Push fit connectors for EHT/ RF, high current?

[Al (astral highway)]

Hey folks,

I am re-designing a bit heavyweight tank circuit that will need taps on the main inductor for tuning. Also, it is ideal if the whole thing can easily be detached and re-attached at will.

Some folk have used croc clips for clipping to successive taps before making a more permanent arrangement once they find the sweet spot, this but it's obviously far from ideal as in the event of a poor connection, all sorts of problems can occur, some of them potentially very destructive.

So any ideas for push-fit options that are good to 20KV HF RF (only around 210KHz) at very high peak current? I will use copper strap for the main conductor. I had thought of bolting the thing under high torque but at this prototype phase it is too fiddly to keep undoing if anything needs rebuilding.

Thanks folks

[G6Tanuki]

Two ways I've done it on Kilowatt-rated HF linear amps:

1] Get some 30-amp or 45-amp "choc-blocks". Take the brass barrel-and-screws out and cut each one in half, so you have a short barrel and a single screw. Thread these in appropriate places on to the turns of the coil as you wind it. Replace the standard screws with short lengths of brass studding: you can then use a couple of nuts/washers to attach the taps.

2] When winding the coil, at tap-points wind the wire round a 1/4-inch drill-bit pull it out and twist it to form a loop before continuing. So you get loops of wire on a twisted 'stub' sticking out of the coil. As shown here:

http://w5jgv.com/1665tx/1665tx.htm

[I like this guy - he's using one of the same 6-way WWII era General Electric ceramic switches that I've got in my homebrew 40/80-Metre-band base-loading-coil matching unit!]

[GrimJosef]

Lemo Y series connectors are lovely for this kind of thing, but you'd need to be spending somebody else's money.

Cheers,

GJ

[TonyDuell]

I was going to suggest Lemo as well, having worked in particle physics I got to play with quite a few versions. My comment would have been 'make sure you are sitting down when you hear the price'. As GrimJosef says, they are not cheap (but are beautifully made...)

[Al (astral highway)]

Originally Posted by G6Tanuki

Two ways I've done it on Kilowatt-rated HF linear amps:

That's great, really helpful. Thank you. The choc-block method is ingenious.
I guess it would also be possible to solder the brass to one of the wires so only one screw is involved.


That's also a great link. As a P.S, what's the distinctive green wire he is using in his two tank circuits?

Cheers!

GF and TonyD, lolZ! at nearly £50 a pop, I think I'll do it the other way!

[G6Tanuki]

Yes I guess you could solder [preferably silver-solder!] the chopped-down choc-block to the turns of the coil for better connectivity.

I don't know what the green wire he's using is: when I did this for a living I specified wire with clear "Kapton" insulation which was back then happy to work at 200-plus-Centigrade for a decade.

I used Kapton-insulated wire on various toroidal transformers and it survived a couple of situations where mismatch meant the balun's hysteresis was dissipating most of the power [to the point where its magnetic characteristics permanently changed].

These days the usual wire for these things is "FEP" insulated - it's got brilliant dielectric strength and again works happily at 200+ Centigrade. Teledyne is the company which comes to mind as the manufacturer of this.

[Argus25]

What is the diameter of the the wire you are using on the tank coil ?

[Al (astral highway)]

Originally Posted by Argus25

What is the diameter of the the wire you are using on the tank coil ?

I have 2mm enamelled single strand, but I'm considering that 2.5mm has more ampacity and skin effect is less noteable at the operating frequency of around 210 KHz. I think circulating current is 55A.

Some builders use 2.5mm PVC covered but I think it will break down quickly with heating losses.

What do you think? There is no advantage in forcing a lowest possible inductance in this application.

[russell_w_b]

Ever considered using Mini brake-pipe or microbore heating tube for your tank-coil? You could squidge the coil in and out to fine-tune it before supporting it with PTFE or whatever.

You wouldn't need a solid conductor at that sort of frequency in any case.

[Al (astral highway)]

Originally Posted by russell_w_b

Ever considered using Mini brake-pipe or microbore heating tube for your tank-coil?

Ahah, yes, Russell, good call. It's appealing and constructors of coils in the mid kW range (usually spark-gap excited, rather than valve models) often do this.

In my case, I need around 35 metres of windings in the tank circuit, and that would mean spending a small fortune on microbore heating tube!

So good candidates are robust and heat-resistant but won't break the bank.

[russell_w_b]

Are you using just the 150pF vac cap in the tank cct?

[Al (astral highway)]

Hi Russell, no, the design has completely changed.

I originally wound a smaller secondary coil, with a very low DC resistance, resonant at around 1.9MHz. After some problems with a lot of destructive power emerging where I didn't want it to, I redid some design calculations to prove that results would be unsatisfying. I'm beyond being content with any kind of result: I want very low losses and very high efficiency and I'll go the extra mile to get this.

The next iteration has a far lower operating frequency. It will be physically as large as I can accommodate where I live. To answer your Q, the new tank cap will be around 1nF.
The 150pF vaccuum cap will either be kept in a hall of fame or I'll be offering it to a new home at the end of the project.

[russell_w_b]

If you did something with a coil diameter about 2.5 x its length (more bangs for your 'Q') and made it about 400mm long with a diameter of about 1 metre, you'd need about 22.5 turns of brake-pipe, or seventy metres. This comes in 25' (7.6 mtr lengths) would cost you typically 120 quid, so a tad expensive.

Then you'd need material to support it... It would certainly fill your living room! I can see where you're coming from.

[Argus25]

Originally Posted by astral highway

I have 2mm enamelled single strand, but I'm considering that 2.5mm has more ampacity and skin effect is less noteable at the operating frequency of around 210 KHz. I think circulating current is 55A.

Some builders use 2.5mm PVC covered but I think it will break down quickly with heating losses.

What do you think? There is no advantage in forcing a lowest possible inductance in this application.

I think then the answer to a reversible wire termination with wire this thick is this:

Where you want the wire tap to be, bend the wire out with a sharp a bend as possible, so it is perpendicular to the former surface for perhaps 6mm, make sure first that the enamel is cleaned off for the right length . Then form it into a perfect loop (circle) with about a 3mm internal diameter, return the wire with another sharp bend to the former. It could help to make a small forming tool.

Then to attach a wire, simply use a screw/washer/nut in the range of 2.5 to 3mm diameter with a ground type lug soldered to the wire to be connected to the tap. Later, when you have found the right tap, you can simply solder to the loop directly.

I would guess no jacket for the wire, why put a blanket over something that is going to be getting hot unless there are insulation concerns. It raises the question whether you might need a former with a spiral groove to give a small spacing air spacing between the turns, but it might not be needed.

[Argus25]

Actually, off hand, I think the current carrying capacity of 2mm diameter enamel copper wire, with a conservative heating effect , is about 25 to 30A. Though I have not looked this up. The reason I think this is that Lucas used 2mm dia wire close wound on regulator current coils for currents in this region. But it would pay to check the official current carrying capacity of the enameled copper wire you intend to use from the tables. If its really 55A you may need thicker wire. Obviously though it helps with the cooling that its a single layer coil (I assume) so you might be able to push it harder than the table suggests for a multi layer coil, and it would help if the enamel is grade 2.

[Diabolical Artificer]

Here's a table for current capacity of wire and here - https://www.engineeringtoolbox.com/w...ges-d_419.html

2mm is given as 41A or 34A for chassis wiring or 9.3A for power transmission.

Andy.

[Al (astral highway)]

Originally Posted by Diabolical Artificer

Here's a table for current capacity of wire...

2mm is given as 41A or 34A for chassis wiring or 9.3A for power transmission.

Hey Andy, yes. That's PVC covered wire, though - it's patently more sensitive to overheating and consequent degradation, including smoking and catching fire. Also that table has to include DC (it states chassis wiring), not relevant to my application. I think pulsed current ampacity is harder to evaluate in terms of a true maximum vs average. I haven't found a table for high frequency and enamelled; I'm still mulling this.

I did encourage some discussion of wire diameter earlier but can we now please keep the thread on topic, which is about the challenge of the physical connection for the stated application, thanks. folks.

[Al (astral highway)]

Originally Posted by Argus25

Where you want the wire tap to be, bend the wire out with a sharp a bend as possible, so it is perpendicular to the former surface for perhaps 6mm, make sure first that the enamel is cleaned off for the right length . Then form it into a perfect loop (circle) with about a 3mm internal diameter, return the wire with another sharp bend to the former. It could help to make a small forming tool.

Then to attach a wire, simply use a screw/washer/nut in the range of 2.5 to 3mm diameter with a ground type lug soldered to the wire to be connected to the tap. Later, when you have found the right tap, you can simply solder to the loop directly...

This is a neat solution, thank you, Argus, I like it.

[russell_w_b]

Originally Posted by Diabolical Artificer

2mm is given as 41A or 34A for chassis wiring or 9.3A for power transmission.

Remember that the ratio of a.c resistance at RF to d.c. (or 50Hz) resistance will be higher because of skin effect: in this case, for 201kHz and a 2mm diameter wire the resistance will be 3.7 x the d.c. or power freq. tabulated resistance value.

[Al (astral highway)]

Thanks Russell, it's helpful to see that effect quantified exactly for the design f.