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Any advice with this schematic

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Benjamin

Tue Aug 23 2016, 12:02AM

Registered Member #54655 Joined: Thu Mar 19 2015, 05:56PM
Location: BC, Canada
Posts: 82

You could try a propeller gap design where the the tungsten rod is pressed into a polyethylene rotor on the motor shaft and multiple stationary electrodes like this

It is quite easy to build compared to other designs. Also you will want to aim for at least 300-400 BPS (breaks per second) for the gap. I don't see why you don't want to use the doubler, it will improve output and reduce spark gap heating, and I've never had any problems with it.

Neukyhm

Tue Aug 23 2016, 12:39AM

Registered Member #57520 Joined: Thu Oct 15 2015, 11:21PM
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Posts: 15

I agree, that's a very simple design, I will go for it. And Benjamin, I don't like the idea of a doubler because of the diodes. I have read that they become hot and they can explode, and I don't like silicon for high power things lol. Also, using a doubler will turn the coil to DC.

Benjamin

Tue Aug 23 2016, 01:35AM

Registered Member #54655 Joined: Thu Mar 19 2015, 05:56PM
Location: BC, Canada
Posts: 82

I have also read that they can become hot and unreliable but this has not been the case in my experience. The trick is to make sure that the diode string is rated for at least double the peak supply voltage because of high voltage spikes coming from the tank circuit. These are what cause the diode heating. I use four microwave diodes in the string and it barely gets warm at all. With your rotary gap, DC will be fine and because it is a half wave rectified instead of full wave rectified power supply, no charging reactor is needed. All you have to do is add the diode string and 2 caps and you're done!

Neukyhm

Tue Aug 23 2016, 02:05AM

Registered Member #57520 Joined: Thu Oct 15 2015, 11:21PM
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Posts: 15

Ok Benjamin, I will do this: I will build the coil without doubler, then see how it works, and try the doubler later. I still have some questions about the mots. They seem to have the secondary welded to the core so, should I remove it and weld it to a cable? or is it better to have it welded to the core and then weld a cable to it?

And another question, I have seen many people welding a ground to the middle connection of the MOTs, and that's something I don't understand. I'm a physics student, not an electric engineering one, so I don't know the reason for that. (is that ground the same ground as the mains?)

I have been reading tesla coil stuff for two years, I wanted to have as much information I could before starting to build one, but I still have a few questions without answer like that one haha.

PS: thank you very much Benjamin, without you, this thread would be empty like my wallet.

Benjamin

Tue Aug 23 2016, 03:50AM

Registered Member #54655 Joined: Thu Mar 19 2015, 05:56PM
Location: BC, Canada
Posts: 82

Don't remove the middle core connection, that's only necessary when you have more than four mots in series which you don't have. Connect the cores together and make sure the mots are in phase by flipping on of the primaries if you don't get any output. Also put one capacitor on each hv terminal when using a doubler. You don't have to ground the cores but I do just to be safe. In the microwave one side of the secondary is grounded to the core and the mains ground, you are just adding a second transformer to effectively make a center tapped one. I just bolt the mains ground to both cores. Also how big are your secondary and topload going to be?

Neukyhm

Tue Aug 23 2016, 03:00PM

Registered Member #57520 Joined: Thu Oct 15 2015, 11:21PM
Location:
Posts: 15

The secondary is big, like 60cm of PVC, diameter is 11cm. I'm using AWG27, wich is very very thin, I expect my secondary to have a lot of turns because of that. The topload is a thoroid and it is already built. It's big, 45/75 cm of inner/outer diameter.

Benjamin

Tue Aug 23 2016, 04:07PM

Registered Member #54655 Joined: Thu Mar 19 2015, 05:56PM
Location: BC, Canada
Posts: 82

That should work, although the high number of turns means that you will need a large number of turns for the primary to tune it. If you already have the wire, use it but if you don't I would use 24 gauge instead. my coil uses an 8.9 cm by 50 cm form with 24 gauge wire and it has put out sparks three times it's length. The key to getting large sparks out of small coils is a high break rate spark gap and a very large toroid (mine is 8 by 36 inches, 20 by 91 cm). Also you don't have to use copper tube for the primary coil, I used 10 gauge stranded insulated wire and just stripped out tap points and it gets a little warm but it seems to work fine.
Edit: Just saw your question back there. Yes AC motors can have brushes, they are called universal motors and can run on AC or DC. They are usually high speed (which is good for a rotary gap) and found in things like blenders and vacuum cleaners.

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