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Secondary Coil Terminations

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Wilson

Tue Jul 04 2006, 01:48PM

Registered Member #78 Joined: Thu Feb 09 2006, 11:27AM
Location: Sydney, Australia
Posts: 133

I've been wondering about this for a week now, and i haven't been able to find an answer out there on the web:
Do you guys usually terminate the secondary winding by leading the wire along the outer surface of the secondary former, and to some sort of attachment, ie a screw, at the base/top? OR
Drill a small hole in the secondary former, and lead the wire through the centre of the former, to some form of connection?

Is there any difference between the two? [Note: I've found in a test SSTC that a kink in the secondary wire near where it terminates leads to promoted streamer formation in that area when no topload is used]

Also, is there a stark difference in performance if the secondary wire makes a poor electrical connection to the topload or RF ground? ie. the magnet wire hasn't been "stripped" and just wrapped around a bolt?

Thanks,
Wilson

Marko

Tue Jul 04 2006, 02:31PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I usually drill a small hole and stick the wire trough it.
Sometimes two holes can be drilled, so wire goes inside and out wich makes a very good connection.

When winding I usually put a small piece of doble-sided sticky tape for first few turns, wich eases things a lot.

I even tought if it could be used on entire secondary length ( with thicker gauges, it would be visible with thin wires) in order to prevent unwinding/overlaping of wire if secondary isn't immediately warnished ( exscept myselfI saw this happening to Steve Conner and TDU, very frustrating experience

).

Regarding poor electrical connections:
I had such one on my class E SSTC secondary.
To make things worse Secondary was filled with poliester goop to prevent melting of former at enormous frequecies. Top wire of secondary was soldered onto a brass bolt, and it looked pretty good when I put everything in.
After poliester dried I measured it and figured out it shows infinite resistance

, god knows what happened. (wire tripped, bad solder?) I saw it arcing and smoking a bit under poliester but I could do nothing at that time.I used ping-pong ball as topload, and continuous arcing finally created enough carbon to weaken the contact to topload too.
Arcs finally ignitedvery flammable celluloid ball, wich burned with low presence of air under the foil and exploded into a cloud of smoke. It stunk out terribly the whole room.

Fortunately wire seems to have welded itself to bolt under the poliester and this saved the secondary for now.

I strongly reccomend any high-voltage connections to be best possible, adviseably soldered and checked using ohmmeter.

PS. with bad contats spark difference usually isn't visible, until you eventually set something to fire..

Avalanche

Tue Jul 04 2006, 02:51PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

I keep the secondary wire on the outside of the former now. The trick is to not bend it too sharply towards the topload, so I kind of spiral it towards a bolt sticking up from the centre of the former, and trap it under there. On small coils, I have started to cut the secondary wire right at the last turn, and solder on a thicker piece of insulated wire. Also make sure the topload is sheilding the top windings with its electric field, this helps a lot.

HV Enthusiast

Tue Jul 04 2006, 07:51PM

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

I simply take a rectangular piece of thin copper sheet (say 0.5" x 2") and glue to the PVC directly (clamping overnight) at the base of the coil. Use some type of superglue. make sure to scratch the surface beforehand.

GimpyJoe

Tue Jul 04 2006, 07:59PM

Registered Member #316 Joined: Mon Mar 13 2006, 01:30PM
Location: Marietta, GA
Posts: 212

Never ever run the secondary wire inside the former. With small sstc's you may be fine but with drsstc's and sgtc's where spark length exceeds secondary length you can get sparks tracking along the inside of your former and burning carbon tracks that ruin your secondary. I find the best way to terminate the secondary winding is to wrap a few layers of electrical tape over the last few turns and cut the former right where the tape ends. Then make a corona ring for the top, the best way I have found is with a foam floral wreath ring covered in aluminum foil. Strip the wire leading from the top of the secondary and secure this to your corona ring by wrapping strips of aluminum tape around it and the ring to hold it tight. Then glue your ring right to the top of your secondary. I always make my toroid supports from PVC couplings wrapped in aluminum tape and set them on top of the corona ring so I get good contact.

Marko

Tue Jul 04 2006, 08:30PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Wire can be safely ran inside as long as it doesn't fall under top turn/hang too high from bottom turn of the secondary. If wire is nicely placed it usually won't be more than few cm long and this isn't a big deal.

It's anyway just for fixing purposes.

Hazmatt_(The Underdog)

Wed Jul 05 2006, 02:48AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

I'm using the copper pad at the bottom, but with a bonus. We have a pair of 6-32 nylon screws threaded through from inside to outside acting as studs. These studs have knurled nuts that snug a forked terminal against the copper pad for termination.

The top lead is terminated to a corona ring supported by ceramic supports. The corona ring serves as a termination and corona suppression. This helps when the system operates without a toroid, I like the crazy streamers with and without. So the ring helps in the sense that you don't cause a lot of stress to that last turn on top.

I'll have some pics soon so you can see what I'm referring to.

Wilson

Wed Jul 05 2006, 12:38PM

Registered Member #78 Joined: Thu Feb 09 2006, 11:27AM
Location: Sydney, Australia
Posts: 133

Pictures would be greatly appreciated :) I'm particularly interested in this corona ring. Is it just as simple as a ring of copper tubing?

Also, are there any effects of having ferrous objects in close proximity to the coil, ie, for the screw connections at the top and bottom? I imagine it might lower the resonant freq. somewhat and can cause some losses by induction, but any other negative effects? [Not that i'll be using iron in my coil of course

]

Steve Conner

Wed Jul 05 2006, 01:21PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I think the only bad thing you can do is passing the wire through a hole in the former. The presence of the hole shortens the tracking distance down the inside of the secondary, and the sharp bend in the wire makes corona start early. Everything else is OK, although I think the little glued-on squares of copper sheet lower the flashover voltage compared to corona rings. I had a secondary made with these glued-on squares top and bottom, and it always flashed over between the squares.

My DRSSTC secondary has the square of sheet at the bottom (though it's glued to the bottom flange, not the tube itself) and a corona ring at the top, and has made sparks 4.4 times its own length. Having said that, when used without a breakout point, it flashes over from the top turn to the bottom turn. The spark breaks out from the bottom turn, tracks over the bottom of the tube, up the inside, out the top, and back to the top turn. I guess I should have left more unwound length at the ends. >_<

Metal hardware is :-/ IMO. I've used steel bolts to mount toroids on coils, and steel woodscrews to screw the secondary mounting flange to the baseboard. They never seemed to induction heat noticeably. But I do try not to use metal hardware in any area where the E-field is high, in case it starts corona. So I make sure nuts and bolts are hidden inside toroids, corona rings, or under the primary or whatever.

I just make corona rings out of 3/8" soft copper pipe that I bend into a circle. I leave a gap in the ends to stop them being shorted turns.

You should make the effort to strip the enamel off the wire and make a good connection. It'll probably not make any difference at high power, but if you're trying to tune it with a signal generator or whatever, it could make a huge difference.

Terry Fritz

Thu Jul 06 2006, 12:37AM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Hi,

I am a bad boy since all my secondaries have holes and connection drilled through the form.

But my coils are not real voltage stressed like those ones that do 3 times the coil length streamers. I think I have one of the largest 12/60 coils out there

)

But what you really have to consider are voltage stress plots like this:

The brighter the image the higher the stress and the more likely it is to arc. The strike rail near the bottom is hanging out there like a lighting rod trying to attract streamers. The secondary is the real arc producer with the toroid radius trying to make the arcs. The top wind of the secondary is always a "hot spot" but only in the "outward" direction, not inward. If we color enhance:

The punctures at the top and bottom "inside" of the coil just don't have anywhere to arc too. The voltage difference is just not real high. Teh bottom of the coil is real dead so no problem there. The top is a little more stressed but there is no place for the arc to find a big potential difference other than the free space capacitance inside the coil which is not very great.

Cool charts :o))

So if your coil is not a super high performer (like Dan's) then you are probably fine.

I would alwasy try to get the connections sound and tight with low resistance. There are a few amps in the secondary and that will burn up a poor joint over time. High voltage will arc over, but that is going to bite you sooner or latter. It is also an energy drain on the normally high Q secondary. These days, I ponder silver filled epoxy...

Also, are there any effects of having ferrous objects in close proximity to the coil, ie, for the screw connections at the top and bottom? I imagine it might lower the resonant freq. somewhat and can cause some losses by induction, but any other negative effects? [Not that i'll be using iron in my coil of course ]

I am going to study this tonight or tomorrow. My new coil will have a lot of things under the primary fairly close. So I am going to set it put an measure the effects on teh primary inductance and Q. Remind me if I forget to report back..

I usually drill a small hole and stick the wire trough it.
Sometimes two holes can be drilled, so wire goes inside and out wich makes a very good connection.

I drill two holes two to wrap the end of the wire to get a solid locked end of wire point that won't come loose. You should try not to make a "bump" toward the outside of the secondary though.

It is far safer not to make holes and keep from making any sharp points. It is far easier to do the opposite, but you have to be a little careful.

Cheers,

Terry

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