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Hazmatt_(The Underdog)

Tue Mar 27 2007, 05:32PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

Two things I'm going to point out that you need to think about right away:
1. You only list 2 electrodes for your spark gap.
I have done it that way for a low power coil, but this is 900W we're talking about here!
2. Use the static gap cap .015 or .016 for best performance even with a rotary. My research, simulations, and real TC testing showed me right away that the large .028 drops your output performance because it loads the transformer too much.

If you want more details, see Calculating LTR thread.

Michael W.

Wed Mar 28 2007, 05:10AM

Registered Member #50 Joined: Thu Feb 09 2006, 04:07AM
Location: Vernon, B.C, Canada
Posts: 324

Ok, so I upgraded my spark gap to a 5 electrode gap...I'm now getting 18-20" arcs. Now for more optimization...

Hazmatt_(The Underdog)

Wed Mar 28 2007, 07:20AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

One way to tell that you're way out of tune that I noticed is by listening to the gap. Now this is very very loud! So be careful! But if the gap fires and its really loud, and your spark length is really short, start moving your primary tap around a lot. I noticed that as you get closer to that 'sweet spot' the gap and the spark are about the same loudness relativily speaking. So if you have almost no noise coming from the arcs, and lots of bang from the gap, move your tap out more and more until the gap quiets down and the arcs start getting longer..and louder.

I know that sounds terribly crazy, its just what I've noticed from SGTC's. Your experience may vary.

Steve Conner

Wed Mar 28 2007, 09:56AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

It sounds crazy, but it makes sense. In a SGTC, most of the energy that doesn't make it to the output spark gets burnt in the primary spark gap. So as you get closer to being in tune, the streamers get bigger while the spark gap dims and quietens down.

The various kinds of pulsed solid-state coils (DRSSTC, OLTC, etc) recycle the unused energy for use in the next bang, instead of wasting it. They have to do it that way because the transistorized "spark gap" can't stand the losses that a real spark gap can. So when tuning these, you can see the current draw from the line reach a maximum when it's roughly in tune.

Michael W.

Wed Mar 28 2007, 02:35PM

Registered Member #50 Joined: Thu Feb 09 2006, 04:07AM
Location: Vernon, B.C, Canada
Posts: 324

Well I'm getting 18ish" arcs and my sparkgap sounds like a cap gun going off every half a second. I wear hearing protection or my ears ring after but I think I'll try Hazmatt's tuning method next....

Marko

Wed Mar 28 2007, 02:43PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Have you tried that bigger secondary you have there?

Regarding the spark gap, it's best to completely remove the primary and mmc and set it so that your nst *just* barely arcs over it. Anything more is dangerous to the transformer if cap is close to resonant value.

Terry Fritz

Wed Mar 28 2007, 07:27PM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Hi,

You may be having a problem with primary "Q". If the primary capacitance is high and the primary inductance is low, the primary circuit will not "ring" like a bell but rather "thud" like a rock.

Q = SQRT(L/C) / 3

Q should be at least 20.

In your case Q = SQRT ( 0.429e-6 / 0.01611e-6) / 3 = 1.7

That is a least an order of magnitude too low!!
Bart should put a "Low Q Warning" in his program since this is easy to miss... I see he does have secondary Q which is almost impossible to mess up...

If so, the primary inductor basically looks like a dead short to the primary capacitor and the current is very high and fast, thus the extremely loud bang. Getting 18 inches still is actually pretty good!!!

So you need more primary inductance since the cap size is sort of fixed by your transformer size. (Ignore the 75,000 Volt recommended cap rating

(Based on one old time silly "rule"...)

So how to fix it...

A Q of say 30 is pretty good for "me":

30 = SQRT( L / 16e-9) / 3 Where L = 130uH !!

Fo would then be:

Fo = 1 / (2 x pi x SQRT (L x C) == 1 / (2 x pi x SQRT( 130e-6 x 16e-9) = 110kHz.

So you need a lot of turns on the primary basically.

You then need a lot of turns on the secondary and a big top load too to match the 110kHz of the primary.

You need a big top load like 25pF would be nice. Then you get thin wire and wind the secondary to get lots of turns (probably 1000 to 1500).

You can see my big coil here but it uses a rotary gap (different primary cap value (27nF):

There is also a formula sheet here with lots of calculation info where the programs get most of their equations:

For what you have now, do everything possible to make the top load "BIG". Add more top loads... The secondary needs to have a lower resonant frequency so you can use more primary turns to increase the primary Q... Using a smaller NST transformer that would have a much lower matched primary capacitor may actually do better than what you have now too. You need more L and less C.

Hope this helps and is not too confusing

Cheers,

Terry

Part Scavenger

Wed Mar 28 2007, 07:50PM

Registered Member #79 Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673

Matt's right about that tuning, you can tell by the way it sounds. You don't necessarily have to have your earplugs out to tell though. Apart from the streamer length, loudness he was talking about... the even-ness of the gap firing should tell you alot as well. Unfortunately, you kind of have to acquire "the ear" which isn't really hard to do, but like I was in your situation, I didn't know what to listen for until I had a coil that worked well.

Marko

Wed Mar 28 2007, 08:00PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Just to clarify myself after terry: Definitely, do use that bigger secondary you have, and put on as much topload as you can before the coil starts looking silly.
Try to get your frequency somewhere to 100..200kHz, lower = better!

You need as much primary inductance as possible because you are on very low end as me and terry have seemingly figured out!

Tuning isn't actually all that hard thing to do, you just get a figure from a calculator adn then
fiddle the tap by trial and error around it.
Increasing primary Q should be your priority now.

Terry Fritz

Wed Mar 28 2007, 08:11PM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Oh! I see Firkragg "already" mentioned the Q thing

I missed his earlier post, but the one turn primary caught my eye

Looks like this is getting figured out

See, we didn't need any test equipment after all

Cheers,

Terry

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