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Blitzorn

Tue May 26 2009, 01:27AM

Registered Member #2128 Joined: Thu May 21 2009, 03:17AM
Location: Seattle, Washington, USA
Posts: 58

SteveC -

Funny thing you mention it, i do. But it's from like 1974 and i am highly dubious of it's functionality. I may be able to repair it though, it's pretty sturdily built and i think the essential components are intact. If you think it's a good idea i can try to get that working for use during my tuning phase...

Herr Zapp -

I'm a little indignant that you have so little faith in me! But i concede, i'm new so i'll do the listening, after all, you did save my butt once already. In that spirit, here are my working stats. And for the record, yes, i do know what Farads and Henries are xD

Secondary Length: 533mm
Wire Gauge: 0.56mm Outside diameter, 0.51mm conductor diameter (24 AWG)
Form Outside Diameter: 11.43cm
Turns: 954
Secondary Coil Capacitance: 8.4pf
Secondary Coil Inductance: 21mH
Top Load Tube diameter: 11.43cm
Toroid span: 29.21cm
Top Load Capacitance: 13.6pF
Secondary Circuit Sum Capacitance: 22pF
Secondary Circuit Resonant Frequency: 234kHz

Now here's where the game really becomes theory crafting, but i think i did this correctly so shoot me down if i'm off track.

I am going to make eleven salt water bottle caps, each one with an estimated 0.825nF of capacitance, giving a total of 9.075nF.

With that much capacitance, it takes 51uH of inductance to match the 234kHz resonant frequency.

My primary coil tubing is 6.35mm diameter, and using that and an interior gap diameter of 16.5cm i messed around on a calculator until i got some decent figures. With 6.35mm tubing, 6.35mm spacing, and an interior gap of 16.5cm, i would need approximately 12.25 turns to make the 51uH. Assuming that my estimates for the capacitors aren't perfect, i'll have plenty of extra room to move the tap point up or down the coil and tune it a little that way.

I'll draw up my spark gap and link the image in an edit.

Coronafix

Tue May 26 2009, 02:39AM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

C=1(2pi x f x Z)
f=line frequency (60Hz in the US)
Z=V/A (secondary side)
This will give you the resonant cap value that your transformer can charge at.
If you are using that Allanson NST then the resonant value will be 7.7nF. At this value, the voltage on the cap bank can go a lot higher and therefore burn the windings on your NST secondary. That is why we use LTR, larger than resonant, usually 1.4-1.5 X.
So it might pay if you aim for a few extra salt water bottles. This will give you less inductance needed on your primary.
Also remember that the sparks breaking out will lower the frequency of the coil a little, so don't go for exact calculations. Tuning will need to be experimentally adjusted. I think Herr Zapp might have said somewhere else that it amounts to another turn or two from measured resonance.

Blitzorn

Tue May 26 2009, 03:47AM

Registered Member #2128 Joined: Thu May 21 2009, 03:17AM
Location: Seattle, Washington, USA
Posts: 58

Thanks for pointing that out, i had that written down somewhere and had adjusted my capacitance up to 10.7, but that didn't make it into this calculation! Let me see, that changes things...
Or does it?
I'll still go for 13 turns on the primary, but i'll tap it earlier, my calc says 11.35 turns is optimal, that also gives me more buffer space to make sure i can tune it properly. Everything else can pretty much stay constant.

Here's my spark gap design, it's pretty straightforward but i don't yet know exactly how far to space the electrodes or how many i should use. I have completed the end cap and attached the funnel segment to the shopvac, that's as far as i've gotten. I don't want to push on further until i figure out how to calibrate it. Right now it is a very open and flexible design, i'm not going to do anything permanent to the main tube until then.

Everything look decent here?

Herr Zapp

Tue May 26 2009, 03:49AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Blitzorn -

Take no offense; it looks like you do understand what's going on, and are on the right track.

I've reviewed your design data, and everything looks OK so far.

If the NST you ordered is a 12kV, 30ma unit, the resonant capacitor value is .0066uF. You will want to use a capacitor value that is ~1.4X or 1.5X the resonant value, or around .009 - .01uF.

I'd go ahead and use a full 50' coil of 1/4" refrigeration tubing for your primary, giving around 14 turns with a 6.5" ID. This will give you plenty of adjustment range for tuning in case your capacitor value ends up smaller than you have calculated. Make all your connections between spark gap. capacitor, and primary coil as short and direct as possible to avoid adding a lof of unwanted inductance.

Plan on how you will make your adjustable "tap" for the primary. A modified standard 3AG fuseholder works well, and it provides a good, solid mechanical grip on the 1/4" dia. primary tubing that can be easily moved around to various positions.

As mentioned by Steve C, a variac is HIGHLY recommended for a controlled "initial power-up" of the coil. A 5A variac will be adequate for your 12/30 NST. A variac is pretty simple device, and as long as the windings are not burned, the commutator is clean and the brush is intact, then it should "work". Test it with a common 100W incandescent light bulb first, to make sure that it works, and that you understand how to connect it up.

Otherwise, press on!

Regards,
Herr Zapp

Blitzorn

Tue May 26 2009, 04:26AM

Registered Member #2128 Joined: Thu May 21 2009, 03:17AM
Location: Seattle, Washington, USA
Posts: 58

Thanks much - Do you know a place i could find such a fuseholder?

I actually have exactly what you recommend, 50' of 1/4' refrigeration tubing, so i'll go ahead and use the whole thing :)

I'll have a look at the Variac and report back about it's status.

Coronafix

Tue May 26 2009, 06:06AM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

Apologies, for some reason I used 35mA in my calculation.
I think that was your old transformer.

Blitzorn

Tue May 26 2009, 01:23PM

Registered Member #2128 Joined: Thu May 21 2009, 03:17AM
Location: Seattle, Washington, USA
Posts: 58

No worries, you didn't trick me!

My variac is totally functional and i intend to make use of it - according to my dad it was cannibalized off a huge old IBM computer from the sixties! all the same, it works well enough.

Also, i should note, i'm considering changing my spark gap design a little bit, i found two spare identical doorknobs lying around and with a little tinkering i could make one heck of a simple static gap.

Herr Zapp

Tue May 26 2009, 03:41PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Blitzorn -

How many individual gap "segments" do you intend to use? If you have found a pair of doorknobs, that will provide only a single gap segment, which will probably not provide the best quenching (or streamer length).

A very simple, cheap and well-proven design is the multiple-parallel-copper-pipe gap. Use 3" or 4" long segments of 1/2", 3/4", or 1" diameter copper pipe. Again, small pieces of scrap copper pipe can be obtained from any plumber, or Home Depot carries 2' lengths.

Design it with 5-6 gap segments, and then during coil tuning you can select the number of gaps that give best performance.

Mount these pipe segments so they are parallel, with about a .030 - .050" gap between them (use business cards or thin plastic, etc as a setting shim). There are many options for mounting the pipe segments, just make sure that your mounting scheme allows air to be blown through the gaps, perpendicular to the axis of the pipes. Use whatever type of small fan you can find to provide the airflow.

Search "static spark gap", "copper pipe spark gap", etc. to see some examples of different mounting methods.

Regards,
Herr Zapp

J. Aaron Holmes

Tue May 26 2009, 03:44PM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

Blitzorn:

As for where to get fuse holders, there are a few electronics stores around Seattle. If good ol' Radio Shack doesn't have them anymore (I haven't been into a Radio Shack in more than five years, so...) you might check out Fry's in Renton. They have an ok supply of the basics like that, and even a few oddities.

As for the gap: Depending on what the doorknobs are actually made of (not finished in), they may disintegrate pretty quickly. Solid brass drawer knobs (Home Depot or Lowes *might* have these, but a specialty hardware store would be best) would work much better. However, even at the powers you'll be running, an actively-quenched gap (air-blast, sucker, RSG, etc.) will reward you with better performance, I think. As I believe I mentioned in a PM, I've pieced together several complete air-blast gaps standing in the aisle of McLendon's Hardware (in Seattle, Woodinville, and a few other nearby places). They have an absolutely awesome selection of PVC and brass pipe fittings.

Cheers,
Aaron, N7OE

Blitzorn

Tue May 26 2009, 09:09PM

Registered Member #2128 Joined: Thu May 21 2009, 03:17AM
Location: Seattle, Washington, USA
Posts: 58

Did either of you spot my first spark gap design? it's basically exactly what you are describing.

There it is again. Anyhow, i'm poised to start work on either one, i was just holding off until i got a second opinion. As per your recommendations i'll stick with my original design. Unless of course, you think i could achieve sufficient quenching with a 1.5 horsepower shopvac with a narrowed air chanel directly across the gap...

I have a 4.5"id 5"od x 3-3/4" piece of black ABS tubing, it's a coupler segment so i am sanding the center rib out of the middle of it. I have an endpiece in which i have drilled holes to only permit air in where it's needed, and thus increase the draw across the gaps. The base of the housing will rest in a funnel that brings the gauge down to the size of my shopvac hose, with which i will move the air. I have 3/4" copper pipe (2' of it, enough to make extras) that i plan on cutting down to 2" segments. I have all the hardware i need to attach them and calibrate them, and really all i was waiting for was someone to clue me in on how to space them. I'll get on that tonight, it shouldn't bee too too difficult.

Thanks J.A.H. i'll pop over and see if radio shack has anything like that. If i can't find a fuseholder, are there any other fixtures that would make good adjustable tap points? I was thinking alligator clips, but between how wide they are potentially shorting the primary and how strong the grip is potentially crushing it, i shyed away from that.

Maybe i'll play around with the doorknobs later, just to see how they perform....

finally got my hands on a file, a hacksaw, and some epoxy today - that opens up some new tasks that i was waiting on for those. Busy week ahead, science fair in T-7 days.

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