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My first coil : 572B VTTC

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PhilGood

Fri Jun 08 2012, 07:53PM

Registered Member #3806 Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259

I just finished to build my first TC and I'm glad it ran well at first power on.

Specs are:
- Tube: 572B
- MOT : 2480V 1715VA
- Secondary : 1" x 6" with 480 turns of Ã˜ 0.30mm wire
- Top load : 3.5 HDD tray + 3" sphere
- Primary : 2" with 14 turns of Ã˜ 1.50mm wire
- Tank Capacitor : 650pF Sprague doorknob
- grid leak : 10k 25W / 2.2nF
- feedback coil : 25 turns of Ã˜ 0.35mm

When I first built it, I had a grid leak circuit with 5k / 650pF and primary was 30 turns of Ã˜ 1.00mm with taps at turns 10 to 30.

I was getting the best results at turn 10 (first tap) but couldn't go lower :/

It was working but everything was getting hot very fast, especyally the primary (smoking), and I wasn't able to push further than 200V input, because current was 10A at 200V and my higher fuses were 10A.

Changing the grid leak circuit for 10k / 2.2nF reduced consumption and components heating a lot, allowing me to push input voltage to 250V.

I made another primary with 14 turns of Ã˜ 1.50mm wire and taps from turn 5 to 14, but now I'm getting best results at turn 14 (last tap), lol, no luck, I would need some turns more.

I can now run the coil for longer time at low power, but at full power the tube plate starts reddening rather quicly (30 seconds).

Max sparks lenght is around 15cm.
Well it's not bad, but still needs better tuning and further improvments :

- winding another primary with more turns, and maybe using Ã˜ 2.00mm wire
- lowering consumtion to allow long runs
- adding steve's staccato controller

I also have a few questions :

- I'm pushing more than 2.5kW into the MOT (10+A @ 250V). Is that normal ? seems a lot to me

- What are you guys using as breakpoint ? I'm using blue steel nails (those that come with painting hooks). They seem quite strong but they melt very quicly at full power
Clou10

- I noticed internal tube arcing when feedback coil is too close to primary. Maybe I should reduce feedback coil turns (actually 25) ?

- Any suggestion to improve performance and reduce tube heating would be much appreciated :)

Dr. Dark Current

Fri Jun 08 2012, 08:52PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Hi,
Nice coil, especially for a first one

Don't use steel as a break out point, it just burns and sparks. I have had good experience with copper and brass.

It is probably normal the MOT draws so much current, transformers in general don't like half wave rectification on the output, which saturates the core and makes the primary draw a huge magnetizing current.

Edit: Yes you should probably reduce the feed back turns. Tube sparking is very bad and means a heavy voltage overload.

Sigurthr

Fri Jun 08 2012, 11:53PM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

Wow, that is a really, really impressive first coil! Your layout looks fantastic and clean!

When you remake the primary make sure you don't change the wire diameter or you may find you need a different value/range than you originally intended. Wire diameter matters!

Stick with Tungsten or Copper for the breakout point, everything else will melt/burn.

PhilGood

Sat Jun 16 2012, 07:21PM

Registered Member #3806 Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259

Thanks guys, I'm glad you like it :)

I used copper as break out point, it is indeed a lot better !

@Sigurthr: yep, I forgot that using higher diameter wire gives a lower inductance per turn.

I made a new primary with more turns and same wire diameter, but that was useless cause I get the best results at turn 14, which is the last tap of my previous secondary, lol

ok, I spent a lot of time trying to tune and improve coil's performance, I tried to vary:

- tube: 811A and 572B

- topload with 3.5" HDD tray, 3" and 4" spheres, all combinations - best results is with Hdd tray + 3" sphere. Weird thing I noticed is changing toplaod doesn't seems to affect secondary resonant frequency, well at least it doesn't affects primary tap that gives best results.

- feedback coil number of turns with 15, 20 and 25 - best results with 15 turns and feedback coil close to primary.

- grid leak circuit - resistor from 2.5k to 20k - capacitor from 680pF to 3.3nF - best result with 10k / 2.2nF

- position of primary and feedback coils - doesn't affects much results.

- I moved tank capacitor away from primary, I thought tank capacitor beeing very close to primary could see some current inducted.

Well my coil was very close to good tuning at first run, the only two real improvements I could make are:

- using higher diamerter wire for primary, the first one was smoking after 30 seconds.

- changing the grid leak circuit from 5k / 650pF to 10k / 2.2nF. This reduced a lot current drawn by the MOT and thus reduced tube and coils heating.

But I still have a big problem with primary, secondary and tube heating.

- at 150V (3A) the tube is fine, but secondary heats after a few minutes and becomes flabby. Sparks are 7cm long

- at 220V (7A) the tube plate starts reddening after 30 seconds. Sparks are 10cm long.

- at 250V (12A) the tube plate starts reddening after 20 seconds. Sparks are 14cm long.

- If I plug a 811A instead of the 572B, the tube plate becomes bright orange before I can even reach max power with the variac !

Now, my coil is inspired from Arcstarter's coil posted here. I built mine as close as possible to his. I'm wondering why my coils and tube are heating that much, while Arcstarter's coil remains almost cool and can run for long times with a 811A.

I really don't know how to reduce tube and primary/secondary heating (without using a staccato), I tried everything that came to my mind... any help on this topic would be greatly appreciated

Here you can see how the 811A suffered from a few very short runs:

The epoxy coating at the bottom of my secondary broke due to the PVC getting flabby and bending... But I wonder why the enamelled copper wire solder became black like that...

Dr. Dark Current

Sat Jun 16 2012, 09:23PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

The Tesla coils are unique in a way that even if you try to reproduce a good design exactly, it never runs the same

How thick is the wire on your secondary coil?

Tips to reduce coil heating:
-Wind the primary to a greater length, with interturn spacing, or use PVC insulated wire used for electrical installations. Spreading the winding also decreases magnetic flux density and so eddy current heating of the coils.
-The heating of the secondary seems to be less severe with lower coupling - with a relatively large diameter of the primary form to the secondary form.
-Thinner wire on the secondary seems to help, with a lower resonance
-And of course larger coils do help.

Then, your tank cap is maybe too big for the frequency you're running, with too few primary turns. This results in a too high Q of the circuit, high currents and strong magnetic fields, which contribute to coil heating.

PhilGood

Mon Jun 18 2012, 09:49AM

Registered Member #3806 Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259

Dr. Dark Current wrote ...

The Tesla coils are unique in a way that even if you try to reproduce a good design exactly, it never runs the same

How thick is the wire on your secondary coil?

Seems true, lol

My secondary wire is 0.3mm, with 480 turns on Ã˜ 1" x 6" long.

Before building the coil, I made some calculations using Teslamap and JavaTC and I get:

Calculated Secondary resonant frequency = 1.95 MHz
Calculated Q = 19.16

Teslamap calculations:

Dr. Dark Current wrote ...

Tips to reduce coil heating:
-Wind the primary to a greater length, with interturn spacing, or use PVC insulated wire used for electrical installations. Spreading the winding also decreases magnetic flux density and so eddy current heating of the coils.
-The heating of the secondary seems to be less severe with lower coupling - with a relatively large diameter of the primary form to the secondary form.
-Thinner wire on the secondary seems to help, with a lower resonance
-And of course larger coils do help.

Thanks for all these advices, I'm pretty sure these will help a lot, and I'm gonna try them. I also plan to add a staccato.

When you suggest thinner wire on the secondary with lower resonance, you mean more turns for same winding lenght, right ?

But as a beginner who stills need to learn a lot, I would really like to understand what makes my coil run so different than Arcstarter's one ...

Dr. Dark Current

Mon Jun 18 2012, 10:33AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Well, 0.3 mm is way too thick for such a coil, it will overheat from eddy current losses.

I would rewind the secondary with something like 0.15 mm wire, then retune the primary - you will need more turns. Keep the tank cap the same. With lower frequency, you will also get longer sparks for the same input power.

As for the differences - I don't know, have you asked Arcstarter for all the specs? I'm sure you know that changing wire diameters or coil sizes just a bit, detunes the coil.

PhilGood

Mon Jun 18 2012, 01:49PM

Registered Member #3806 Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259

Yep, Arcstarter gave me full details of his coil, his secondary is ~300 turns of ~AWG 28 for 5" winding lenght.

I was suprised by so few turns of thick wire, as most secondaries are usually 1000+ turns, but seeing the nice result he got I thought Arcstarter found a kind of sweet spot.

I used a little bit thinner wire with more turns (480 instead of 300), maybe I should have stick to his exact specs.

For the primary, Arcstarter used ~AWG 20 but he said it was too thin and heating, so I used a higher diameter (1.0mm on my first primary, and 1.5mm on me 2nd primary).

Everything else is exactly the same as Arcstarter's coil.

Anyway, this is my first coil so I'm gonna play a lot with it and try every possible change.
I will try 300 turns of AWG 28, as well as lot more turns of 0.15mm as you suggest.

Good news, I received this morning some good quality russian capacitors.

I replaced my 650pF doorknob cap with a K15Y-1 470pF 15kV rated 40kVAR

Curiously I still get best results on same primary tap (maybe due to the 20% tolerance both capacitors are very close ?)

There is really a great improvement ! Now I can run my coil at full power for longer time.
- I'm now getting 6" streamers at full power with 10A (MOT input current) instead of 12A previously.
- Tube plate isn't reddening at all.
- This new tank capacitor stays cool (the doorknob one was getting very hot)
- secondary is heating less (but still too much, becoming flabby after 3-4 minutes). This is the last problem I have to fix...

PhilGood

Wed Jun 27 2012, 12:53PM

Registered Member #3806 Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259

Some nice pics and videos that I would never have been able to do without this:

EDIT (2012/03/08) :

I recently got a scope and I've been able to make some measurements on this TC:

I built something quick and dirty (C=10nF, R=68, 75Î© cable) :

I didn't expect to accurately measure voltage or current, but just to display waveforms properly, and it did well !

Measured resonnant frequency is 1.948MHz (very close to the 1.953MHz initally calculated with TeslaMap !)

Thanks Sulaiman, Dan and Steve for your help with the high voltage probe

PhilGood

Fri May 31 2013, 01:51PM

Registered Member #3806 Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259

Today I took my 572B VTTC out of the cabinet for some experiments.

The change I made was adding a classic voltage doubler (MOC+MOD)

I noticed something weird with the input voltage/current ratio while increasing input voltage with the variac:

From 0 to 170V, current increases up to 7A
From 170V to 220V current decreases down to 5.5A
From 220V to 250V current increase again up to 8A

Now I understand that this transformer is built to run in an optimized fashion with this MOC and at 220V, but I would like some more "scientific" explanation of this phenomenom

Another weird thing: The votage doubler doesn't improve at all the coil output, aside from this input current change described above, there is no other change at all in the coil behavior, is this normal ?
Is there anything I should tune due to the voltage doubler, like grid leak circuit, resonnant frequency, or anything else ?

Shall I add a second MOC in parallel with the first one ? (I've seen that on some schematics)

Thanks for your help :)

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