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Hi guys. I actually couldn't believe I found a forum for Tesla coils. So I was pretty excited to see this!
Anyways, I'm nick. Believe it or not I randomly saw a tesla coil on America's got Talent and decided to build one of these. After 3 months of gathering parts, it has finally come together. Over the weekend, just a few days ago, I ran my first test.
On Sunday I ran my coil again and well I blew out 3 capacitors on the Terry Filter. The .0033Uf 1600v capacitors. 2 on one side, and 1 on the other side.
Tonight I tried running it again and blew 2 more caps. 1 on each side of the terry filter.
Im using a 12k 30ma NST with no GFI. Everything is grounded out. I'm using a standard spark gap. The gap is set at 1/4". the secondary coil is wrapped with exactly 1500ft of 26g magnet wire. Around a 4" PVC pipe. Coil wrapping is 19 1/2". Primary coil was created with 50ft of 1/4" copper tubing. Spark gap has a fan for it. Using a 20amo 250v line filter and a variac is being used.
Discharge off toroid is only about 4-5" long and very thin.
Ive tried to search Google for an answer on why these capacitors would be blowing out but I cant find anything on it. Ive checked all my connections. Everything seems to be hooked up.
If someone may be able to help me, maybe give me some advice on what I need to look for on this as to why these caps are blowing that would be great!
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
IR -
These are just functioning as RF bypass capacitors, so during normal coil operation they should not see either excessive current or excessive voltage (IF you've assembled your Terry filter correctly). You need to provide some additional info before we can help diagnose your problem:
1. Who is the cap manufacturer, and what is the specific part number (verify actual P/N marked on the capacitor body)? 2. How many series-connected caps are you using between each HV transformer bushing and RF ground? 3. Is your variac configured in step-up mode to supply MORE than line voltage? If so, what is the maximum MEASURED input voltage to your NST? 4. What do you mean the capacitors "blew out"?? Please describe EXACTLY how they failed, and the appearance of the failed capacitors. Did they discolor, swell, crack open, split, or ?? Exactly where on the capacitor body is the failure: at the ends, or in the center? 5. How much clearance do you have between each capacitor and the nearest other component or structure of your coil?
Some well-focused macro photos of the failed caps would be a great help in determining what's going on here.
These are just functioning as RF bypass capacitors, so during normal coil operation they should not see either excessive current or excessive voltage (IF you've assembled your Terry filter correctly). You need to provide some additional info before we can help diagnose your problem:
1. Who is the cap manufacturer, and what is the specific part number (verify actual P/N marked on the capacitor body)? 2. How many series-connected caps are you using between each HV transformer bushing and RF ground? 3. Is your variac configured in step-up mode to supply MORE than line voltage? If so, what is the maximum MEASURED input voltage to your NST? 4. What do you mean the capacitors "blew out"?? Please describe EXACTLY how they failed, and the appearance of the failed capacitors. Did they discolor, swell, crack open, split, or ?? Exactly where on the capacitor body is the failure: at the ends, or in the center? 5. How much clearance do you have between each capacitor and the nearest other component or structure of your coil?
Some well-focused macro photos of the failed caps would be a great help in determining what's going on here.
Herr Zapp
Well I not sure on a few of these questions, but I built the terry filter using terry fritz's plans.
All I can see on it is whv 332J, 1.6kVDC This is what I found on my invoice when I purchased them.
Panasonic Polypropylene Film Capacitors 0.0033uF 1600VDC 5% US HTS: 8532250070 ECCN:EAR99 COO:JP
Im using 6 of these caps on each side of the filter with 7 MOV's on each side for a total of 14.
The variac is I guess you would call a step up mode variac. Is has a dial on it that goes up to 130. I havent taken it above 100 though.
I have it laid out that the power from the house goes to the variac, then the 20amp 250v line filter, then to the NST then connects to the terry filter.
The caps blew by swelling, the ceramic? cracked and one actually blew a piece off. And on top of the swelling it has a black hole burn in it. Only one cap had a black hole, the others swelled and cracked. Sometimes you could see arcing! between caps. The caps failures were in the center of them.
Ive attached a few pictures o the terry filter installed and the blown caps.
The filter inst really close to anything else that could arc from it. Ive just seen some arcing between the caps.
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
IR -
I believe these caps utilize a "floating plate" construction, where a failure in the center of the cap indicates an overvoltage condition that results in internal arcing at the gap between the ends of the primary plates.
So, how is it possible that you are overvolting these caps with a 12kV NST? The most likely scenario is because your primary circuit is "resonant" with the AC supply frequency (50 or 60 Hz). How did you determine what the "safe" (a non-resonant) value needed to be for your tank capacitor? What is its "actual "(measured) value?
If its anywhere near .0066uF with a 60 Hz supply (.008uF with a 50 Hz supply), then you're in real danger of experiencing uncontrolled "resonant voltage rise" in your tank circuit that can within milliseconds "ring up" to extremely high voltages capable of destroying your NST, your tank capacitor, or your bypass caps (depending on how you've adjusted your safety gaps).
So, aside from letting us know what the value of your tank cap is, please provide more details on the "cap-to-cap arcing you mentioned. Between which points on the capacitors is this arcing taking place? Are you sbsolutely sure yu are seeing "cap-to-cap" arcing?
well to make things easier, I can send you a pdf file of plans I got. And it will actually tell you everything on exactly how I built this.
Honestly Im still learning about these and just wanted to see If I could build one. (Im only 21) So basically I bought some plans online.
So If there is a way too, I can send you the PDF file that had the plans in it that I followed to build my first coil.
But to throw this out there, I have kinda figured out why I blew caps so quickly and also why I had small discharges coming off the toroid.
I read somewhere on this forum that You have to have the safety gap spread out so that it barely contacts or something along those lines.
Before I read this, My safety gap was prolly only 1/4" apart, I'm assuming this was way to close. So after adjusting the safety gap farther apart I was actually able to get about 1 foot streamers off the toroid and also didn't blow any caps very quickly UNTIL I turned it on after like the 6th time and had a huge arc on the terry filter, which in turn blew 2 more caps=/. You can see it slightly in a video I took. If I can somehow send or post it on here, Id be glad to show you guys!
As far as the arcing, It looks like its arcing cap to cap, now what it could be is the cap blowing and the voltage shooting out of it. Honestly I dont want to get that close to look ha.
But as I said, to be completely honest, I am still learning about these. There very interesting!
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
IR -
Just answer the critical question: what is the value of your tank cap?
I suspect you may have utilized a value that results in a resonant condition, and if you blindly opened up your safety gaps so they could no longer function as safety gaps, then YES, you'll destroy components. You're lucky that you haven't already destroyed your NST and/or tank cap.
There's plenty of info available on how to properly adjust your safety gaps to match the output voltage of your NST. Then, research "larger than resonant" tank cap value to find out what you need to do with your current tank cap to obtain a "larger than resonant" value.
If you haven't already been there, go to Richie Burnett's exceptional Tesla coil website () and learn some of the basics of Tesla coil design and tuning, including all you need to know about "resonant" cap values.
The Capacitor bank is using 16 Cornell Dubillier C942C20P15K-F running in series with bleed resistors with a rating of 0.15 uf at 2000 vdc with a final value of 9.3 nf at 32,0000 volts
I actually only spread the safety gap apart last night for the first time after reading this from a member on a thread here.
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-- Re:Terry Filter Questions « Reply #9 on: February 06, 2006, 11:58:02 AM » Reply with quote Yep, you could definitely kill it. Voltage could build up across the secondary much higher than the rated voltage of the transformer.
For setting a safety gap, simply connect your spark gap to the output of your NST with nothing else connected. Start with the electrodes real close together so you have a continuous arc. Keep moving the electrodes apart until it stops arcing. (OF COURSE, do this with the power off!) Now move the electrodes apart just a smidgeon more and you should be good. If during actual operation you find the safety gap arcing considerably, you may want to spread the electrodes out just a little more.
Dan ---------------------------------------
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Ill be looking at the site today, I haven't seen that site yet before!
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Sorry to hear the Irish luck isn't working for you! ;-(
I see you've built your Terry Filter on some perforated board. Is it the kind with copper strips on the underside?
If so, can you run the coil with it flipped over, and look for arcing between the strips? That could be your problem. If some of the capacitors are arcing over, that would put excess voltage across the remaining ones.
Herr Zapp's suggestions are also worth checking out, but I don't see how the caps can be getting overvolted as a whole, as long as they have MOVs across them.
The best way to post videos here is to put them on Youtube as "unlisted" and then post a link. You'll see a Youtube button in the editor, it will embed the video right in the post.
I think ill eventually get it working, It worked a whole lot better last night when I spread the safety gap a little bit and I was so excited to see it. But i want it to perform at its best!
No, no copper on the board at all. I read that you do not want the copper on it as it does arc across.
I do have MOV's on the terry filter. The MOV's are all soldered together but not individually soldered to the other caps and resistors. The MOV's, caps and the resistors only attach at the very ends of each other. but the resitors are soldered together to the 1600 caps in between them.
I wouldn't think this at all, but could it in fact be that the caps are just to close together in general? I wouldnt think this would be a problem but.....what do you all think?
Ill try to get a video of my last shot last night of the huge arc that shot across the whole entire filter.
So is he suggesting that the terry filter inst I guess "Big" enough to handle the voltage?
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First time Tesla builder, Terry Filter caps popping?
