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what to do with these MOTs and caps

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eniyuki

Tue Oct 06 2009, 01:01PM

Registered Member #2148 Joined: Tue Jun 02 2009, 02:22AM
Location:
Posts: 62

hi
i scavenged some 5 mots rated 2100V. 1200watts each and 10 microwave ovens from 0.7uF to 1uF at our local junk shop. do you guys have any suggestions what to do with these?
im planning to build a new tesla coil with these mots.
maybe a DC tesla coil with charging choke and deQing diode.
i recently looked at richie burnett site and found that he provided a schematic diagram about resonant charging without the need of a ballast.
Ive also stumbled at greg's DC tesla coil and found a reliable schematic there based on richieburnett site.

really there so many schematics ive seen and dont know which to follow.

regards
eni

Crunchy Frog

Tue Oct 06 2009, 03:39PM

Registered Member #2422 Joined: Tue Oct 06 2009, 02:41AM
Location:
Posts: 85

I've seen some shematics (I have the link, but at home) with two mots wired back-to-back to allow larger stacks and output voltages without the need to submerge everything in oil.

You could also arrange the caps and HV diodes into a voltage multiplier of some sort.

Bird-Dawg

Tue Oct 06 2009, 09:53PM

Registered Member #2360 Joined: Sun Sept 13 2009, 05:43PM
Location: Kennesaw, Ga USA
Posts: 14

Don't use the microvave caps in the coil. I had some and i knew they would probally fail but i put them in a box (to abosrb the possible explosion) arranged in mmc on my smallest coil. And they bulged and popped and smoked, rather interesting though

. I used some microve caps in a coil gun though and they work great.

Herr Zapp

Tue Oct 06 2009, 10:09PM

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

eniyuki -

I think a DC resonant-charged coil would be interesting. You could use 2 or three MOTs, depending on your AC line circuit breaker rating (20A? 15?), with a voltage doubler and a variable-speed rotary spark gap.

I think a DC coil with variable speed RSG makes a spectacular display as the sound and the appearance of the streamers changes dramatically with the break rate.

Richie's site has the best technical analysis of DC resonant charged coils that I know of; I wish he could find time to develop his JavaScript "calculator" for easily determining all the parameters in a DC resonant charging system. (At one time Richie mentioned developing a calculator for his website "if there was enough interest".)

Richie: There IS interest!! A JavaScript calculator might be enough to trigger a LOT more interest in DC resonant-charging systems,

Regards,
Herr Zapp

GeordieBoy

Tue Oct 06 2009, 10:16PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

> Richie: There IS interest!! A JavaScript calculator might be enough to trigger a LOT more interest in DC resonant-charging systems,

LOL. I'd love to finish that off, but my time is severely limited these days.

The DC resonant charging equations are all on the site though, and to be honest, they're not difficult to work out by hand.

At least DC resonant charging is a lot easier to get your head round than AC resonant charging. AC has one additional variable: The mains line frequency, which interacts with the ballast, bps and resonant frequency of the charging frequency. With a DC system the supply is DC so this interaction doesn't take place!

-Richie,

Arcstarter

Wed Oct 07 2009, 04:04AM

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

By far the most out of the ordinary coil i built was a sgtc with a single MOT. That is a very fun thing to make! I used lytics and made a voltage quadrupler. The chokes always died, which is why i do not have is today. I was forced to use MOTs as the chokes. The microwave caps could be used for the quadrupler, though they may not have enough capacitance. I got about 3 foot sparks (i think, cannot remember) untuned, up to around 3000BPS! 8 electrodes, 20krpm motor.

If you make a DC resonant charging coil with a multiplier, you must use a choke. It will limit the current that flows to the cap bank from the multiplier. Otherwise, you will get hot or dead multiplier caps, hot or dead de-Qing diodes, a firey arc at the gap, and alot of power loss. What calculations suggest is wrong! It always comes out to be something like 3H. You need something like 20H-40H. Sometimes more, as i figured out the hard way... For a simply rectified DC resonant coil, you do not need a choke that big, only enough to get rid of RF to protect the diodes.

eniyuki

Wed Oct 07 2009, 04:17AM

Registered Member #2148 Joined: Tue Jun 02 2009, 02:22AM
Location:
Posts: 62

herrzapp,
Thats exactly what in planning right now. It is easy to understand all the formulas and explanations given by richie, ive read it many many times until i undestand but my main problem right now is how to build and design my charging choke and deQing diode. Any suggestions? Greg didnt build a charging choke tesla coil.

Our circuit breaker can Ã¶nly hold of about 10amp max. Do i need a ballast with dc resonant charging? How can i compute output current with a mot-ballast mot?

My plan is right now is to series 2 mots with voltage doubler and rectify it. 11.28kvdc times 2 equals 24kv roughly because of charging choke. 38nf 24kv cde caps. Or is there any better idea? Input voltage of mot is 120v, our wall output is 220v so i cant use 3 m0ts. 4 mots in series can be done easily but im worrying about its insulation.
I hope richie finish his calculator soon.

Steve Conner

Wed Oct 07 2009, 01:22PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Richie is right. A DC resonant charging system is really easy to design, you don't need a calculator.

The first thing to remember is that the tank capacitor always charges to twice the DC bus voltage.

The second thing to remember is, the charging inductance is not at all critical. Like I said above, the tank capacitor always charges to twice the DC bus voltage, irrespective of the values of tank capacitance and charging inductance. The inductor just needs to be big enough to stop power arcing at your gap, but unfortunately that is pretty big. Arcstarter's 20-40H figure seems reasonable to me. Bigger, beefier spark gaps and larger tank capacitors may well get away with smaller inductances.

The third thing is that the bang energy depends only on the DC bus voltage, not on any gap timing considerations, and so the power throughput is easy to control by rotary gap speed. The minimum power output is limited by how slow your rotary gap can spin before it starts power arcing, and the maximum is limited by how much power your DC supply can deliver. (Again, the tank cap always charges to twice the DC bus voltage, no matter what speed the gap is running.) The DC supply is a separate design problem, and this separation is one of the things that makes DC coil design easy.

My favourite DC resonant charging design is of course the Tesla-2 that I came up with myself.

raff

Wed Oct 07 2009, 05:21PM

Registered Member #2315 Joined: Tue Aug 25 2009, 02:35AM
Location: Leyte, PH
Posts: 161

steve,

is flyback driven SGTC another DC resonant charging tesla?

Herr Zapp

Wed Oct 07 2009, 05:44PM

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

eniyuki -

The only challenge will probably be the charging inductor. The de-Q'ing diode can simply be a string of (25) 1kV, 3A or 5A diodes.

Occasionally, 10-20H HV charging inductors show up on eBay, but not very frequently. Pictures are attached of a few I've found, but their current ratings may be marginal for use with a MOT-powered voltage multiplier.

Steve C used (4X) conventional 7.5H DC filter chokes connected in series. These are probably not rated for anything near the applied voltage, but they seem to have survived.

Or, you can build your own inductor, either air-core or iron core. Here is a link to an inductance calculator for designing multi-layer air-core inductors:

Four or five reasonably sized inductor "disks" could be wound and series connected to get 20-25 Henries. Kurt Schraner's techniques for winding the disk inductors for his induction coils could be used, with insulation between each layer of wire.

Regards,
Herr Zapp

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