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"too much" topload?

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LAN_403

HM_Murdock

Wed Sept 15 2010, 07:35PM

Registered Member #3075 Joined: Fri Aug 06 2010, 02:44PM
Location: Athens, GA
Posts: 148

I am going to buy my parts for my SGTC toroid, and and considering doing the double stack...

If I have the CDE 942 MMC, a 3.5" secondary (24AWG) that has 19" high winding, and a 9kV/30mA NST, what is considered "best" for a topload?

I plan on making it out of 4" ducting, with cake pans for the center and 1" aluminum tape to cover...

Thought of doing a small one with a 4" center, then a larger one with a 6" center...that would give me:

1 @ 4" x 12"
1 @ 4" x 14"

Is this overkill for this coil? would I be better off doing one with the 6" (or even one with a 9" pan?)

teravolt

Wed Sept 15 2010, 09:44PM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

you will have to try it, more toride lower F Res. your coil will have to drive that capacitance once you get it running. maby you will want to add another transformer later. Don't forget to use chokes and safty gap on the outputs of your tranformers.

HM_Murdock

Wed Sept 15 2010, 11:06PM

Registered Member #3075 Joined: Fri Aug 06 2010, 02:44PM
Location: Athens, GA
Posts: 148

Thanks Nathan...

Not looking to break new ground on this one, more trying to hit the sweet spot of the components I have in place...

I am going to start with a single toroid based on dual 8.5" cake pans (cause that's what I came across at the dolloar store size-wise) and 4" ducting, and see what I get...

Herr Zapp

Thu Sept 16 2010, 01:29AM

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

HMM -

Are you "simulating" your proposed topload changes using JAVATC (or some other TC design application) to make sure your secondary resonant frequency doesn't drops below the tuning range of your primary system?

In my opinion, there's no benefit to a "stacked toroid" unless you have made your secondary so short that you are constantly getting primary strikes, and you just need to add distance between your topload and primary (or strike rail).

Also, (opinion again) I don't think a stacked toroid setup looks nearly as "clean" as a single, correctly-proportioned toroid.

Here are some of the tradeoffs: A larger topload may ultimately generate the longest streamer, but it may just be a single streamer, rather than multiple simultaneous streamers that a slightly smaller topload may generate. Also, a "big-for-the-power-level-of-the-coil" toroid may not start to break out until you are near 100% power input level on your variac, where a smaller toroid will start to generate streamers at only a fraction of input power, and allows you to smoothly and continuously "ramp-up" the display from small streamers to large.

Also, I've seen more coils with "oversize" toroids suffer secondary insulation breakdown (surface flashovers, etc.) during power ramp-up when the spark gap was firing, but there was no discharge from the topload because the minimum toroid breakout voltage had not yet been reached.

Herr Zapp

HM_Murdock

Thu Sept 16 2010, 02:46AM

Registered Member #3075 Joined: Fri Aug 06 2010, 02:44PM
Location: Athens, GA
Posts: 148

Zapp,

I am using a combination of WinTesla and Tesla CAD to simulate my design...

going with the 1 big toroid (4" x 16"), I get a frequency of 278.23 on my secondary. With my MMC I a building this weekend, that gives me a projected tap between 9 and 10 turns on my primary.

I only have 9 turns max on my current primary coil, but with a brass coupler I can extend it out to 11, maybe 12 turns. (had a 20' section of pipe when I wound it, and ran out at turn 9).

Here's what I have for my toroid...still needs some TLC with a wooden spoon to smooth it out some...how does it look proportionately?

Feels a bit big to me, but I haven't seen mine with a real toroid yet...definitely gonna need a strike rail I think...

Herr Zapp

Thu Sept 16 2010, 05:03AM

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

HMM -

Your toroid looks nicely proportional to your 3.5" secondary, but is possibly a little big for a 9/30 NST power supply. It just depends how efficient your coil is, and how well you can tune it. Start with a breakout point to aid initial tuning, then see if it will breakout without a point.

Keep your eye open for a second 9/30 NST; paralleling two will give a very nice boost in performance. Design your MMC so you can add additional parallel strings later in case you find a higher-current power supply.

Herr Zapp

HM_Murdock

Thu Sept 16 2010, 12:16PM

Registered Member #3075 Joined: Fri Aug 06 2010, 02:44PM
Location: Athens, GA
Posts: 148

Zapp...thank you for that information, it is exactly the sort of thing I was looking for (a starting point based on experience).

I have two options here...

1- build a smaller toroid

2- use one of my three 15kV/30mA NST's (or two, or all three)

I purposely built this TC to be smaller...in fact, the secondary is a good deal bigger than I originally had planned...I want this one to be a good candidate for indoor running.

I think I will build a smaller toroid to be able to swap them out for testing...that way, when I do build a much bigger TC I have the parts for a double stack if need be...and it was fun building this one

I have an old vacuum that died...I think the attachment hose and one cake plate would make a really nice (2.5" x 13"?) toroid size.

I have 12 of the CDE 942 caps (and resistors) in transit, so I will be building my MMC hopefully this weekend. Will be sure to leave room to expand it to the (19, I believe? Will look it up later) number of caps needed for a 15/30 NST...

HM_Murdock

Thu Sept 16 2010, 11:35PM

Registered Member #3075 Joined: Fri Aug 06 2010, 02:44PM
Location: Athens, GA
Posts: 148

New toroid built...this one was made from one cake pan and the hose off an old vacuum. New size is:

1 3/4" x 11"

HV Enthusiast

Fri Sept 17 2010, 11:58AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Nice toroid. That should work nicely for that set-up.
Just keep in mind your tuning point will change on the primary tap.

HM_Murdock

Fri Sept 17 2010, 01:01PM

Registered Member #3075 Joined: Fri Aug 06 2010, 02:44PM
Location: Athens, GA
Posts: 148

Thank you sir!

With the smaller toroid, I believe I may have enough turns to tune as-is...will find out. I have made several adjustments, and will be posting a Mark II thread later for validation of some of my parts and design assumptions...

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