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Terry Filter buzzing... corona issue?

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Blitzorn

Fri Jun 07 2013, 10:48PM

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

So. I'm withing the final day or two of getting my new coil up and running! The components are done at this point, I only have to finish setting up the stand and laying out the wiring, then it's on to tuning. I took the parts to class yesterday to show them off and discuss the history/theory of Tesla coils, but I didn't get to shoot any sparks so they're all waiting on a youtube vid now! I'll have more images and info about it soon, but for the moment I have a smallish problem(?). More like a question.

My Terry filter is finally all assembled, and I'm working on getting the spacing right on the safety gap. The circuit is just a Tripplite line conditioner/power strip, my variac, then my line filter, followed by the transformers, and then capped off by the filter assembly. Here are some pictures of the setup: http://imgur.com/a/4ZRBs#0

One thing that's kind of hard to tell from the pictures is that the ceramic insulators supporting the Ohmite resistors are kind of just there for looks (plus some thermal mass) because the screw that goes through the bottom of the board goes through the whole insulator and makes contact at the top. Still, I don't thing this is a big problem so long as I get it up on a halfway decent insulating platform. Correct me if I'm wrong...

I connected the transformers at the end they would be connected during nominal operation, with the RC and MOV elements and the 1kOhm 100W resistors between the transformers and the gap. When I get the electrodes far enough apart that there is no visible arc across them, instead I hear a sort of faint buzz. At first I thought it was just my transformers humming or the variac or one of my filtering elements, but after moving my head around the whole assembly and just listening directionally, I completely ruled that out. Something on the filter board is humming. My first thought is that it's arcing/contacting the wood of my porch and traveling through that to the filter chassis a foot away, but lifting it up on 4" ABS couplings only slightly reduced the sound (or it could be my imagination that it was any different). So now I'm thinking it MUST be corona discharge, and after a minute I catch a tiny whiff of ozone, adding to my suspicion. This is something I was worried about because the solder joints on the bottom of the punchboard are a bit jagged just due to the geometry of the wires I was joining... the discharge may not be coming from there, but that's my primary suspect. Anyhow, I might be able to put some kind of coating on the bottom of the board if that would help and if it's a serious problem. I really don't want to have to disassemble the whole thing to get at the solders (it's a bitch and a half, that's why I didn't take it apart to take pictures of it) but I will if you guys think this is a real problem.

Based on my description, do you think it's probably corona? If so, is this going to be source of possible wear/damage or a power sink when the coil is running? If you think it's coming from the pointy corners of the solders, could I sand/file them or coat them in polyeurethane or something to help reduce the discharge? Any other design critique also more than welcome!

Thanks much!

Blitzorn

Fri Jun 07 2013, 11:14PM

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

I just double-checked whether lifting it up reduced the buzzing, and it did make a big impact - but there is some residual buzz. It is, however, extremely faint. I am a bit less worried about this now, I just need to get it on some kind of insulating legs and I think that should help significantly. I think the circuit was shorting to the wood surface it was on via those screws I mentioned... Knew that was a bad design choice!

Sulaiman

Sat Jun 08 2013, 05:28AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

With great care observe your powered up transformer/filter in the dark,
you will be able to see where the problem/corona/arcing is.

Blitzorn

Sat Jun 08 2013, 08:48PM

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

So I ran the coil for the first time last night and it's a winner! Great fun, great performance on the whole, much more lively than my first build. However, in order to get it to work properly I had to disconnect the central punchboard of the terry filter from the circuit because of a dramatic amount of corona at operating voltage for the coil. Rings of discharge were visible at the two little machine screws that fastened the input leads from the transformer onto the board. I will have to do some research into the best ways to mitigate this, but I suspect that the nuts I used are serving as breakout points.

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