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HM_Murdock

Sat Sept 18 2010, 06:40PM

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

I got my MMC made (thanks to super fast shipping from EVR!) with 11 CDE 942's, and man, what a difference a proper cap bank makes!

I kept everything the same, switched out just the caps, re-tapped the primary, and got 3x the output. Spark and safety gap firing fine.

Then I built my 5 section (1.5" copper pipe joints), and got even more output...BUT, now my safety gap is firing constantly. I widened it up a good bit, and shortened my spark gap to 4 (and 3) sections, but the safety gap is firing off as much as the spark gap...I have tried re-tapping, increasing and decreasing coupling of the primary and secondary by up to 2 inches either way (which didn't seem to affect output very much)...

Any suggestions?

HM_Murdock

Sat Sept 18 2010, 09:33PM

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

Here's a pic showing what's happening...

Safety gaps are set to 7mm each side, HV ground in the middle

Also, some specs...

I set up the safety gap with it hooked up to the NST alone...the safety gap stops arcing at about 3mm each side. They were set at 4mm per side (which worked fine with the 2 piece spark gap set to 5mm.) The current multi piece gap is about 1mm per section

Power is from 9kV/30mA NST

I am concerned that something is not right, and I hesitant to widen the safety gap any more for fear of frying the NST...

modularduck

Sun Sept 19 2010, 02:17AM

Registered Member #3094 Joined: Tue Aug 10 2010, 03:12AM
Location: portland, or
Posts: 30

our coils are very similar, i would check the total SG spacing and make sure it is in line with javatc, if you have to much gap you can over charge the caps and that would certainly cause the described. I am certainly not expert on these things, lol. but its a place to start.

ScotchTapeLord

Sun Sept 19 2010, 03:22AM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635

"Although the total gap distance is comparable to one big gap, there are more arc to electrode interfaces. Since there is a fixed voltage drop at each interface the total voltage drop across the whole gap will be higher resulting in a higher power dissipation."

So that means that multiple segment gaps drop more voltage than single gaps with the same total spacing... so maybe make them closer? I guess you just have to hit the sweet spot.

From your picture it looks like your gaps are in parallel. It would be very odd for such drastically different gaps in parallel to both be firing. What schematic are you using for your coil?

If worst comes to worst you could pop in a resistive ballast. Or, with a little math you might be able to figure out what value inductor you could use to ballast the system so as to make your capacitor 2x resonant, which would help the keep the voltage rise down, while blocking transients... but I don't think I've ever seen that done before on an AC coil, so there may be something fundamentally wrong with that idea!

HM_Murdock

Sun Sept 19 2010, 01:30PM

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

Schematic I am using is this one...I could not find one to fit what I was building, so I modified an existing one and added the safety gap where I thought it was suppos3ed to go

Please feel free to correct me if this is wrong:

ScotchTapeLord

Sun Sept 19 2010, 02:34PM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635

Yes, that's the best configuration. Very strange that this would happen, even when shortening the gap to three segments. You connections appear good and short, too. How long are the connections from the NST to the safety gap?

HM_Murdock

Sun Sept 19 2010, 02:50PM

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

Wires from the NST to safety gap are GTO wire, about 6" on one side and 10" on the other...

**EDIT**

ok, I think the mystery is solved...

was advised to try remove the middle (RF) terminal from the safety gap...

I did so, reset the safety gap, and now all 5 sections of the spark gap fire and the safety gap does not...and I have pretty serious arcing between the primary and secondary.

Considering the middle terminal was connected physically to the base of the secondary via the RF ground, it makes sense it would cause arcing in the safety gap.

Now I have arcs jumping 2" between the secondary and primary (secondary is encased in Envirotex Lite). I think I am going to have to remove my first turn or two on the innner side of my primary to get the decoupling I need.

HM_Murdock

Mon Sept 20 2010, 01:42AM

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

Nope...my RF ground was weak...

Doubled the size of my counterpoise, and solved two issues...my racing arcs went away, and my streamers got bigger.

I know this isn't much compared to the big boys, but I am getting 14" streamers to the air with a breakout point (screwdriver is 10" long)

Closing in on this thing...pic is from screenshot of video below...video also has two mountain dew cans stacked in the dark. Looks cool , but too much corona from the top of the secondary.

]tuned_day1.wmv[/file]

ScotchTapeLord

Mon Sept 20 2010, 02:30AM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635

Now it all makes sense. It was the middle terminal that was at the high potential. I should have deduced that! Let's hope the safety gap didn't do damage to the NST... that would be ironic. You're supposed to attach the ground terminal of the safety gap to the NST's ground, which is usually attached to the case. As for the secondary, that should have its own dedicated RF ground, separate from everything else, but with this power level you should be fine...

14" is pretty darn good for 13.6 nF of tank capacitance! Congratulations!

I don't know how big of a deal the corona is on the top... I know it happens with a lot of people's coils, and it happens with mine, too. If you could lower the toroid or get a wider diameter for the "donut" part so that it comes closer to the top turns, that might help. Or maybe just a bigger toroid altogether.... (since I think you have another one if I recall correctly from another post)

HM_Murdock

Mon Sept 20 2010, 02:40AM

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

Thanks...pretty happy with it so far!

I wasn't clear in my previous post...the corona around the top of the secondary only happens with the mountain dew cans (as shown in the video). With my toroid on (small one), I get no breakouts from the secondary.

And yes, I have a much bigger (3x?) toroid, but I haven't tuned for it yet. I did try it, but I think it is going to be too big. Also, this so far is with my wires clipped on with alligator clips (spark gap and secondary taps). Hopefully will pick up a tad more output when I connect them proper like...

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