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Beginner building first TC. Requesting advice.

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JSpark311

Sun Feb 26 2006, 12:55AM

Registered Member #252 Joined: Sat Feb 25 2006, 07:21AM
Location:
Posts: 3

Hey there, I'm new to this forum and couldn't find an faq type page, but I have been reading over the forum for days now and have collected a few questions that I hope no one has a problem with me asking.
Situation: I'm building my first TC and have decided to use 2 MOTs (total output of 4kv @ ~500mA).
* Why do I not really see people using microwave oven caps as their tank caps?
* In order to minimize losses incurred during switching, I wanted to use SCRs, but before I fry very expensive silicon, I wanted advice about how to estimate the amount of current that normally gets shoved thru a spark-gap.
* What does "bbs" mean?
* And I saw reference to an "h-bridge using IGBTs". I've done very little HV work, but I have built h-bridges for motor control. If IGBTs are the switching devices (wikipedia tells me they might be), is it common to drive the primary as if it were a motor in an h-bridge, or am I way off-base?

Thanks for your time. I hope to be a regular contributor to this forum, but first, I'd like to catch up to the standard level of craftsmanship. Very nice coils here.

...

Sun Feb 26 2006, 01:28AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

OK, a few things
Be very careful with the mots, they are lethal. One touch with the output will probably be your last.
For a first Tesla coil, I very strongly recommend using a simple spark gap; do not try to make a solid state coil.

As to your questions...
No one uses microwave caps in the primaries because they are afraid that they will explode, and they are the wrong size. You are looking at a tank cap of around .1uf, so in order to make the microwave caps work you would need a lot in series.

You could (in theory) use a scr in the place of a spark gap, but you would need a friggin huge one. It is really not worth it to use them.

By BBS I am guessing you meant bps, which means breaks per second. It refers to how often the spark gap fires (usually 100 or 120 breaks per second depending where you are located)

Yes IGBT's are switching devices, and they have been used in Tesla coils; with impressive results. The only problem is that constructing the drivers for them requires a ton of work and skill, and you will spend a lot of $$$ in parts (and replacing the parts you blow out).
It is relatively common to use the same type of h-bridge used in a motor controller to drive a Tesla coil, this is referred to as a 'sstc' (solid state Tesla coil).

This forum is only a few weeks old, so there is not much information yet. Most of our information is currently hidden in the archives or the wiki (both can be found in the 'main menu')

Good luck and be safe!

HV Enthusiast

Sun Feb 26 2006, 01:50AM

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

wrote ...

Situation: I'm building my first TC and have decided to use 2 MOTs (total output of 4kv @ ~500mA).

Not to be negative, but if you are building a first TC, i would strongly recommend against using MOTs, especially at the current level you are talking about. Not only are MOTs absolutely lethal, but you are going to have some great difficultly with a spark gap design to operate at those high of currents. 500mA is A LOT.

I would recommend a NST powered Tesla coil. Any NST rated over 4kV would be ideal for a beginner's first coil. I used a 4kV/20mA NST for my minicoils and these work great. 12/30 or 15/30's tend to be the most common, so look for these as well.

Kolas

Sun Feb 26 2006, 02:24AM

Registered Member #102 Joined: Thu Feb 09 2006, 08:15PM
Location: Raleigh, NC
Posts: 169

yes, i concur with the two above.
most people, when making their first coil, dont funny understand theory.
the advantage of using a 12-15 kv NST, is that they are forgiving, and that makes the spark gap easy to set.
for capacitors, i strongly reccomend bottle caps. with extreamly high voltage ratings, they are perfect for a first timer

kolas

HV Enthusiast

Sun Feb 26 2006, 02:57AM

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

wrote ...

i strongly reccomend bottle caps. with extreamly high voltage ratings, they are perfect for a first timer

I think bottle caps are the worst thing for a first timer. Many years ago, it was basically the only alternative for high voltage capacitors that were within reach of amateurs, but today there are so many more options out there just as economical and practically foolproof.

Bottle caps are marginally reliable whose values are not easily controlled. The beginner tesla coil builder should be more involved in the theory, operation, tuning, and general design of the first tesla coil rather than playing around with bottle water caps. And most of the time, these first coils fail to work because of improperly built water caps, which unfortunately the builder doesn't even realize at the time.

Go with MMCs.

Kolas

Sun Feb 26 2006, 03:14AM

Registered Member #102 Joined: Thu Feb 09 2006, 08:15PM
Location: Raleigh, NC
Posts: 169

bahh! rubish!
while *salt* water caps arent the best, but if you live in a college town, they are basicly free and very fast to make
they work
im not sure why you think the values need to be played with here...
you make your caps, and then tune the primary to that value

kolas

Mike

Sun Feb 26 2006, 03:28AM

Registered Member #58 Joined: Thu Feb 09 2006, 05:40AM
Location: Tri-Cities, Washington, US
Posts: 317

People love to hate bottle caps for some reason. In fact they are quite reliable, and pretty hard to break unless your using a pole pig on them. And Kolas is right, they are easy and cheap as hell to make, especially with the lack of the 942C caps now. Besides, if the person is a beginner, then they should build their first TC out of parts that they won't care about being damaged by mistakes!

Hazmatt_(The Underdog)

Sun Feb 26 2006, 05:49AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

Did you guys stop to consider, that a beginner has no clue how to 'Tune' his system!

The capacitor is one of the most critical variables to the system. It's value alone, not considering construction, can make or break the TC!

My early days were governed by oil caps that did fail after they got hot, but hey, I was operating them without derating them for AC pulse.

If you can get ahold of some glass oil caps for a couple of bucks each they work okay for short runs. They get hot and the envelope bursts, and you swap in another one. Yes its expensive compared to the bottle cap, but I'm telling you right now that if you don't know how to tune or optimize, the best you can do as a beginner is get a few inches or so of sparks.

Now if you know you want to do this for sure, the hard way is getting a pulse cap for half the retail on ebay, or doorknob laser caps.
Doorknob laser caps go for, if you're lucky, $22 each. I have acquired 37 of these 2000pF caps for between $18-$22 each depending on my purchase.
Do not use Mica Doorknob capacitors for anything over 300W. I have a bunch of these old Spragues and I am certian one shorted from my 'misuse' in my higher power 720VA system.

IF you see price as a major limiting factor, AND you must go with a bottle cap system, get yourself some kind of meter with a capacitence range! this is a must! AND better yet, spend the money on an LCR meter of any type. Some of the guys here spent $60 on a LCR meter which is a super deal, considering I spent $200 on my Wavetek, and they're practically identical.
So consider this part the most valuable advice.

JSpark311

Sun Feb 26 2006, 06:21AM

Registered Member #252 Joined: Sat Feb 25 2006, 07:21AM
Location:
Posts: 3

Wow, lots of response!
As for the transformer, MOT's are all I really have sitting around in quantity (5 on hand). I reallize the output voltage is on the low side and I can't safely chain more than two of them (secondary tied to core). Would it then be better to build a voltage doubler between the transformers and the tank cap (lowering the peak current, and raising the voltage)? Also, EastVoltResearch, you've sold me on the gap, but if I could find a diode with high enough (yes, very high) current capacity and put it inline with the gap, could I achieve a perfect quench? It seems like the fewer times the primary oscillates per break, the more energy is transfered to the secondary, yet I don't really see this done. Can someone give an explianation?

The caps...
I have the materials on hand to construct rolled polyethelene caps in the event that MO caps didn't work out, but if risk of explosion is high enough, I will evaluate alternatives. This is exactly the kind of thing I don't want to have to learn on accident. :)

Thanks much for all the help!

vasil

Sun Feb 26 2006, 10:56AM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

Yep, you can still use 2 MOTs for a TC, with good results and good quenching in the gap with proper ballasting on the mains side. You can use for that 1 or 2 other MOTs you have, in different combinations (serie/parallel primaries) to get the best current limiting. The secondary of the limiting MOTs have to be shorted.
I would not use any silicon in the primary tank. It will be quickly distroyed and it will not allow the RF oscillation in the primary circuit. So stay away from this ideas. A simple multi gap is all you want, preferable with massive electrodes (copper bars on heatsinks) to dissipate all the heat.
Here is my 2 MOT SGTC:

http://www.geocities.com/livvasil/4kvtc.html

Using some more beefy primary caps it works ok:

http://d.turboupload.com/d/389858/AVSEQ01.MPG.html

Greg Hunter credited the 355 ml bottles of Miller Genuine Draft beer to be very good in the cap construction, with good results even for a pole pig driven tesla coil. See here:

http://hot-streamer.com/greg/swc.htm

Good luck

vasil

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