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Re: TC Limits


> I'm envisioning a bipolar model;
> helical primary 2" long, 4" dia, secondary about 10" long, 2" dia,
> running at 1 MHz. It isn't intended for spark generation, but rather
> to power a small particle accelerator in pulse mode (I may not even
> want a TC; that's why I'm here).

You could make a one-shot TC, but even then you'll have a hard time trying
to rectify the output - you'd need quite a lot of diodes, or a vacuum
rectifier of some sort. The voltage on both terminals of the bipolar coil
is always opposite, but it changes polarity wrt time. So if it is your
intention to have fast particles of same polarity shooting out from both
ends, and slower particles trapped inside, then just a plain TC could be
ok.

> I'm looking for about 50KV.

If it is critical to have exactly 50kV (and DC), you better go to some
form of flyback transformer circuit. If power is not needed, you can use a
TV flyback or a self built one and connect a voltage doubler,x4,x... to
it to get those 50kV.

What are you going to use the accelerator for, btw?  Home experimentation
probably? 8-)

>   What are the length/diameter ratio limits for a "good" TC?

For your small, handheld/tabletop/accelerator coil, maybe 6:1. Or 5:1. But
best not smaller. And somewhere around 900 turns total. Personally I
like flat primaries more (no sec->pri flashovers and strikes from coil
bottom or topload), with small outer diameter, say 1/3 of coil height.
It's all just some vague rule of thumb, and I'm sure someone disagrees on
my H/D ratio values and such because he has built a brilliantly working
coil with totally different estimates... :o)

>   As extreme cases, I can see that a very long secondary would have
> too many windings out of the influence of a very short primary to be
> worth the trouble to wind, 

What I don't undestand is why should the secondary be enclosed in the
primary field - wouldn't just driving the coil bottom turns, in magnifier
or bottom-fed coil fashion, be enough? The bottom turns would act as
the HF supply source for the rest of the secondary (?). Just use heavier
AWG# wire on the bottom (?).

>   What are the operating frequency limits, and why? I was told a TC
> couldn't operate satisfactorily at 1 MHz. Why won't they scale?

You can very well build a small TC up to 1Mhz or a bit more, and run it 
successfully, but with the higher frequency the energy losses in the
materials will be higher (in the tank capacitor, secondary turns, ...).

>   How do you estimate coefficient of coupling for a particular
> geometry (preferably without using calculus)? <math pun>

Just by experimenting. ;o)

>   Anybody think a standard Xenon flashtube
> They're designed to switch currents quickly at fairly high voltages,
> their electrodes are fairly erosion-resistant, and they can be triggered
> reliably.

You'd need many of them...

Also, the arc resistance is not nice. There's too low pressure and thus 
too few current carriers (suggest you read
http://www.pupman-dot-com/listarchives/1999/March/msg00571.html post by Bert
Hickman)

--
*************************************************
 Jan Florian Wagner
 http://www.hut.fi/~jwagner