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Spark Length with Salt Water Caps vs. Factory Made Caps?

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Chris Maness

Tue Jul 07 2015, 02:35AM

Registered Member #55389 Joined: Mon Jul 06 2015, 12:38AM
Location: Grand Terrace, CA
Posts: 29

I just finished my TC. I have a 240W OBIT, according to the JavaTC simulator I should be getting 24" sparcs, but instead it doesn't even break out at the strongest tuning point, and I get about 4" sparks using the ground wire. I am using body builder whey bottles for my salt water caps. The SG arc is about the loudest I have heard, so plenty of energy there. This is my first TC, but I have helped a couple of students tune their TC's and get them working. I had a student with a smallish NST get his to break out with salt water caps. Is the efficiency loss with salt water caps so bad that it would shrink them all the way down to wimpy 4" sparks that don't even break out with a 240VA xformer?

update: it is breaking out, but I had to wait until it was very dark to see it -- little 1.5" streamers.

Thanks,
Chris

Part Scavenger

Wed Jul 08 2015, 07:58PM

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

I see a lot of things going on here, maybe I can help. First off, the JavaTC simulator assumes you are using manufactured capacitors in an MMC. You won't get that out of saltwater caps. I'd be pretty happy with 10-12" sparks with saltwater caps.

POWER SUPPLY:
1. Knowing the output voltage of the OBIT would be helpful.
2. OBIT transformers usually have low voltage output. Anything under 8kV I wouldn't even bother with. Invest in a MOT stack or a 12kV or higher NST. I won't go into all the reasons, just trust me it will make your life easier in so many ways.

CAPACITORS:
1. Your caps will be pretty inefficient built that way. First, they're too big physically. Imagine the energy having to propagate through all that saltwater and surface area, and think of ways to improve that. Also, that black plastic needs to get away from everything tesla. It's slightly conductive at high voltages and it traps a lot of RF energy.

The way you're using capacitors, it's much better to have many smaller ones in parallel, than two large ones. Remember putting capacitors in parallel maximizes their value, but inductance and resistances in parallel are minimized! This is exactly what we want.

If I were designing the capacitor, I'd first determine how much capacitor I needed. Test the resistance of the secondary of the transformer, and then C = .0833 / 4R should get you pretty close. JavaTC should have a capacitor matching tool also.

Then I would make the salt water caps out of beer bottles, 20-30Oz and put them in parallel until I got the capacitor bank size I need. Brown glass is better than clear, green, and blue in my experience - I have tested brown 20Oz and 40Oz budweizer glass to 40kV+. If you don't have a way to test the capacitance (an LCR meter or Oscilloscope), Radioshack has their DMM's on sale for $35 right now, and the Harbor Freight http://www.harborfreight.com/ac-dc-digital-multimeter-37772.html works surprisingly well. If you're just wanting to guess, I would start with 6 20oz beer bottles and add subtract from there.

Watch for electrode corrosion inside the beer bottles. Once they get dirty you lose performance. Make the electrodes as wide as they will fit in the mouth of the bottle, and 1.5" from the bottom of the glass.

Make sure your salt water is saturated by mixing until it starts to fall out, then discarding the fall out. Then put 1/2" oil over the top of the salt water. This will keep the water from evaporating.

When putting aluminum foil on the outside, use spray contact cement and a wallpaper roller.

Cover the top edge of the foil with electrical tape to minimize corona loss.

Bottom line, think about ways to decrease resistance and inductance.

WIRING:
Shorten EVERYTHING!!!! Nice and neat is imperative when it comes to performance. Alligator clips are the devil.
Run the coil, shut it off, and check for hot spots. These are areas you need to improve.
Use multi stranded wire for everything in the tank circuit, preferably 12AWG or greater.

SPARK GAP:
Try cooling your spark gap. Start with a muffin fan or something. If the air in the spark gap is too hot, the gap will stay on too long, and transfer energy back into the primary circuit instead of trapping it in the secondary circuit. This is the primary reason I won't use a transformer under 8kV.

TOPLOAD:
Try using a smaller topload, or rather one with equal volume but smaller cross-section. This is why the toroid is so popular. Alternatively use a breakout point, but that's no fun.. :)

Also, it could be argued you could increase your coupling by taking out the first coil of the primary, and dropping the secondary down into the primary, but I would definitely do the other things first.

Give that a try. Start with the wiring and capacitors and I'll bet you have much better success in no time.

Chris Maness

Fri Jul 10 2015, 06:51PM

Registered Member #55389 Joined: Mon Jul 06 2015, 12:38AM
Location: Grand Terrace, CA
Posts: 29

Thanks, Part scavenger. I have some 40kV caps coming from China that total the 6nF that the JavaTC spec calls for. It would be nice to play with some better caps in the mean time, so I will Freecycle for some beer bottles. It did cross my mind that the large volume bottles would cause an issue. I will work on it, as I am not getting anywhere near 10" sparks. Also, my OBIT is as beefy as they come. 10kV at 23mA, yes not huge, but it is not that big of a coil, but that is big for an OBIT.

Also, my tank circuit is with heavy #10 and no gator clips accept for the tap on the primary. The two gator clips coming off the transformer are there because that is not where the high current is present. If you still think this is an issue for another reason unknown to me, let me know. I am now running pretty low on cash from the project.

For the torroid, do you think I should use 3" ducting, or purchase one of these when I get the funds:

Thanks,
Chris Maness

Part Scavenger

Sun Jul 12 2015, 12:43PM

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

That OBIT should work fine. I would definitely get rid of the clips coming off the transformer and shorten all wires. No professional torpid necessary, I would use a ducting diameter about the size of the toroid. If possible. I still think your biggest problems are the caps and a hot spark gap.

Chris Maness

Sun Jul 12 2015, 02:07PM

Registered Member #55389 Joined: Mon Jul 06 2015, 12:38AM
Location: Grand Terrace, CA
Posts: 29

Yep, I agree. According to the tracking on the package, my caps are in LA right now, so any day.

Thanks,
Chris

Chris Maness

Mon Jul 13 2015, 06:28AM

Registered Member #55389 Joined: Mon Jul 06 2015, 12:38AM
Location: Grand Terrace, CA
Posts: 29

Ok, I out of shear curiosity. I tried the beer bottle caps even though I have some commercial caps on the way. It improved the coil a tiny bit. I am getting about a 15% improvement on spark length. Still pretty feeble break out without bringing the ground wire near the top load. I am getting a solid 4" from it to the gnd. Should be better tho.

Chris

Part Scavenger

Mon Jul 13 2015, 09:55PM

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

Pics?

Chris Maness

Tue Jul 14 2015, 12:51AM

Registered Member #55389 Joined: Mon Jul 06 2015, 12:38AM
Location: Grand Terrace, CA
Posts: 29

Point is moot now. My commercial caps are now in place. I got about a 20% boost from the commercial caps. I will post pics later.

Chris

Chris Maness

Tue Jul 14 2015, 04:58AM

Registered Member #55389 Joined: Mon Jul 06 2015, 12:38AM
Location: Grand Terrace, CA
Posts: 29

It really lit up when I spaced the gap just a hair wider. My new cap bank died. I need to use different caps. 40kV my butt. I put the beer bottles back on for now, not too bad. Have a look:

What is a really reliable and efficient MMC capacitor to use?

Chris

GrantX

Tue Jul 14 2015, 05:18AM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

Chris Maness wrote ...

It really lit up when I spaced the gap just a hair wider. My new cap bank died. I need to use different caps. 40kV my butt. I put the beer bottles back on for now, not too bad. Have a look:

What is a really reliable and efficient MMC capacitor to use?

Chris

What commercial capacitors did you use (pics or part number and manufacturer) and how was the bank connected (how many in series or parallel)?

Everybody's favourite capacitors for spark gap coils are the Cornel Dubilier 942C. Unfortunately, they are rather expensive and you'll need at least 20 of them!

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