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Some strange questions about SSTC

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Thelias

Sun Nov 04 2012, 06:52PM

Registered Member #7773 Joined: Thu Nov 01 2012, 03:35PM
Location: Milan
Posts: 3

Hi all!
I'm new here, but thanks to you (particularly Steve for sstc5 schematic!), I successfully built a working SSTC with IRFP460 (my main is 230v).
My TC (after some adjustments) can produce arcs of about 5" from TC to air. If I put something grounded, closer to the breakout point, the arc became weaker and shorter. Don't know why.

I also discovered that the 470uF Steve used in his project, is a little understimated... using 2 470uF, @70VDC, TC was assorbing power to fast and voltage dropped from 70VDC to 30-40VDC.
Switched to 2 2500uF and 35A bridge rectifier, problem solved! No voltage drop even at 150VDC (max voltage I ran the TC until now).

Now, I have some some specific questions that I can't find or what I found didn't answer completly:

1- According to Steve SSTC-5 schematic, C7 and C8 caps capacity are 0.68uF. Can they be, for example, 4.7uF? Since I can't find polyprop caps easly, I bought 2 polyester. I made some test and they don't seem to warm up. I have (from various disassembled equipment) a lot of 4.7uf/275vac mkp x2 caps and some 600vdc 10uF. What happen if I use a bigger caps instead of 0.68uf?

2- I have 3/4 ferrite toroids (one of them is already used for GDT), but they're "small". With ethernet cable I was able to make 16 wounds, but only with 3 wires. Since I'd like to build a full-bridge, I need 2 more wires. Is it possible to use 2 toroids builds for an H-bridge (3 wires) coupled (one for the first half and one for the other half, both connected to the driver)?

3- The actual TC bridge is built with 2 IRFP460 with the gate connected to 5ohm 2W resistor. After some read on this forum, I found out a lot of you recommend a 10ohm. So I bought some 10ohm 5W ceramic resistors (my local store was out of stock for 2 e 3W) and build a H-Bridge using them and a MUR1560 diode for each mosfet. When I first tried this new bridge, the only thing that happend is that the 2 MUR diode's where cooked up and nothing came out from TC. Why? (I used the Steve schematic to the letter in this case, only with a MUR a little bigger then Steves).

4- Topload is optional, but if I want to use it, what should care about? I'm using 4 turns of non-stranded copper wire (~1.7mm) as primary and 1000 turns of 30awg copper wire on a PVC tube of 11.1cm of external diameter (wounds excluded).

5- (This the lastest and dumbest question of all!) How can I increase the arc length? These days I'm doing some tiny improvements, but I'd like to use a full-bridge, a topload and get to use the full main, but arc can't became too much longer (6-7" max?). If I rebuild the secondary with a lot more wounds (1500-2000), should I need to make some change to the bridge?

When I get more familiar with this SSTC I'd like to try to build a DRSSTC :)

Thanks to anyone who will enlight my way!
And sorry for my English.

Physics Junkie

Sun Nov 04 2012, 08:38PM

Registered Member #7267 Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407

Hi Thelias. The first tesla coil I built was the sstc5 from Steve. I can make a few recommendations from my personal experience with this design. I'm not sure I understand your second question about the GDT toroids but if you look here you can see two GDT's, one for each side. And if you look around Steve's site you can find more information on using the ethernet cable for GDT. Regarding your last question, thats never a dumb question

improving spark length is what it's all about!! When I built mine I first started without a topload. Then I decided to make a small one and found that my sparks increased in length but not significantly. The more I learned about the sizing and capacitance of toploads I found that the one I had built was still tiny for my coil. My secondary is an 18" winding on 4.5"OD PVC form using 28AWG wire. You generally want your topload to be as wide as your secondary is long. So for my 18" secondary I made a new 18" wide topload. This greatly increased my spark length by 2-2.5", total spark length I have achieved with this coil is 8" to grounded object. For this specific design, you should not need to change anything if you decide to rewind a secondary, although your secondary is just fine and you should not need to wind a new one for any reason other then your own experimental purposes. I plan on rewinding a smaller secondary in the future to see how it changes performance. I hope sharing my experience will help you a little bit, there are very knowledgeable people on this forum that will give you better insight on your other questions.

Thelias

Sun Nov 04 2012, 11:50PM

Registered Member #7773 Joined: Thu Nov 01 2012, 03:35PM
Location: Milan
Posts: 3

Thanks Physics!

I didn't noticed that photo with 2 GDT! Analyzing the schematic, seems doable to build a full bridge with 2 GDTs in parallel! This is a good news.

For the topload you have gave me some more useful info! And thanks to them I found out that the topload should be above the secondary of about half of the height of the toroid (I have some flexible aluminum pipe).

Thanks again Physics, now I'm waiting to see if someone can answer to the other questions.

Graham Armitage

Tue Nov 06 2012, 10:30PM

Registered Member #6038 Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248

Hi,

I built the same circuit and tried a few things to get longer spark length. increasing the torroid size definitely had a big impact. With a 3" diam ducting about 12" diam i got about 7" sparks. Going to 4" duct about 20" across, it bumped up to 11" of arc. I tried 6" ducting and it dropped right down. I run consistently now at 11" without anything getting warm. I did replace the MOSFETS with some cheap IGBTs (TO-220 package).

I also played with the primary, and put 3 turns of 8G stranded copper with VERY tight coupling. With the 4" torroid I got over 19" sparks, but the MOSFETs ran really hot, so I back off a little. Tighter coupling improves arc length too.

All of these were sparks measured from a breakout point to a grounded contact.

Thelias

Thu Nov 08 2012, 12:18PM

Registered Member #7773 Joined: Thu Nov 01 2012, 03:35PM
Location: Milan
Posts: 3

My primary coupling is as tight as possible, using a rubber tape as isolation from secondary (tape is soft rubber and about 2-3mm thick). Until now, it seems to do the job, but after I ran for a few minutes my tesla, primary got a little warm.

In the next days I should be able to build a full bridge, but I need to know why I killed 2 MUR using Steve's schematic.
In the mean time I building the audio modulation part using kaizer (CTC-Lab actually) audio modulator schematic.

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