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Re: An internal primary

Original poster: "K. C. Herrick" <kchdlh@xxxxxxx>

I conveyed my thanks to Finn for this info and I now thank the others who've commented. With Finn's encouragement (and good info and a couple of IGBTs from Steve Ward) I'll press ahead! Here's what I plan to do:

1. Mount a 7-turn primary, made from paired 1/4"-dia soft Cu tubing on plastic tubing-mounting strips, on the inside surface of a commercial polyethelyne bucket (cutting the bucket appropriately to length). The mounting strips to be held to the bucket with nylon screws so there are no open holes. Place that bucket assembly tightly inside another, similar, bucket that itself will fit closely inside my (existing) 12"-dia secondary (this, for added voltage-withstanding capability).

2. I plan to use a resonant primary with drive-feedback derived from current-transformer monitoring of the primary current. The driving elements will be an H-bridge of IGBT "bricks". Steve Ward has recently done this very successfully as he's reported--and I will use a variation of his driving circuit as well as his current-transformer scheme. Happily, I can adapt the controlling electronics I had made for my original, now-deceased, sstc. At my age (77) I no longer have the energy to build that again!

3. Given room for them, I will mount the MMC capacitors, circumferentially and at the nominal secondary diameter, directly below the primary coil assembly. That way, their magnetic flux will to some degree pass through the secondary rather than out to nowhere.

I take note of a "Rev. Sci. Instrum." article cited some time ago on the List, "Greater voltage gain for Tesla-transformer accelerators". It concludes that the optimum k, primary:secondary is ~0.55. As I read it (perhaps imperfectly; I can't wade thru all the math), one determines k using 2 measurements of the secondary's Fr: when the primary is shorted and when it is open; here, "wsc" and "woc". The formula is k = sqrt{1-(woc/wsc)^2}. Once k is realized, then tune the primary circuit for two Fr's differing by a factor of two--and presumably then run the coil at the lower of the two. When & if I can get it all up & running, I'll shoot for that.

Comments appreciated...

Ken Herrick

Tesla list wrote:
Original poster: Finn Hammer <mailto:f-h@xxxx><f-h@xxxx>


I have successfully used internal primary`s in 2 OLTC`s and also in 2 identical SSTC twins. So I have got 6 internals under my belt.

In OLTC`s, Solid state disruptive, there is no reason to use any precaution. The single turn primary fits elegantly and snugly into the base of the secondary coil.

I have had one problem with the internal primary in the SSTC. During one unintentionally long burst, the insulation on the primary wire toasted. I thought I had experienced a breakdown trough the secondary former material (1/4" PVC) but after stripping the sec. wire off, inspection showed no zoot under the windings. So the sec. was ok, still the pri. was toast.
I am going to use the internal of RG213 coax as primary wire in the future. In pulsed mode, with bursts up to 3mS and rep rates up to 500bps, I have no problems with internal primary even in SSTC.

Go for it! It`s sweet, easy to adjust coupling on the fly (which has been a great asset to learning about the effect of different coupling in twin coils, it being non intuitive) I have a nice design of an OLTC which you can see here:

and almost at the bottom of this page, there are pictures of the twin coils in the prototype stage.

Cheers, Finn Hammer

Tesla list wrote:

Original poster: "K. C. Herrick" <mailto:kchdlh@xxxxxxx><kchdlh@xxxxxxx>

Who was it who implemented an internal primary? Successfully, as I recall. I'm thinking of trying that but perhaps that someone, or someone else, would warn me off.

Ken Herrick