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My first Tesla Coil

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Micha0_0

Sun Jul 10 2011, 03:42PM

Registered Member #2990 Joined: Mon Jul 12 2010, 11:22AM
Location:
Posts: 40

Hey guys .

I want to build my first Tesla Coil .
But i have some questions.

Powersupply :
I thought about a 4x MOT stack , does it work well under oil ?
Would a 3x stack also work ? ( because a 4x stack pulls very much of current)

Some people said that DC makes many problems , if it works with dc too , I could use my 6 MOT 3phase supply which is rectified.

Spark Gap:
At first I planned to use a multiple spark gap made of brass.
Later I will build a rsg.

Cap:

According to the calculations i'll need a 100nF cap .
I found one on ebay.

would this one work ?

Secondary:

15cm diameter
65cm winding length
0.45 wire diameter
1450 turns

Dr. ISOTOP

Sun Jul 10 2011, 04:02PM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

(1) Be warned that MOT stacks are highly lethal.
(2) DC charging works fine.
(3) MOT power demands an RSG.
(4) That cap will not work.

Micha0_0

Sun Jul 10 2011, 11:00PM

Registered Member #2990 Joined: Mon Jul 12 2010, 11:22AM
Location:
Posts: 40

hey...

1)I know but it's very hard to get a NST here , so I use mots.

2)what I have to do to run it on DC ?

3)So a static gap or multi gap wouldnt work with mots ?

4)What caps should I use ?

Dr. ISOTOP

Sun Jul 10 2011, 11:19PM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

(1) KK, but be careful!
(2) Nothing.
(3) Not at high powers.
(4) Cornell-Dubilier CDE942 series film/foil caps.

Herr Zapp

Sun Jul 10 2011, 11:34PM

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

Micha0 0 -

Based on your posts here, it doesn't sound like you have much understanding of Tesla coil design or construction (e.g. using one (!) CD 942 series cap in a tank circuit for a Tesla coil powered by 4 MOTs).

Why not spend some time perusing the 4HV Archives to see what other first-time Tesla coil builders have done: how they designed, constructed, and tuned their coils?

All Tesla coils can be potentially lethal because of the energy stored in a fully-charged primary capacitor. MOT-powered Tesla coil are even more dangerous because just the output of the MOT itself can be instantly lethal.

You may encounter some reluctance here for people to provide you with instructions on building a MOT powered coil if it seems like you have little understanding of what you are doing.

Regardless of where you live, a little time spent searching will turn up an oil-burner ignition transformer, a neon-sign transformer, or some other high-voltage, low-current AC power source that's much more appropriate for use by a first-time TC builder.

If you can spend the time to search for capacitors on eBay, you can also spend the time to search for NSTs or OBITs.

Starting off to build a DC resonant-charged TC powered by multiple MOTs is not trivial task; designing and building a spark gap capable of quenching at these power levsls is not easy, nor is designing and building a multi-Henry charging inductor, or the large MMC that would be required.

You'll have a much greater chance of success starting small, gaining valuable knowledge, and then moving up to higher power levels.

Herr Zapp

Micha0_0

Sun Jul 10 2011, 11:36PM

Registered Member #2990 Joined: Mon Jul 12 2010, 11:22AM
Location:
Posts: 40

thanks man ...
2) so i just run it like ac ? and i could use my rectified 6x stack ? ( today i burned one of the mots :P )
3) ok so ill build an asrsg this week
4) ill get them from ebay ?

Arcstarter

Mon Jul 11 2011, 12:04AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Herr Zapp, that is actually a CJE cap, it is higher voltage, and terrible quality. Never use one of those capacitors for a Tesla coil, i learned the hard way that you will spend more money on cheap caps than a bank of CDE caps rated correctly in the end. I used one of those with a 6kv 30ma NST coil, and it died almost instantly, and that isn't stretching the truth at all. 6kvac is far more than a 10kvdc cap is rated for, but had it been CDE rated the same im sure it would have lasted far longer than the (used) NST.

I stay away from using any MOTs in series, MOTs are not very good quality transformers in the first place. The are insulated fine for their voltage, and they may even have enough insulation to reliably run two in series. But, this is a Tesla coil circuit, and there can be some pretty mean voltage spikes and other interference. They are also rather inefficient, most of my MOTs pull around 500w with no load, and the cores heat a lot. Not to mention the aluminum windings most of the MOTs i have seen have. It is only around 15% higher resistance, and even my 50Kw pole transformer had all aluminum windings, but it is just another nail in the coffin.

I loved the idea of using a single MOT for a coil, 4 MOTs pull a crap ton of power, and the voltage seems a little low for the amount of current (all comes down to spark gap quenching). A power arc in a spark gap is sure not a good thing! I'm not saying that MOTs can't make a great coil, they really can. I am just more interested in the efficiency aspect of a coil than the spark length, and to make a MOT coil efficient it would take some real work. A srsg with MOTs and a cap bank rated correctly for the voltage/current would probably work very well, but a static gap would likely waste alot of energy.

A single MOT with a voltage multiplier is, however, more expensive and complex than MOTs in series. Not to mention, the voltage spikes i mentioned above are as big, if not bigger of a problem with a DC supply due to all of the silicon (diodes) involved. However, leaving a large headroom for voltage, and using avalanche rated diodes can make for a reliable coil. You also need a DC charging choke that can withstand 2 times the charging voltage plus some headroom, and should be something like 20H or so (that is the approx. inductance of a MOT secondary, for example). You can replace the inductor with some large resistors, but again, i prefer higher efficiency over spark length any day, and the resistors would waste hundreds of watts pretty much invariably.

One thing i loved about DC coils is that you could use any breakrate without having to worry about the tank cap being charged all the way (a tank cap being charged with 60hz ac will reverse polarity after every half cycle, and if you don't discharge the cap in time, that is energy wasted). That means that you can use an arsg with as good efficiency as an AC coil with a srsg.

I think AC and DC is a pretty fair compromise. I also think using MOTs takes more work than an NST is worth. This is once again, from an efficiency standpoint, but anyone can make 5 foot arcs using 10kw of power!

Micha0_0

Mon Jul 11 2011, 09:51AM

Registered Member #2990 Joined: Mon Jul 12 2010, 11:22AM
Location:
Posts: 40

I'm not very pleased to use MOTs for the coil , but i often tried to get a NSt or an OBIT , but the most obits I could get were in terrible condition.
So they could only be thrown away.

And i never got a NST , they are very rare in Austria .

Dr. ISOTOP

Mon Jul 11 2011, 10:03AM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

As much as I believe in starting big, I doubt any of us are willing to help you with a MOT coil unless you can show us you have sufficient experience with high voltage to not kill/maim yourself/other people.
@Arcstarter: IMHO the (spark length)/(secondary height) ratio is more important, as it greatly affects the visual display put on by the coil.

Micha0_0

Mon Jul 11 2011, 02:01PM

Registered Member #2990 Joined: Mon Jul 12 2010, 11:22AM
Location:
Posts: 40

i have some expirience with high voltage .
but not really experience with TC , so i want so build one to learn how to tune it and so on .

Should the cap rated for more voltage than the powersupply ?

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