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Almost finished need some last minute advice

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Jonathan Dennis

Sun Aug 02 2009, 03:46AM

Registered Member #2153 Joined: Wed Jun 03 2009, 03:19PM
Location:
Posts: 69

Ok, everything is in the bag now. I'm finishing up with wiring. I got the resistors recommended by Herr Zapp, and my line filter and variac are taken care of. As for a terry filter, I decided it is too much extra money just for a precaution to protect my NST. It was free anyway, so if it does blow, I can get another very cheap. If it were more expensive, however, I would definately get a terry filter.

I have one problem! I accidentally goofed on my primary. I started winding too far from the center, and ran out of tubing about 6 inches out. I only need another 7-8' to finish, which I bought today. I was thinking I could stretch the end just enough to allow it to fit snugly on the existing tubing. This would make a seamless transition b/tween the two. If there is a tiny bump where they connect, will it affect the coupling or anything else, I didn't think so but I am not positive?

I'm interested in your answers, so please... Help me out forum!
I'll post a picture of my almost completed tesla soon, I promise I'm so excited I almost have it running!

Arcstarter

Sun Aug 02 2009, 03:56AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Well, of course you need a safety gap on your NST, and if you have some 1.8kv MOVs they would not be a bad idea.

About the primary, just get one of those sleeves from a hardware store that are almost a perfect fit for your pipe. Then, clean it, perhaps rough it up with some course sandpaper (this is probably not necessary, but i do it and it helps me a lot) and flux it. Then get a propane torch and heat it, and put some solder across the seem. That would be ideal. Good electrical connection, and it would look pretty good.

And no, a small bump, as long as it is not sharp, should not effect anything. A sharp bump could make corona leakage from secondary to primary occur more easily.

Jonathan Dennis

Sun Aug 02 2009, 04:01AM

Registered Member #2153 Joined: Wed Jun 03 2009, 03:19PM
Location:
Posts: 69

Alright thanks. The reason I didn't want to make a terry filter is mainly the cost. Plus I don't have any MOV's around, wish I did. But this being my first electronics project, I don't have any parts laying around yet. :) As for the primary, I thought about simply crushing the end enough to fit inside the other pipe, it would make a solid connection, and wouldn't really be to noticeable. I agree with you, your idea is preferable, however my goal is to not spend any more money. I am getting ready to pay for my first semester in college and I am trying to Save SAve SAVE!!! haha

Jonathan Dennis

Sun Aug 02 2009, 04:58AM

Registered Member #2153 Joined: Wed Jun 03 2009, 03:19PM
Location:
Posts: 69

I cannot upload my pictures for some reason, it keeps saying error, but they are in standard JPG format and only 138kB? That's weird

Herr Zapp

Sun Aug 02 2009, 06:02AM

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

Jonathon -

Before you get too wrapped up in adding another 6 or so inches of tubing to your primary, let's make sure that you have included enough "extra" primary turns to allow tuning.

First, how did you calculate the required length os primary tubing, and what was the resulting total number of turns and overall length?

After finding the required number of turns to "theoretically" achieve resonance with your secondary and topload, you need to add at least another 1.5 to 2 complete turns to give some additional adjustment range to compensate for misc. variables in your construction.

Unless you have test equipment (RF signal generator, oscilloscpe, frequency counte, or Terry Fritz "Tesla Coil Tuner) you will not know the exact resonant frequency of your secondary circuit. Therefore, you will not know the exact frequency (tapping point) required by your primary. To make sure that your primary circuit can be brought into resonance might require + or - one or more turns from your calculated tap point. It's always easy to tap further "inward" on the primary, but unless you've got extra tuns beyond the calculated tapping point, you won't be able to tap further out.

Bottom line, add two complete primary turns MORE than what was specified by your TC design calculations.

Regards,
Herr Zapp

Jonathan Dennis

Mon Aug 03 2009, 06:15PM

Registered Member #2153 Joined: Wed Jun 03 2009, 03:19PM
Location:
Posts: 69

Thanks everyone. Regarding my primary: I calculated from the beginning to have 2 extra turns. I changed my design to include 24" secondary rather than 20" and I changed the primary because I goofed. I started winding the primary at the end(furthest from center) or my plexiglass supports. I didn't realize that I had calculated to get 15 turns (tapped at 12-13) spaced at .25", out of 50' of tubing. I needed to start @7" from the center. I started 14" from center, and spacing ended up being about .7" because the tubing was to hard to work with and nearly impossible to space that close together in a spiral. I have 10 more feet of tubing now. I simply stretched the end of one, and used a wire crimper to slightly crush the other. They fit snugly together now. No problem. With my new primary spacing and new secondary height, tesla map says that I need to tap at turn 8-9. This is no problem because I will have @11 turns! Also, here is a picture! please feel free to comment on my design.

Herr Zapp

Mon Aug 03 2009, 07:02PM

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

Jonathon -

Based on your comments, you should have adequate range of tuning adjustment, assuming that you built the coil "to spec".

A few questions/comments:
1. Why the large (and inconsistant) turn-turn spacing on the primary? It looks like you are using 1/4" diameter copper tubing, and that can be wound with no more than 3/8" to 1/2" gap between turns. Your primary is so large in diameter that you may not be able to get optimum coupling to the secondary.

2. It looks like your spark gap and MMC are lying on the "floor" of your coil base. You will have at least 24" - 30" of additional wiring connecting the MMC to the primary and spark gap. Raise the MMC and spark gap closer to the primary to minimize the length of interconnecting wiring, but place the blower as low as possible to keep it out of the primary's magnetic field.

3. Your toroid is quite a bit higher than the end of the secondary winding.You may find that the last turn at the top of the secondary generates a lot of corona. If so, you may need to lower the toroid slightly, or increase the toroid's diameter to provide better electrostatic shielding of the top of the secondary. Start testing with everything as-is; if you find your optimum-performance tapping point leaves some of the primary unused, then you can increase the diameter of the toroid a bit.

Regards,
Herr Zapp

Jonathan Dennis

Mon Aug 03 2009, 07:33PM

Registered Member #2153 Joined: Wed Jun 03 2009, 03:19PM
Location:
Posts: 69

alright thanks for the good review Herr Zapp. As for the primar. Only the outer 4 rings were in place when that picture was taken. The inside coils have not been bent and glued down yet. I was considering attaching my MMC to the wooden support column in the middle so that it could be closer to the primary. As for the spark gap, I attached my blower to it for quenching. can it not just sit on the floor of the lower platform? If I have trouble with the space btween toroid and top of the secondary, I can try to stretch the toroids diameter a bit, but the ducting I am using will not give much.
Another question:

To ground my secondary, can I use 3-4" of the 28 gauge wire to connect to a larger wire (4, 20 guage strands of copper wire WELL insulated). This larger wire will go to a metal stake located on the bottom platform of my design, and from this metal stake, will go through a much thicker wire to my grounding rod. Will this work, or will it just melt the 3-4" of 28 gauge? Would I better off trying to find some copper tape?

Herr Zapp

Mon Aug 03 2009, 08:59PM

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

Jonathan -

The objective is just to keep all the connecting wiring between primary, MMC, and spark gap as short as possible, while also keeping any large metal objects (like the blower assembly) out of close proximity of the primary. It should e easy to mount the spark gap and MMC up closer to the primary, and re-orient the blower so it is 10" or so away from the primary.

Your ground wire doesn't need to be WELL insulated, it can be bare wire as it's essentially at ground potential. It's recommended that you terminate the actual magnet wire from bottom of the secondary to a fixed stud or terminal block as a strain relief so someone tripping over the ground wire doesn't break off the magnet wire at the secondary.

The RMS current at the base of your coil will not be able to melt #28 wire (check a wire gage chart to see the actual current that a #28 AWG wire can carry).

Also, there's no need to use a "much thicker" ground wire anywhere in your ground path. If it's only a few feet to your earth ground rod, just use a single piece of #12 or #14 AWG house wiring, solid or stranded. No need to use 4 X #20 AWG for 4", then transition to something else.

Regards,
Herr Zapp

Jonathan Dennis

Mon Aug 03 2009, 11:03PM

Registered Member #2153 Joined: Wed Jun 03 2009, 03:19PM
Location:
Posts: 69

Alright I will do that. As for the blower I actually mounted it on the wooden box that the spark gap is in. Is that too close? Should I connect the blower to a plastic hose and run it up to the spark gap? I re-measured my primary spacing and it's not very consistent. It ranges between .9" max and .4". However the spacing looks relatively even when eyed carefully. Will this incosistent spacing greatly reduce my performance? You mentioned that the max spacing for copper tubing was 3/8" to 1/2". However the lage tesla coil where I volunteer has a primary spaced at around an inch. It works great. So I guess i'm saying, elaborate please. I will post some more pics for you to look at. Since I haven't finished gluing my primary, I could make the spacing of the last 4 or 5 turns closer together. Would that help or no?

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