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What to do now... (partially completed Tesla coil)

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Steve Conner

Wed Mar 21 2007, 11:37AM

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

Like you guys just figured out, there are two ways to drive a Tesla coil with MOTs.

The way recommended by Greg Hunter uses a voltage multiplier circuit with no filter capacitors on the output, and undersized capacitors everywhere else. This makes it deliver pulsating DC at 60 or 120Hz, depending on how you wire it, and with a certain amount of inherent ballasting because of the reactance of the too-small capacitors, and the leakage inductance of the MOTs. The pulsating voltage that dips to zero 120 times a second, and the ballasting, both help the spark gap to quench, to the extent that this circuit can just about drive a static gap, especially if it's got forced air quenching from a shop vac or similar.

The other way, that was popularised by me with the Tesla-2, uses a multiplier with large capacitor and heavy filtering that produces a steady DC voltage. I then use a choke and de-Qing diode in series with the DC output. If it weren't for these, the large filter caps would discharge through the spark gap and blow everything. This requires a rotary spark gap or triggered gap: it's unstable with a static spark gap. It's also more expensive than Greg Hunter's system, because you need more and bigger capacitors. But it gives true DC resonant charging. In my Tesla-2 I started with a single MOT and ended up charging the tank capacitor to nearly 20kV.

If you put chokes on the output of a Greg Hunter style supply, that is like a cross between the two methods. I'm not sure what would happen, but I wouldn't be too surprised if it improved it like a previous poster said. You might be as well to just experiment with whatever components you have, and see what you can make that works well.

Marko

Wed Mar 21 2007, 01:35PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I apologize, I got confused with the upper schematic but now I figure you don't use any filter caps exactly as with greg's schematic.

Level shifter alone can't actually cause power arcing because it's 'ballasted' by secondary inductance of MOT's. There are 100W resistors in series with output wich are seemingly there just to protect the level shifter diodes when primary voltage rings under zero.

So, the circuit is more or less fine if you are happy with 100..200W of dissipated power in the resistors.

As steve already said, level shifter outputs only halfwave rectified voltage and thus I'm not sure if it is perfect for TC supply, but it is simple and cheap!

I imagine you could have trouble with synchronizing an ARSG to it, and maybe it will give somewhat less streamer growth because output is 'modulated' at 50/60Hz.

sparky

Wed Mar 21 2007, 05:49PM

Registered Member #530 Joined: Sat Feb 17 2007, 07:56AM
Location: Victoria BC, Canada
Posts: 178

check out roger smiths' DC coil... might be worth the view :)

Sam

Wed Mar 21 2007, 07:21PM

Registered Member #227 Joined: Mon Feb 20 2006, 10:47PM
Location: Cambridge Ontario, Canada!!
Posts: 127

I have been working...

Got some cool stuff from school and built it all in class
100 22991

Thanks for all Your help guys :D

And another Question: what kind of motors should be used for the RSG motor? I have one here, its 120AC but not variable, just on or off. I was told its only variable when changing the frequency.

Steve Ward

Wed Mar 21 2007, 08:03PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

What is the plastic you are using for the disk? Might want to be careful about the hot spark gap electrodes and melting of the plastic. The electrodes will get as hot as a soldering iron (if not hotter), so keep this in mind. Most people use fiberglass/epoxy composites (garolite). G-10 is a really popular material for RSG uses. In any case, put a shield around your gap so you dont have accidental projectiles flying about the room if that plastic disk gets a little soft and lets go of the electrodes

Marko

Wed Mar 21 2007, 08:15PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

And another Question: what kind of motors should be used for the RSG motor? I have one here, its 120AC but not variable, just on or off. I was told its only variable when changing the frequency.

Universal motors are often picked for ASRG's because they are easily variable by most ordinary dimmer, and usually have very high max RPM. Such are motors from drills, angle grinders, vacuum cleaners and etc.

Guessing you have an asynchronous AC motor, it is pretty slow (3000..3600 rpm max) and you can't regulate it without a frequency converter. Such motors are, ofcourse, far more rugged than universals but I guess it just isn't needed in TC duty with short runs.

Regarding your setup using halfwave rectified input, I imagine your firing rate is going to be wierd.. but probably not much of a problem since it's still a DC coil.

Sam

Wed Mar 21 2007, 09:35PM

Registered Member #227 Joined: Mon Feb 20 2006, 10:47PM
Location: Cambridge Ontario, Canada!!
Posts: 127

I could have large aluminium bits with a tapped center to act kind of like a heat sink...?

maybe like 1" dia.

Edit: some pics

I'm getting sick of pop, but at least its not bear...

100 23011

8 more bottles tonight, gets me to about 16nF

Steve Conner

Thu Mar 22 2007, 10:21AM

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

Yea, like Steve Ward said, if that rotor is made of Lexan, you could be in for a nasty surprise.

Marko

Thu Mar 22 2007, 11:52AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Yes, and definitely be careful about your gap. You don't want it flying apart!

I would use some kind of good fibre impregnated plastic wich is resistant to heat and abuse.

in lack of imagination, I think that even few stacked dud PCB's would work! You can even leave the copper in an outside 'ring' to act as a conductor!

Sam

Thu Mar 22 2007, 11:54AM

Registered Member #227 Joined: Mon Feb 20 2006, 10:47PM
Location: Cambridge Ontario, Canada!!
Posts: 127

Im stuped. I cant really get ahold of any other material. I'm trying to think of a way to disipate the heat, like using larger bolts or...then if they let get its not very good. If I put two nuts to hold it in place on both sides...

any other ideas?

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