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Round Two: Begin! (And a heartfelt thank you!)

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Blitzorn

Sun May 12 2013, 07:07AM

Registered Member #2128 Joined: Thu May 21 2009, 03:17AM
Location: Seattle, Washington, USA
Posts: 58

Salutations, dear coilers!

Some of the more regular and veteran among you might remember me from a thread I posted in 2009 regarding my first-ever Tesla Coil, which I somewhat miraculously threw together for a school science fair. I was 17 at the time and never would have been able to complete that build if it weren't for all the incredible support and guidance that I got from this forum's members. I can't thank you all enough, it was an incredible experience riddled with trial-and-error, the satisfaction of hard work, the thrill of discovery, and plenty of blood, sweat and tears. In retrospect, that project was almost solely responsible for helping pull me out of a depressed high school stupor and get me excited about science and engineering, which led me to join my school's rocketry team and participated in the NASA Student Launch Initiative, where we built this rocket! I am currently studying calculus and electromagnetism in college, and it's mostly thanks to you lot (along with my dad)! So that's the first reason for this thread - I just wanted to say thanks to everyone for keeping this community alive - I am so glad to see it hasn't tapered out. You have been an inspiration to me. I will be donating to the site's upkeep costs as a sign of my gratitude

That brings me to my second point - I want to get my old coil up and running again as a project for my current school program! I learned the basics of coil construction the first time around, but I didn't grasp a lot of the physics/math behind the design. I lacked a good theoretical grasp of what I was doing, beyond what I had to know to make it work. This time I want to do less of a hack job, and maybe build some components I'll be proud to keep going forward. It won't really be the same coil, because I had to scrap a lot of parts during a move a couple years back - the primary, spark gap, and capacitor bank all seem to be gone, wish I had kept the first two - but I have a lot of the important components, and now I have some money to throw at the design, which means I'll be able to spring for a real MMC, build a better spark gap, maybe even throw in a Terry filter and a PFC Cap to milk it for efficiency. I want to learn some new skills (effective soldering, for example) and I generally just have a good time.

I can handle most of the process on my own, but like last time, questions are cropping up that are best answered by responsive, real humans with personal experience, and I would be honored to get a nudge in the right direction - which is where you come in!

My first capacitor was a giant bank of beer bottle saltwater caps - it was a pretty sketchy build all around but it worked well enough. It was massive, burdensome, and probably a safety hazard, and I'm in no mood to put in the 16+ hours required to make another one from scratch when I can afford to put together an MMC this time. So first off I'd love to get any pointers you have for MMC construction, and I'd appreciate any recommendations for MMC calculators, the best cap manufacturers and designs, sites to order parts from, etc. I am also looking at putting a PFC Cap into the circuit and have been having some difficulties finding good documentation on how best to go about this. I've been testing out various software and, if interface is worth anything, I'm tempted to put my faith in TeslaMap and the JavaTC apps over anything else I've played with so far. Thoughts?

For what it's worth, my transformer rig is a set of two NST's each delivering 7.5kVAC at 30mA connected in-phase. I have a variac, line filter, will likely be using a static sucker spark gap similar to my last build.

Look forward to working with you guys again, it feels good to be pulling out the HV hardware again!

Steve Conner

Sun May 12 2013, 09:33AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

People always ask me "What are Tesla coils actually good for?" There's your answer right there.

I use JavaTC too. It's based on Paul Nicholson's numerical model of the Tesla resonator and is the most accurate calculator I know of. The last coil I designed with it, I was so confident I didn't even bother to include a primary tap for tuning. It worked first time. Never tried TeslaMap though.

For MMC capacitors, the 942C series of polypropylene film/foil caps from CDE are very popular. Most people seem to prefer the 0.15uF 2000V part. EasternVoltage sells them for $4.99 each

but you might be able to get them cheaper direct from a CDE distributor, especially if you want a large quantity. I got mine from Richardson Electronics.

Blitzorn

Mon May 13 2013, 02:58PM

Registered Member #2128 Joined: Thu May 21 2009, 03:17AM
Location: Seattle, Washington, USA
Posts: 58

I've seen so many recommendations of that particular CDE cap that I've decided to just spring for them - also did some poking around regarding what to use for the parallel bleeder resistors, and almost universally I see the use of 10Mohm 0.5W 5% tolerance axial resistors, although less brand consensus... I'm looking at this model, and I will probably get both sets of components from this site (this is their page for the capacitors) - unless anyone has horror stories or better recommendations!

I'm also curious if anyone has an opinion to lend on the Tesla Stuff store on ebay... I was looking at their Terry Filter kit and thought it looked like a decent value. Thoughts?

Steve Conner

Mon May 13 2013, 04:26PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Those Ohmite carbon comp resistors have a quoted max voltage of 500V. For bleeder resistors I prefer the Vishay VR37 series.

Blitzorn

Tue May 14 2013, 12:57AM

Registered Member #2128 Joined: Thu May 21 2009, 03:17AM
Location: Seattle, Washington, USA
Posts: 58

Thought something was off about those - good catch. Those VR37's are exactly what I was looking for - and they are also called for in the filter design, so I'll just get some extras...

I can't find any theory/calculation guides for Terry Filters other than a blurb on hvtesla.com regarding how to calculate the number of MOV's to wire in parallel. I have found plenty of schematics of how to assemble the filters, just no treatments of how to customize them for your particular system's specs. Is there any further customization required beyond having the correct number of MOVs?

Steve Conner

Tue May 14 2013, 08:48AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Not really, the R and C sections of the filter are non-critical and will cover a wide range of coils. A really high current transformer might burn out the resistors, so if you're running a huge bank of NSTs or a small pole pig, you might need to lower the values or increase the wattage.

Blitzorn

Mon May 20 2013, 10:04AM

Registered Member #2128 Joined: Thu May 21 2009, 03:17AM
Location: Seattle, Washington, USA
Posts: 58

Is there any particularly excellent way to make wiring connections in terms of safety, minimizing losses, ease of modification, tidiness, etc? Any little bits of hardware you've discovered that are personal favorites? I'm trying to be better about making soldered connections wherever it makes sense (a lot of the connections in my first coil were shoddy to the point of being a little scary, frankly.) And is liberally-applied electrical tape an adequate (if ugly) insulator for most connections?

Also, is it true that EMI line filters should be re-wired so that they are oriented in the circuit opposite to how they would be in normal use? I am seeing some dismissive attitudes toward this practice and I am not certain if all filters are created equal (or directional).

My last question is about spark gaps... how much airflow is required to achieve sufficient quenching? my last design revolved around a shopvac (the whole thing, not just the motor) and I am planning on using a similar perpendicular single static gap design, but I don't have access to a shopvac anymore (although I probably could find one, in a pinch). Would fixing a couple small fans on it like I'm seeing in a lot of people's designs suffice, or with my two little tungsten electrodes resting on PVC do I need to be more worried about keeping the assembly cool? I can draw up a diagram if that would help!

Blitzorn

Thu May 23 2013, 04:47AM

Registered Member #2128 Joined: Thu May 21 2009, 03:17AM
Location: Seattle, Washington, USA
Posts: 58

For the filter, does it make more sense to round up or down one MOV if my calculations based on the recommendation on HVTesla.com put me at '5.3' MOV's per leg? I have been searching for any detailed references regarding the design philosophy and engineering behind varying the number of MOV's in each leg and can't seem to figure this one out once and for all.

Also, is it unwise to make bridges in my filter or MMC by soldering in shaped steel wire? I don't know much about the conductivity of steel compared to copper, or how well it takes a solder, but I tested it out and it seems plenty tough mechanically, I don't know about the electrical properties of the joint though. Would copper be better? I can get copper wire instead but I have steel already (and am colliding with my budget).

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