4Kva Dual GMI-7 VTTC - FIRST SPARKS / Tesla Coils / Forums

Forums

4hv.org :: Forums :: Tesla Coils

« Previous topic | Next topic »

4Kva Dual GMI-7 VTTC - FIRST SPARKS

1 2 3 4 next last

Move Thread

LAN_403

Dr. H.

Tue Oct 30 2007, 06:29PM

Registered Member #931 Joined: Mon Jul 30 2007, 05:25PM
Location: Bulgaria
Posts: 486

Long ago i built this - my first Tesla Coil ever. It actually turnd to be quet good for a first one. It was running in CW mode and it was capable of producing flemalike sparks up to 45sm (18 inch). It was using 2.5 kAV to do that. The arc was so hot that everything except tungston was mealting. But the problem with the setup was that it is very difficult to move it arround

. So i finally decidet to rebuild it.

So here are the parts that i have:
valve - GMI 7 (russina lamp , 20kV on the plate, 240W fillament 125ma current)
mmc - russian doorknobs - 15kV 470pF (i have 26 of them)
secondary (the existing - fres around 200kHz )
power supply - two MOTs

So here are the questions

- shoud i use the mots in paralel or series 9in both cases i am intending to use level shifter to double the voltage ). I was concerned about the voltage in the mmc. If i use the two MOTs in parallel i will have 4kV and with doubler around 7Kv. So will 15kv doorknobs be enough. Also could i use two different MOT transformers for the power supply, or they must be identical?
here some pictures of the old setup and the parts for the new one. Sadly i can't post pictures of the biggest sparks because of a computer crash only this left

and excuse my english, its awful

J. Aaron Holmes

Wed Oct 31 2007, 04:12AM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

Hi, Huben!

That's a nice collection of goodies!

What are your goals for the new setup? Those will determine everything. 20kV on the plate is way up there... and 125mA... makes me wonder if you should be using a big NST instead of MOTs!

Most VTTC's seem to use tubes in the 2-4kV and 200-500mA ranges. Unless MOTs are all you have, they may be a hard way to get where you're going. Plus I've never been too into daisy-chaining MOTs endlessly like some people like to do. At some [relatively early] point, other types of transformers become more sensible choices.

On the caps: Without data, it's almost impossible to know. But if they survived the last coil, they may be ok. The voltage rating would be my biggest concern. Steve W. and others have suggested that high tank Q factors mean that the tank cap may see voltages of two to three times the plate supply. If you ran your 20kV tube at 20kV, three times that would be 60kV! That's four of your caps in series. Much smaller than this, and you may risk blowing caps. Correspondingly, most VTTC designs I've seen on the web--even those which employ only a single MOT with level shifter (5-6kV peak)--employ 30-40kV tank caps.

Cheers,
Aaron, N7OE

Steve Ward

Wed Oct 31 2007, 05:45AM

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

Yeah, build in a lot of headroom on the plate cap voltage rating. Id say 3X the plate supply voltage is probably OK. It looks like you have a lot of those capacitors, so it should be possible.

Dr. H.

Wed Oct 31 2007, 06:20AM

Registered Member #931 Joined: Mon Jul 30 2007, 05:25PM
Location: Bulgaria
Posts: 486

Hi Jeason, hi Steve
on the old coil i didn't used level shiftet supply because when i was building it i didn't know about that so i used regular rectifer (4kv 2A ) and a smoothening cap. So it was running at 2kv. After time i found about doublers, and staccato and i implemented it, or sort of. On the test i was achieveng up to 80sm (32 inch) of spatks, but that was hitting my cealing so i think that i was capable of more. But then the transformer arcedt internaly end cought fire

. I was also having problems with the diode.It was 12kV 3A and it died. After some tests it took up to 22kV rectifier on 3A not to blow

. So after that, because i had only one MOT left i put it back on the old way. I still haven't undarstood why i destroid the MOT. As for the tube and the 125ma .... we've conducted a little experiment back there. Because i have a lot of tubes (40 or 50 i don't know exactly), we've left the coil on only one tube, power on the max (about 2.5kAV on 2Kv - so about 1A through the tube). It was running in CW. It took 7 minutes to destroy the lamp, without any kind of cooling

. Here some pics of the destroid tube

Dr. H.

Wed Oct 31 2007, 07:20PM

Registered Member #931 Joined: Mon Jul 30 2007, 05:25PM
Location: Bulgaria
Posts: 486

**UPDATE**

So today i started building the coil. I've made the stand for the tubes, i made the MMC (30Kv - 2.8nF - it's quite low compared to it's size but thats what i have

). I've also rewound a MOT for the fillement but there the trouble started. Because i live very close to the neighbourhood power transformer, the voltage in the main is extremely high- avarege 240v . When i was testing the MOT with no load ot draw 5,6A from the main !!!! heating it up very fast. After that i hook it to a variav and started measuring. It turns out that at 210V it was using 1A, 220- 1.2A , 230- 2.5A , 245(the actual voltage in the mains) - 5.6A

. It has no trouble heating the tubes (with load the current actuly decreases ). Here some pics and if someone have any idea how to fix the problem with the heating of the MOT, please write.

...

Wed Oct 31 2007, 10:15PM

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

I think that mots are generall intentionally designed to be like that, since it makes them slightly more compact and there is a big fan on them anyway.

To fix it you need to add about 10% more turns to the primary/secondary. It looks like there is probably enough room to add them...

With that MMC, it seems like there is a lot more than 1.3J of energy in that thing... Are you sure you did your math right? I suppose you won't have to worry about it overheating

In any case, looking good!

Electroholic

Wed Oct 31 2007, 11:10PM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

energy density for ceramic doorknobs are really low, 1.3J sounds about right.
ESL and ESR are extremely low tho, so powerdensity is not bad.

thats why they sometimes use it in N2/CO2 TEA lasers.

Dr. H.

Thu Nov 01 2007, 11:26AM

Registered Member #931 Joined: Mon Jul 30 2007, 05:25PM
Location: Bulgaria
Posts: 486

Hi all. Electroholic is right. Doorknobs have very poor at energy density. Each of the doorknobs is 15kV and 470pF, so the whole thing is 30kV at 2.7n, and for the energy its actualy 1.215J

. And for the heating... yep the wom't break a sweat. The capacitors are from some sort of industrial induction heater and yesterday i was told that they were oparating at 34A. Here is a pic, if you know russian.

Dr. H.

Thu Nov 08 2007, 07:48PM

Registered Member #931 Joined: Mon Jul 30 2007, 05:25PM
Location: Bulgaria
Posts: 486

Hi guys i have almost finished the supply for the fillament but i have a question. Is it OK to use DC on the fillament, because my transformer's voltage is a bit low, so i use full wave rectifier and a capacitor and they raise the voltage just enough.

Steve Ward

Fri Nov 09 2007, 06:21AM

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

DC is fine for the filaments. Just be sure to put an AC bypass cap across the filaments, perhaps .1uF film type.

1 2 3 4 next last

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob