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serge petiot

Sun Aug 21 2011, 09:15AM

Registered Member #4054 Joined: Sun Aug 14 2011, 01:39PM
Location: The boonies, France
Posts: 26

Hi there,
E. Goodchild was very kind to answer some of my questions regarding secondary wiring. On my coil, the end wires of my secondary were ending up on the inside of the tube. Because of that I was wondering whether this would creates sparks on the inside of the tube. What goodchild told me is that he learned the hard way that this was the case and that the wires had to run on the outside, to topload and to earth. here is a picture at the winding ending on my secondary just now:
TCD

as you can see the wire goes inside to connect to the threaded rod supporting the top load.
and here is a picture of the secondary:
TCs

Please tell me what you think, the idea is to have the sparks coming off the ball on top... Now I am going to swap the threaded rod supporting the topload assembly with a nylon one, and I'll run the wire on the outside to connect to the base of the topload.

I think this is an extremelly important point when building TC, and in an attempt to make a coil look "pretty" it's also an easy mistake to make; which is why I launched this thread...
Now I am interested to hear what you guys do for wire termination?

Thx and Rgds

Serge

[Edit: Picture size]

Proud Mary

Sun Aug 21 2011, 10:36AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Comment Ã§a va, Serge?

You must consider the surface leakage path down the inside of the coil former as a resistance in parallel with the whole secondary, and not just between adjacent turns as it is on the outside.

It is less effort for a current to flow over the surface of a dielectric - the inside of your tube - than it is to cause dielectric breakdown of the air to make your sparks.

Ionisable surface contamination on any plastic insulator can lead to the irreversible carbon channel formation known as tracking.

axelro

Sun Aug 21 2011, 10:37AM

Registered Member #3640 Joined: Sat Jan 22 2011, 12:16PM
Location: Germany close to Heidelberg
Posts: 39

Yes, that's a common mistake, have it even with one of my (commercially) bought secondaries. It should only matter, when you start reaching the point, when spark length is in the area of the length of secondary.

As for the rod, it might suffice to just insulate the inner end, by some plastic disk or so.

doctor electrons

Sun Aug 21 2011, 12:28PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

Here is what i did to kick the appearance up a little bit!
Notice the nylon screw that comes up through the toroid. Theres also a nylon nut that holds the toroid on.
In the bottom of the breakout point you can see the darker color, that is also nylon. When the point
is screwed onto that nylon screw only the outside edge of the point makes contact with the top of the toroid.
Seems to work pretty well and has not caused any performance issues. Sometimes i remove the point and just use
a wire breakout point taped to the side of the toroid, also works equally well.

serge petiot

Sun Aug 21 2011, 02:03PM

Registered Member #4054 Joined: Sun Aug 14 2011, 01:39PM
Location: The boonies, France
Posts: 26

Thanx guys, it's just as I tought!
I know it's a little off-thread, but I've got a problem with my MMC cap... Basically, Java TC tells me I require 0.018 uF with 8 turn primary to tune my TC. I have 24x 942C20P15K worth 0.15 each. The only way to reach this value is to use a single sting of caps with 8 caps in series!!! great for voltage rating, but, for current, I'd really like to use more than on string. What can I do?

Herr Zapp

Sun Aug 21 2011, 05:30PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Serge -

I've experienced the internal surface tracking problem on several secondary coils I wound, and have seen several other PVC-pipe secondaries damaged or destroyed by this type of insulation failure. Installing internal baffles inside the secondary will absolutely prevent this failure IF the baffles are hermetically sealed to the ID of the coilform.

I use a router with circle cutter attachment to cut a perfectly circular disk from 1/4" or 3/8" thick acrylic stock, and bond the baffle in place using electronic-grade RTV silicone sealant. I usually install 2 baffles, each about 6-8" from the ends of the coilform. I've never had a problem with internal arcing after adopting the internal baffle design. I "draw" a bead of sealant inside the coilform about 1/2" above the desired baffle location, push the baffle into place, and then add a second bead of sealant above the baffle after the first bead cured. This creates a seal on both sides of the baffle, and guarantees a hermetic seal. You can mount a standoff insulator in the center of the upper baffle to terminate the end of the secondary wire, and then run a separate wire up through the center of your secondary cap or plug to your topload. This protects your secondary wire from mechanical damage, and gives a very clean appearance with all the wiring and connections concealed.

Do not use standard "houshold" type RTV sealants, as most of these materials release free acetic acid during curing, and that might become trapped between the baffles.

You haven't provided any info on your HV power supply ratings, but look here for MMC configurations compatible with the most commonly available neon sign transformers. You need to be aware of the peak voltage and peak current ratings required for your MMC, based on the power supply and type of spark gap you will using:

Herr Zapp

serge petiot

Sun Aug 21 2011, 07:16PM

Registered Member #4054 Joined: Sun Aug 14 2011, 01:39PM
Location: The boonies, France
Posts: 26

Mein Herr (Zapp),
the secondary is meant for a DRSSTC, so no NST here...
The resonant frequency is about 250 Khz since the secondary is about 4x22 inches with 1100 turns of awg26 wire. The toplaoad is a 3x12@ tore plus the cylinders andthe little sphere visible on pictures.
Your baffles are an interresting concept, especially since you do not find this info anywhere on the net...
As for the caps, I'd like to put in about 10 turns of pancake primary, but that would lead to a rather small MMC caps...
so, I guess the solution would be to go for 2x 6 caps in series which gives 0.05uF MMC with about 5 turns of primary...
I was just wondering what impact the shorter primary would have on current...
Thx for your answer

Serge

Tetris

Sun Aug 21 2011, 07:28PM

Registered Member #4016 Joined: Thu Jul 21 2011, 01:52AM
Location: Gainesville, FL
Posts: 660

Tu parles franÃ§ais, Serge? C'est gÃ©nial! Well anyays you could end up with coronal discharge in the pipe and eventually destroy secondary.

Herr Zapp

Sun Aug 21 2011, 09:52PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Serge -

OK, not a spark-gap coil, but a DRSSTC. Have you calculated the peak current in your primary circuit? The peak current rating for the C-D 942C, .15uF/2KV cap is 432A, so you need to make sure that you have enough parallel strings to handle the actual current.

Curiously, years ago I wound a secondary that's quite similar to yours: 4" X 20" PVC coilform, wound 17.5" with #30AWG wire, ~1600 turns, overcoated with clear epoxy. This secondary has a single internal baffle about 1-1/2" from the top, and a quick-disconnect "giant banana plug" interface with the spun aluminum toroid. As you can see, the last turn at the top just disappears through the sidewall of the coilform, and is completely "potted" in the epoxy and protected from damage, especially important with smaller gage wire. Likewise at the bottom, the last turn is soldered to an epoxied-on copper plate, with the wire encased in a bead of epoxy.

Herr Zapp

1313963544 480 FT122823 4x20 Secondary Top Turn

1313963544 480 FT122823 4 X 20 Secondary Top Plate

1313963544 480 FT122823 4x20 Secondary Internal Detail

1313963544 480 FT122823 4x20 Secondary Grounding Plate

serge petiot

Mon Aug 22 2011, 09:09AM

Registered Member #4054 Joined: Sun Aug 14 2011, 01:39PM
Location: The boonies, France
Posts: 26

HVchick, beeing French, yes, I naturally speak it. Well, not really wanting to spend time winding my own secondary, having carbon tracking occur inside it is the last think I want!
Herr Zapp, that's a pretty nifty setup you got there... Very nicely done indeed... Well, I'll guess I'll go with the wires on the outside; saves me alot of work!
Regarding caps current, I have no idea how much current the primary is going to absorb. The VBUS will be rectified 240 V, so in essence the primary will be driven by 2x340 V=720 V. The primary being driven at resonance, I have absolutely no idea what current will be... I can't imagine it to be higher than 400Apeak as that would be really pusshing my IGBTs... But having a single string of caps seems a bit risky... I'd rather live with at least 2 strings to carry the current confortably.

Serge

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