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Questions about my First Tesla Coil

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bomberguy92

Tue Dec 18 2007, 12:07AM

Registered Member #1181 Joined: Mon Dec 17 2007, 10:48PM
Location: Iowa USA
Posts: 17

Hey,

I am in the beginning stages of building my first Tesla Coil and I have a few questions.
First a little about my coil: it is going to be a 4x20 inch secondary, 9000 V 30 mA NST, spark gap Tesla Coil.

I have some questions about wiring like do you connect the primary to the secondary? I don't think you do but i have read it both ways.

I also want to know about protection. To protect my house I am planning on using 2x20amp, 115-225 VAC, RFI filter on ebay. Will this filter work?

this is the link:

I am planning on putting it in my control box before the NST.

I also want to protect my NST but I can't afford to buy or make a Terry filter. I am going to use a safety gap but i don't know if they actually do anything and I don't know for sure how to wire it up. I have heard about using chokes I'm not exactally sure what they do, how they work, or how to make them, so I could use all the help i can get.

Any comments about any of the issues I have talked about would be great. Let me know if you need any more info. about my rig. I will be posting pics and stats later to keep you updated.

THANKS!!!!

P.S.
I designed my coil using TeslaMap5 I can send you the file if you would like to get more detail about my design

Shaun

Tue Dec 18 2007, 12:29AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

My first coil was almost exactly like yours; same secondary, made on teslamap, except I used a 15/30 NST. One thing I will tell you is don't expect ground strikes or anything. The most I could ever manage was just over half of TeslaMap's estimated 3 feet of spark.

Other than that, yes thats a fine RF filter to use; as for NST protection, in my crudely drawn schematic you can see exactly how it is wired up.

The filter capacitors should be a few tens to a few hundreds of picofarards, rated for at least 7kV. For filter inductors I used about 200 turns each of secondary wire on a paper towel tube.
This inductance/capacitance combo will keep the HF oscillations out of your NST.

As for the safety gap, it goes as shown. The electrodes must be round! otherwise it will fire unexpectedly. Set the distances so it just barely doesn't fire with the NST only, then attach it too the coil.

Unless you have already wound the secondary or begun building, my only suggestion would be to make the coil physically smaller; that way the sparks look bigger in comparison.

bomberguy92

Tue Dec 18 2007, 02:09AM

Registered Member #1181 Joined: Mon Dec 17 2007, 10:48PM
Location: Iowa USA
Posts: 17

OK

Thanks alot!! I'm kinda new to schematics Does the middle safety gap ball get connected to NST? And what is the PFC Cap between the RFI filter and NST?

On the website where I got most of my information they had the spark Gap wired before Cap this is the link to their website:

I am open to other suggestions as well

Thanks again.

MOT_man

Tue Dec 18 2007, 02:10AM

Registered Member #1127 Joined: Mon Nov 19 2007, 12:08AM
Location:
Posts: 139

You might want to change the position of your tank capacitor. Rather than having the main capacitor across circuit, you could move the cap so that it is on one leg of the high voltage circuit and have the gap across circuit. This reduces transformer RF feedback I find.

I ran my 3" Coil off 2 kVA - and strikes were 5' + a few inches. - If you could find a 15 kV 60mA or a 12kV 60mA NPF - it will be a BIG wow!

bomberguy92

Tue Dec 18 2007, 02:11AM

Registered Member #1181 Joined: Mon Dec 17 2007, 10:48PM
Location: Iowa USA
Posts: 17

Also I have already pushed my budget so cheap suggestions would be really helpful
Can I use the Saltwater bottle caps I was going to use as the tank cap (before I decided on an MMC) for the filter caps?

thanks

Shaun

Tue Dec 18 2007, 02:32AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

TeslaMap should tell you a "PFC capacitance" or something similar; its just a capacitor that goes across the power inputs of the NST.

Also, the middle electrode of the safety gap should connect to the NST's center tap, which will be connected to the case of the NST. Same place you connect mains ground to the NST.

I Agree with MOT man, it would probably be better to switch the locations of the main gap and capacitor. For power, if you could get your hands on one (preferably two!) 15/60 NSTs, I guarantee you wont regret it! I seem to remember a 15/120 NST on ebay awhile back when I was shopping for mine; dont know if its still around.

bomberguy92

Tue Dec 18 2007, 02:38AM

Registered Member #1181 Joined: Mon Dec 17 2007, 10:48PM
Location: Iowa USA
Posts: 17

what should the voltage rating be for the PFC cap?
How important is it that it have the same uf as what TeslaMap says?

Steve Ward

Tue Dec 18 2007, 04:07AM

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

what should the voltage rating be for the PFC cap?
How important is it that it have the same uf as what TeslaMap says?

IT should be rated for the line voltage you operate at (either 120 or 240VAC id presume). The capacitance can be less than what is suggested, and it simply wont fully correct the power factor. No harm done, but it wont yield the minimum possible line current.

On a technical note: tesla coils dont simply present a phase lag like say an induction motor, thus power factor correction by means of adding parallel capacitors is not a true fix. You will never get perfect power factor in this manner, and in my opinion its pointless to try and correct fully. Adding *too much* capacitance just pushes the phase angle of the current positive (leading) and doesnt really hurt anything, but again it causes higher current draw than what is required for the REAL power being processed.

Tetrafluoroethane

Tue Dec 18 2007, 11:56AM

Registered Member #127 Joined: Fri Feb 10 2006, 03:36PM
Location: Cincinnati, OH - USA
Posts: 44

Yes, the center ball of the safety gap should be attached to mains ground. The center tap (case) of your NST is connected to mains ground so each HV terminal will want to arc there. This makes sure neither half of the NST gets over-volted regardless of the voltage across the other half.

Save yourself the hassle. I know it is more expensive up-front, but build an MMC from the CDE caps everybody uses (CDE142...). Your chances of getting great performance out of salt water caps it pretty low and it takes a fair amount of effort to build them. Coincidentally, I used the CDE140... caps. I wasn't paying attention and ordered them by mistake. I have over 3 hours of run time on them with no problem, but I am not pushing them to their voltage limits either (I actually calculated my cap based on their AC voltage rating, which is much more expensive since you need more caps).

Your coil is pretty much my first, but I used a 15/30 NST. I got 36" out of that baby once I got her tuned, and I think she is capable of more (should I ever dig her out of the garage). I think most of that was luck. I wound the secondary on 4" PVC using 22GA wire. Toroid from 3" aluminum dryer duct. So you have the potential for some great arcs there. And you can always add another 9/30 NST down the road for more input power.

The filter you are looking at on eBay should work fine. Just don't expect the cheap ones to last forever. Make sure you have a fuse on the input somewhere so if you happen to blow the filter short you won't be relying on your mains breaker to save everything.

You can usually find some cheap motor run caps on eBay. These work great for your PFC capacitors. Just parallel enough to get close to your calculated PFC value. Like Steve said, close is fine.

I did use a Terry filter, sort of. I didn't include the MOVs since I couldn't find them. Nobody had a stock. So I just used two big resistors and a series of caps. Everything came from Mouser and wasn't all that expensive. But quite a few people say they have run without a filter just fine. Whatever you do, make sure your spark gap is across the NST terminals, not your tank capacitor. And still use a safety gap even if it looks like it will just be another spark gap (remember, it gets set wider than your main gap and protects each NST half individually).

Good luck and happy coiling.

HV Enthusiast

Tue Dec 18 2007, 02:41PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

bomberguy92 wrote ...

what should the voltage rating be for the PFC cap?
How important is it that it have the same uf as what TeslaMap says?

PFC cap really isn't necessary for the power levels you are running.

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