[TCML] Problems with secondary - internal arcing
quarkster at att.net
Thu Jul 31 15:14:26 MDT 2008
Well, you've certainly found the root cause of the poor performance.
You MUST NOT run the ends of of the secondary winding inside the coil form unless you install sealed insulating baffles (as in hermetically-sealed, gas-tight ) inside the coilform to isolate the high-voltage end from the grounded end. If this is the design that was recommended in your "kit", then this was an extremely poor design from the outset. Running BOTH the HV and ground end of the secondary inside the form will almost guarantee insulation breakdown, as you found. This is just very bad design practive for any HV equipment, especially at 80-150KV and high-frequency RF.
Your secondary may be salvagable with some work, but in the end you will either have to add baffles, or remove one or both ends of the secondary wire from inside the coilform.
Here are my recommendations:
1. Sand the interior of the the secondary to remove ALL TRACES of the carbon tracking. This may take a bit of work, depending on how deep the carbonization is. You MUST remove all visible carbon deposits. Finish up with 220 grit or finer paper so you get a smooth finish, and can visually verify that all carbonized and discolored plastic has been removed.
2. You may want to apply one or two coats of insulating varnish over the previously burned areas, just to give some additional protection.
3. Remove the ground wire from inside the coilform, and make an attachment point for the wire on the outside of the coilform, or on the outside of the lower end cap. I've used an external copper plate for years, without any problems. Basically, you will bond a small piece of copper strap to the outside of the coilform, solder the secondary wire to the strap, and attach your ground wire to a nut soldered to the strap.
Get a thin piece of copper (.025-.040") and cut a piece approx 1" X 3". Form the copper to the radius of the coilform, so it fits very closely. Solder or silver-braze a common machine nut (#10-32, or similar) to the center of the copper strap. Roughen the back side of the coppper strap and the area on the coilform that you will bond it to, and secure the strap with epoxy adhesive. Use heavy rubber bands or masking tape to hold the strap in close contact with the coilform until the epoxy cures.
4. Just removing the ground wire from inside the secondary will probably eliminate the internal arcing, but if you want to make sure, I'd add a sealed internal baffle towards the top of the secondary, a few inches under the top cap. If your top cap is removable, this will be relatively easy. If you've permanently bonded the top cap in place, this will be more difficult. In any case, you want to install a plastic baffle inside, and seal it using silicone RTV adhesive (preferable automotive grade) or something similar that will create an absolute gas-tight seal, free from even microscopic holes. High voltage will easily find and penetrate even micron-sized holes.
5. Also, looking at your photos it looks like you've used some large diameter white wire (I can't quite make out the markings) for all your HV interconnections, including the tank circuit. If this is common 18 AWG GTO neon sign wiring, it's really not large enough gage to handle the high peak currents in the tank. You may be able to improve coil performance by making all the tank wiring as short and direct as possible, using considerably heavier wiring (like #10 AWG or larger). It looks like your total tank wiring may be 3-4 feet long (or longer), so I'd consider relocating the spark gap and MMC as close to the primary as possible, so all connections are short and direct.
I'l send you some photos of several secondary coils with the copper-strap ground plate, and with internal baffles.
"Stephen J. Hobley" <shobley at userfs.com> wrote:
I put together some images of our poorly performing coil here:
it is the tesla coil kit sold by Alan:
I think I've found a problem. When I opened the secondary form there was
a massive smell of "electricity" and the inside of the coil had
undergone some arcing.
So at this point I ask the question, should the secondary wires run
completely outside of the form, or is it acceptable to have a short
length inside to connect to the bolts?
The base of the form was darkened by the arcing, and there was also
evidence of a fluid, almost like oil on the lower bolt. I'm sure the
inside was completely dry when we assembled it.
I've searched the internet, but cannot find any examples of how the
secondary wire should be connected.
So this is probably what's affecting the performance. The coil does not
seem damaged on the outside, just the inside of the tube.
I will clean out the inside tonight trim the wires down so that they are
as short as possible.
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