a riminder that if you have to use the PVC, make certain it is properly prepared, with the baking and the polyurethane before winding.

potatorage

Thu May 14 2009, 12:39PM

Registered Member #1891 Joined: Wed Dec 31 2008, 02:30AM
Location:
Posts: 21

The CDE 942 series are recommended because they have been tested to work with tesla coils-- they are abit pricey, but people recommend them

HV Enthusiast

Thu May 14 2009, 02:36PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Z28Fistergod wrote ...

Just a riminder that if you have to use the PVC, make certain it is properly prepared, with the baking and the polyurethane before winding.

Thats not necessary.
PVC is fine by itself.

Just wind your magnet wire, and then coat it with polyurethane or similar.

Herr Zapp

Fri May 15 2009, 08:56PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

asimov -

Unfortunately, your 12" diameter secondary coilform and 3/8" diameter copper tubing are way, WAY oversized for a Tesla coil powered by a 15/30 NST. You may end up with significantly poorer performance than you could achieve with properly-sized components. Your components are sized for a coil powered by a 10 or 15KVA pole pig, or larger power source.

For a coil powered by a 30ma NST, a secondary coilform diameter of 3" to 4" is far more appropriate, with about a 5:1 length:diameter ratio. Your primary conductor can be just #12 or #10 AWG solid copper wire, or 1/8" diameter copper tubing.

The performance of a Tesla coil is dictated (primarily) by the input power, not by the physical size of the secondary.

There are may different types of home-made capacitors that will "work" in a Tesla coil circuit, but none will be as efficient or as reliable as a commercial pulse capacitor, or an MMC assembled with high-quality, pulse-rated capacitors with low-loss dielectric and "self-healing" design.

Because you are starting with a very low input power, you will want to select a tank capacitor and spark gap design that have the lowest possible losses. A high-quality MMC and a synchronous rotary spark gap would be optimum, but you could start with a simple fan-cooled two or three segment static gap.

For Tesla coil use, probably the "best" capacitors are the Cornell-Dubilier 942 series. These are specifically designed for high-voltage, high-current, high-frequency pulse discharge use. These can be purchased directly from any of C-D's distributors for around $3.50, depending on quantity purchased. C-D's website will show you which distributors have stock, and what the current price is.

Regards,
Herr Zapp

asimov13647

Fri May 15 2009, 11:39PM

Registered Member #2048 Joined: Wed Mar 25 2009, 08:58PM
Location:
Posts: 8

I realize a 15kv NST may be a little too lean for a 12-inch secondary which I why I am already scouting for utility transformer. I know where there are several . . . It's just they are so HEAVY.
Please give an opinion on this: let's say you have an adjustable spark gap inside a glass tube of air. Around the glass tube is loosely wound a small-gauge wire which is connected to the tank capacitor or primary coil. The air inside the tube would be excited by the voltage in the capacitor (or coil) and the spark gap adjusted to fire when the voltage was at its maximum. Is this a sound idea or do I suffer from a GCE (gross conceptual error)?

StevenCaton

Sat May 16 2009, 04:51AM

Registered Member #1845 Joined: Fri Dec 05 2008, 05:38AM
Location: California
Posts: 211

let's say you have an adjustable spark gap inside a glass tube of air. Around the glass tube is loosely wound a small-gauge wire which is connected to the tank capacitor or primary coil. The air inside the tube would be excited by the voltage in the capacitor (or coil) and the spark gap adjusted to fire when the voltage was at its maximum. Is this a sound idea or do I suffer from a GCE (gross conceptual error)?

I'm not entirely sure what you mean by that, or what you are even thinking, but enclosing the spark gap is not good.
Look up "quenching" This involves blowing air over you gap, so enclosing it is a terrible idea. I would recommend that you build a quality static gap, with copper tubes and a fan for quenching.

Or, if cost is not an issue, you could build one of Terry Blake's rotary spark gaps. I used his design and it has been working great for over a year now. See Terry's designs on his site.

Good luck with your coil
Steve Caton

Sulaiman

Sat May 16 2009, 08:40AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

To 'match' the large secondary coil the topload will be physically large too,
the combined capacitance of the coil and topload mean that
to get breakout you will need a breakout point.
(or a few joules per bang in the primary capacitor)

I think that a large coil gives slightly longer arcs than a small one for the same power input but 15" of arcing from a small TC looks more impressive than 18" from a large one.
Although if you do plan on HIGH power later I think it's a good plan.
When you get your high power transformer you will have a well tuned/measured TC.

IF you can TOTALLY seal/enclose the spark gap then after a while all the oxygen will get used up and the gap will perform well - good luck!
I do like Tungsten rods (e.g.

eBay etc.) as electrodes, a single gap with LOTS of air works well. For later with a high power transformer I guess you'll have to go rotary.
(I put the rods in my drill and sandpapered the ends to dome them a little)

For capacitors, anything that's (KP or MMKP/KP) polypropylene foil or foil/film
should be ok.
I notice a lot of German coilers also have success with MMKT.
The cost savings (if any) of diy primary capacitors are not worth the hassle.
Do a couple of hours of overtime, do your grandparent's gardening, a paper round etc. ANYTHING (legal/moral) to earn cash for capacitors - quicker and better.

P.S If you ask a question about component sourcing you should add your location to your profile, makes it easier for members to advise.

asimov13647

Sat May 16 2009, 10:03PM

Registered Member #2048 Joined: Wed Mar 25 2009, 08:58PM
Location:
Posts: 8

For capacitors I intend to use a bank of Wima FKP1's, .01mfd 1600v. I also intend to build a homemade bank of caps using plate glass and aluminum flashing. Just to see what will happen. All the major parts of my coil will modular so that I can easily swap them out for experimentation with alternatives.
Everything in my coil was acquired far from here. (That little part of WAY upper NY state in that area between the St. Lawrence River and the Andirondac Mtns.) My NSts are from Little Rock, Arkansas. My PVC is from a public works project in SW Michigan. You see, I travel a great deal. All of the electronics I bought from eBay or from electronics suppliers on the Net. I shopped for electronic components from an electronics firm in Kansas City but was informed that they could not compete with the Net. I would like to find a few stand-off insulators from the utility company, for esthetics.
In the next few days I will mount the PVC on the lathe and wash it off with acetone to remove the ink and sand it smooth. Then I will apply a few layers of urethane. Question: can the speed of a lathe be controlled with a variac? If so, I can use the lathe as a winding jig. If not, I will have to improvise.
What until you hear what I have in mind for a spark gap.

StevenCaton

Sun May 17 2009, 04:36PM

Registered Member #1845 Joined: Fri Dec 05 2008, 05:38AM
Location: California
Posts: 211

Changing the input voltage to a lathe will probably blow a fuse, partially destroy something, etc.

Most lathes have a variable speed knob, or the ability to be set for a certain rpm, but since you asked, I'm assuming your lathe doesn't have that capability. Running a tool below its input voltage tends to blow fuses. I varied the input voltage to a gear motor the other day, and I blew the 20amp fuse. Only certain types of motor will allow for the input voltage to be varied.
The lathe that I wound my coils on didn't reach full rpm until several turns, so I wouldn't worry too much. I was able to control the wire and wind a nice 2000+ turn coil at 140rpm. It takes a bit of practice. People have done it at even higher speeds.

If you don't have a metal lathe, then a reasonably low rpm won't be attainable and the use of a variac is most likely a bad idea.
I would then suggest building a small winding jig with a low rpm gear motor. My jig turns at 25rpm, which is actually pretty slow. I would recommend getting a motor that is a bit faster.
Get a threaded rod to put the pvc on, and tighten it from both ends with two bolts, or something like that.

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