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Toroid design

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crashstudio

Mon Apr 27 2015, 01:00AM

Registered Member #54484 Joined: Sun Feb 15 2015, 06:18PM
Location:
Posts: 46

The coil will be powered by a 15kv 60ma NST with Terry filter. Secondary is 28ga on 4" PVC wound 22". I have 50' of 1/4" refrigeration copper tube for primary. For the toroid I have 4" ducting and 2 9" aluminum pie pans. I found a link where this guy covered his with drywall compound, sanded it smooth, applied a few coats of poly then aluminum tape to achieve a "relatively" smooth surface.

Will this design work? Does the inside diameter need to be smaller? Any suggestions are greatly appreciated. There is just too much different information online. Thank you

Mads Barnkob

Mon Apr 27 2015, 06:39AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

It is a lot of work to make a toroid topload like this.

I made one by the same concept for my DRSSTC 1:

Scroll down 40% of the page to see the construction pictures.

I ran out of filler and had to leave som gaps, sanding it down quickly gave some damage to the alu tubing underneath where the filler didn't cover it even. Covering it in tape is also a tedious work and it is easy to make bumps and kinks, a lot of smoothing with a gentle hard surface is needed.

It is much more sturdy than just using the alu ducting by itself, where the loop connects is much more smooth by this method, but overall there is little gain to a pretty extensive work, except it looks better :)

A neat trick is to use a elastic cord on the inside to keep it tight against the form before applying filler/varnish/tape.

loneoceans

Mon Apr 27 2015, 07:07AM

Registered Member #4098 Joined: Fri Sept 16 2011, 09:26PM
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Posts: 236

I found that if you buy a good quality aluminium ducting and take care when building the toroid, you don't need to make it smooth all around and it'll still perform great.

I simply connected both ends together with a plastic spacer in-between which I used screws to hold them together, and used two large pans for the middle. Try not to unravel the aluminum ducting as much as possible - this will make the resulting toroid much stronger. I found it much easier to connect both pans together, then carefully squeezing it through the toroid. The ducting should have enough play and residual tension to make this work really well.

See this page (somewhere in the middle) where I describe how I made my toroid. It worked out pretty well and I use this method for all my large toroids.

Sulaiman

Mon Apr 27 2015, 10:02AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

In my opinion, the topload is the least critical component, (I once used a cooking pot !)

if you make it nice and smooth and shiny you will need to look after it carefully
and probably need a breakout point
(incidentally, 'coiling' needs quite a lot of storage space)

if you just lash it together it will work just as well and may or may not need a breakout point

I suggest that you initially just lash it together to get the whole T.C. working / tuned
then, when all of the other parts are optimised
(I think that the spark gap is the most difficult component to get right for performance)
then you can beautify your top load, a little re-tuning of the primary may be required.

in other words, I suggest that you build a learning prototype
then a fully optimised system. do not expect to get everything perfect at first.
Usually the primary and secondary coils remain unchanged,
the spark gap, primary capacitor and the top load may change.
(if you get 'hooked' then multiples of everything, hence the need for storage space)

unless you are following some plan exactly,
you will discover many unexpected ways that electricity can flow
so I think that if this is your first sgtc you should build it asap,
don't over-think it (like I did) because as we learn our plans change
it is likely that you will get flash-over, arcing, corona discharge etc.
part of the art is in controlling these things.

I enjoyed constructing SGTC because they can be just six components
eht transformer, spark gap, capacitor, primary, secondary, topload
(safety spark gap, filter(s), variacs etc. are optional, but recommended)
each component, and their interactions, gave me some understanding of many fundamental principles
and sparks !

I suggest that your FIRST task is to construct, test and attach (never to be removed) either
a safety spark gap or a Terry filter or equivalent.
the (stupid .. I knew I shouldn't have tried that) loss of an NST can be frustrating

unless you have plenty of spare cash and can just buy another one.

good luck, have fun, stay safe.

p.s. decades ago,John Freau came up with an empirical formula: spark length in inches = 1.7 x sqrt(VA)
it is only approximate and applies in certain cases only, but you should aim for 1.7 x sqrt(15kV x 60mA) = 30"
i.e. do not forget to add a 'strike rail' to your primary winding.
also
there are many online TC calculators, I haven't used them for a while so I will not choose one.
I used JavaTC before and found very close results.

crashstudio

Tue Apr 28 2015, 02:33AM

Registered Member #54484 Joined: Sun Feb 15 2015, 06:18PM
Location:
Posts: 46

Thank you guys for the help. Sulaiman, you said exactly what I was already thinking. I guess I just needed to hear someone else say it. As far as the spark gap, after reading a lot online I've decided to go with a Richard Quick style static gap and will be needing some advice on that as well. As far as the NST, I am building a Terry Filter. I really don't want to ruin my transformer. Especially cause I got a good deal on it. It's a Franceformer, one of the few models where the GFCI circuit can be eliminated.
Thanks again to all!

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