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First tesla coil thread - Dual Mot

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MrFlatox

Wed Oct 16 2013, 09:17AM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

Hi there, I got some news about my built, and they are bad. I modified the title of the topic so that it is more focused on my actual questions.

So, I have a problem about the homemade cap I was trying to build. I know that homemade caps can be very tedious when it comes to tesla coil, and most coilers usually go along the "MMC path". But considering the cost of the MMC, I'd rather try the homemade one, and if it appears to be a complete failure, I might go with MMC eventually.

My actual capacitor was made in the stacked plate fashion with Al foil and plastic sheets(PP or PVC I guess). insulation sheets were measured to be 70Âµm, and I stacked two of those between each Al foil. I then stacked multiple plates until I reached the desired capacity of 130nF. Two large PE plate press the whole thing together (i will add pictures).

I then plugged the capacitor accross my power supply composed of two MOT with their secondaries in series. The capacitor lasted about 3 seconds in a huge 50hz humming noise before blowing up. After further inspection, the insulation failed and it arced over, melting the whole thing together.

Obviously, my insulation thickness was not good, so I will modify that. I was also surprised by the noise of the thing, is that normal ??
What kind of pressure is needed to be applied on the plate ? the more the better ? I am looking for oil-free, low inductance capacitor design that will work on a tesla coil, I know those parameters are not always compatible but I want to try.

Thank you.

EDIT with pictures :

Ash Small

Wed Oct 16 2013, 11:25AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

First, you're asking for trouble if you don't put it under oil and suck the air out, due to corona issues.

I have successfully made 'dry' capacitors in the past, and they worked ok up to several thousand volts, but they were rolled and taped up very carefully to get rid of any air pockets.

You don't want to use recycled plastic sheet, there are too many impurities in the recycled stuff. Also, try to avoid coloured plastic sheet for the same reason, although some seems to work ok.

They say PP is best, but I usually use PE.

I plan on making some more soon, and have beet looking for suitable dielectric around the local DIY stores. Most of them sell reasonably thick polythene sheeting for 'builders membranes' etc. although most have a rough feel if you run your hand over the surface, due to irregularities, impurities, etc.

I'm sure others have other suggestions.

(I'm pretty certain I used four 2 litre polythene drinks bottles for a 200nF saltwater cap once, that seemed pretty indestructible, and gave huge bangs when discharged after being charged with a DC flyback.)

MrFlatox

Wed Oct 16 2013, 10:49PM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

Thank you for your answer.

I thought corona might not be a major issue with 4200Vac. 200nF is really impressive with plastic bottles, are you sure it is the right capacity ? I might also try bottles if it proves to work great. The problem with my actual design is that the thickness of the plastic roll i used was not constant.

Any thoughts on the sound thing ? I think I just made a 4200vac 50Hz speaker...

redruM69

Thu Oct 17 2013, 05:13AM

Registered Member #31557 Joined: Tue Aug 06 2013, 02:38AM
Location:
Posts: 58

Did you have a spark gap of any kind when testing the cap? You cant just run the cap connected directly to your transformer without some sort way of discharging that energy, else things explode.. :)

Ash Small

Thu Oct 17 2013, 11:17AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

MrFlatox wrote ...

200nF is really impressive with plastic bottles, are you sure it is the right capacity ?

I'm fairly certain each 2 litre bottle was 50nF, but it was a long time ago. I'm certain that most of the caps I rolled were ~50nF (I rolled some smaller, 20nF ones as well). I used four in parallel to get the 200nF I wanted.

They were actually cheap cooking oil bottles from Sainsburys, but they were the best 'shape' to wrap the foil around. The oil could always be used for insulating something, I don't use the cheap stuff for cooking

I used 'rubble sacks' last time, because they felt 'smoother' that any of the other fairly thick polythene sheeting I had available. I think they came from B&Q, they were the more expensive of the two types they sold, and were blue in colour, although I've previously stated that it's generally better to use the clear stuff.

I guess I'll have to make some more, maybe it's time to build a TC?

MrFlatox

Thu Oct 17 2013, 11:37AM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

No, i did not put any spark gap, just the capacitor across the transformer. I thought that pfc caps are connected that way to the mains so i did not saw any problem with that. Is that a mistake ?

I have already done rolled capacitors for a 40kv dc supply and they work just fine, but for a tesla coil, i was afraid about current capabilities and series inductance, so i went with plate style capacitor.

The problem i see with rolled caps is about how to connect my two plates to the rest of the system. Just throwing two wires in there causes air pockets and makes the rolled cap not round anymore. Plus, connecting only the end of each plates might negatively affect current capability compared to a stacked designe where there is a lot of small plates in parallel. Does it matter that much for a tesla coil ? Or am i overdoing it ?

Talking about insulation thickness, for a 4200vac supply, what should be the minimum withstanding voltage of the capacitor ?consedering ac, security margins, ringing with primay coil...

Thanks

Ash Small

Thu Oct 17 2013, 11:51AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

MrFlatox wrote ...

I have already done rolled capacitors for a 40kv dc supply and they work just fine, but for a tesla coil, i was afraid about current capabilities and series inductance, so i went with plate style capacitor.

The problem i see with rolled caps is about how to connect my two plates to the rest of the system. Just throwing two wires in there causes air pockets and makes the rolled cap not round anymore. Plus, connecting only the end of each plates might negatively affect current capability compared to a stacked designe where there is a lot of small plates in parallel. Does it matter that much for a tesla coil ? Or am i overdoing it ?

I overcame this problem by basically using the same system you use in a plate capacitor. I had one roll of foil (plate) overlapping the dielectric on one side, and the other roll (plate) overlapping on the other side, to reduce resistance and inductance, the current basically flows in 'sideways'. I think the only way to completely eliminate air pockets is to place it under oil, and create the best vacuum you can (compressor from an old fridge?). I now have a couple of vacuum pumps, so next time I make some I'll probably go this route.

MrFlatox

Thu Oct 17 2013, 03:35PM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

So if I understood you correctly, after rolling your cap, you end up with a spiral shape of foil at each end of the capacitor ? How do you connect that to the rest of the system in a clean way ?

I know that it would be better with oil and vaccum, but I would like to keep things not so messy.

Ash Small

Thu Oct 17 2013, 03:52PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

MrFlatox wrote ...

So if I understood you correctly, after rolling your cap, you end up with a spiral shape of foil at each end of the capacitor ? How do you connect that to the rest of the system in a clean way ?

Very carefully

I basically just wrap the wire around it, using plenty of copper wire to keep the reistance as low as possible. I use PVC(?) formers (bobbins), made from cheap water pipe or electrical conduit, to wind the capacitors on, and try to eliminate air pockets by winding it as tightly as possible, and using plenty of insulating tape. You can't eliminate all of the trapped air by this method, but you can reduce it to a minimum.

Do you get corona between two sheets of 'same charge' foil, or does it get 'suppressed'?....This is something I've often wondered. It certainly can't 'leak away' out of the capacitor if it's trapped between two 'same charge' plates, can it?

redruM69

Thu Oct 17 2013, 05:23PM

Registered Member #31557 Joined: Tue Aug 06 2013, 02:38AM
Location:
Posts: 58

MrFlatox wrote ...

No, i did not put any spark gap, just the capacitor across the transformer. I thought that pfc caps are connected that way to the mains so i did not saw any problem with that. Is that a mistake ?

Yup! It was overcharged and blew up. Try rebuilding the same cap, and run a 1-2mm spark gap in parallel. Just remember that the bigger the gap, the more stress on the cap.

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