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Carbon brush in DRSSTC primary?

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Daniel Uhrenholt

Mon Nov 23 2009, 03:32PM

Registered Member #125 Joined: Fri Feb 10 2006, 01:52PM
Location: Aalborg, Denmark
Posts: 155

Hi All,

I am designing a DRSSTC primary coil, where I can move the tapping point from distance, with the help from a stepping motor and a arm. It is a little hard to explain exactly how it works, you have to wait until I update my project later this week.

But my question is what will give the best contact between the arm and the primary coil. I am thinking about something like a carbon brush as you see in a variac, but how much current can carbon brushes take? And are they any good for HF applications?

Of course it would take more than one small piece of carbon to transfer 1500 Amps, but my problem is not to make the mechanical part, but to figure out what the best solution would be for the material.

Any help is appreciated!

Cheers, Daniel

brtaman

Mon Nov 23 2009, 03:41PM

Registered Member #2161 Joined: Fri Jun 05 2009, 03:36PM
Location:
Posts: 247

Very interesting idea! The design of the primary will be in a way "screw" up or down by rotating the primary? Sounds cool.

As for the contact, I am afraid I can offer little assistance on that part, though I imagine that at these amps the brushes might offer too much resistance, they will have to be designed with A LOT of contact area in mind. Contact area will definitely be the critical part here.

For the amount of power you plan on pumping through the contacts, it will not be an easy design.

GeordieBoy

Mon Nov 23 2009, 03:57PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

With a moveable contact there is always going to be a lot of resistance there. I would instead consider going for a flexible length of conductor that can either become an extension of the outermost rigid turn to boost the inductance of the primary, or it can fold back along the outermost riging turn to buck some of the inductance. Since the cable is flexible and there is no sliding contact point the resistance will be much lower. Essentially the last quarter turn of the primary is a flying lead that can either continuie in a clockwise direction, or change direction and run back in an anti-clockwise direction giving you an adjustment range of 1/2 a turn.

If you want more inductance adjustment range you could build two primary coils and adjust the sepration between them to control the mutual inductance over a large range. This is similar to how a variometer works, or how those old radio sets used to be tuned by varying the overlap of two pancake coils. I'd always aim to eliminate sliding connections at high power as they are likely to be lossy, overheat and ultimately fail.

-Richie,

Daniel Uhrenholt

Mon Nov 23 2009, 04:53PM

Registered Member #125 Joined: Fri Feb 10 2006, 01:52PM
Location: Aalborg, Denmark
Posts: 155

Hi Richie,

You got a point there, but as I already made the primary holder, I would like to stick to the original design:-)

Maybe I could make some kind of copper brush for it, or go to the local die firm and ask for some copper graphite to make the contacts with. Either way I will need some serious contact area as brtman mention.

I will go to the shop right now, and take a look at the materials we have on the shelf...

Cheers, Daniel

Mathias

Mon Nov 23 2009, 07:16PM

Registered Member #1381 Joined: Fri Mar 07 2008, 05:24PM
Location: Hungary
Posts: 74

Hmm , why not try to "somehow" compress or drag the coil to change the primary L ? Or for even finer tuning maybe use a variable Capacitor(s) (1-2nF :P) ?

But If you really want to use some kind of sliding contacts , Why not fabricate a tube with Ball bearings , (in some Electrolyte maybe) or even better yet some Ferrofluid kept inside with some small magnets? (if money was no object :P)

HV Enthusiast

Mon Nov 23 2009, 07:43PM

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

Just use 0.25" Diameter copper tubing and a BUSS fuse holder. It clips on perfectly.

To be honest, the copper graphite is just adding complexity.

Just machine something out of aluminum and you'll be fine. Even with the graphite, you'll have to machine something to clamp it properly.

Finn Hammer

Mon Nov 23 2009, 07:44PM

Registered Member #205 Joined: Sat Feb 18 2006, 11:59AM
Location: SkÃ¸rping, Denmark
Posts: 741

All,
I talk to Daniel on an almost daily basis, we inspire each other that way. And I have also been thinking about using carbon brushes for the sliding contacts. Something like perhaps 5 or so, from truck starter motors. Individually spring loaded, and lapped to mate with the coils dual curvature, of course.....
You see, there are myriads of possibilities here, materials like a sponge made from a stack of RG 214 braid or ground strap would produce a slightly yielding matrix that could be used as a contact. In the Thumper, doing 4500Amps, the contact was 100mm long and tangentially touching copper plate. Contact area perhaps like 30mm^2.
When I made the first RAT (Realtime Adjustable Teslacoil) coil, the Ambassador, back then there was only about 15x10 mm contact between movable tapping point and primary coil.
I have no idea about the current, but it was lower than what we are running now.
Many drsstcÂ´s use simple fuse holders as contacts, how well do you think they mate with the primary tubing, think about it: Fuse holder is cylindrical, primary is helical, they mate along a thin (red) line.
I think a multible stack of short pieces of carbon brush will do quite nicely.

Cheers, Finn hammer

Daniel Uhrenholt

Mon Nov 23 2009, 09:23PM

Registered Member #125 Joined: Fri Feb 10 2006, 01:52PM
Location: Aalborg, Denmark
Posts: 155

Ham Salad wrote ...

Just use 0.25" Diameter copper tubing and a BUSS fuse holder. It clips on perfectly.

To be honest, the copper graphite is just adding complexity.

Just machine something out of aluminum and you'll be fine. Even with the graphite, you'll have to machine something to clamp it properly.

This is not a table size Tesla coil I'm making, but a rather big one standing approx 220cm tall:-)

I want this â€œscrewâ€ primary tuning arm with stepper motor, and some gear-motor system to change the coupling of the secondary coil. Because at some point I will build the system in Mathlab, to easily change parameters and run the coil:-)

And Finn,

I agree with you, as we talked about 40 minutes ago that the carbon brushes might give even better contact than the fuse holder:-)

I will give it a try, but the results is not before at least 3 months from now... There is still a lot of mechanical stuff to make â€¦

Cheers, Daniel

Killa-X

Mon Nov 23 2009, 11:52PM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

heh funny, Long ago when i was doing SGTC, i thought it would be cool to find a way to make a adjustable warm, with maybe a wheel that goes around the copper flat primary, so when you turn it, even by motor (adjust while running) it would ride the rails, and extend the arm out...

Would be cool if you find a good way to do it, and yeah, i have seen people do fuse holders too...With my design, the supports would get in the way, thus i thought maybe wheel...Never tried it :(
Good luck Daniel!

doctor electrons

Tue Nov 24 2009, 12:24AM

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

I had actually kicked this idea around myself a while ago. Never did anything with it so heres what my plan was.
I was going to use a non conductive shaft going up the height of the primary (threaded) with a guide rod next to it.
Then the idea was to use a tungsten "wheel" to make the contact. Attached to the wheel would be some type of block (copper or al) with a connection
to an insulated wire that wound itself like one of those cord reels when at the bottom. Like high voltage silicone insulated wire, soft enough to "spool".
the block would move up and down the guide rod when the threaded shaft was turned, either by a stepper motor, and a radio shack remote control from
a car or any number of creative ways. (you being the dude who made that awesome antenna, im sure you could come up with something)
Anyway, i think this gives you an idea of what my plan was. I would jump for joy if you put it to practice! Sadly i did not find the time to do so.
If you have any questions and don't want to clog up the thread, go ahead and shoot me a pm. I will surely answer!

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