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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I want to make sure that I'm not missing something:
First, the specifications: NST: 15kv/120ma synch gap 120bps with a phase-shift controller to adjust gap timing Cap: 0.05uf/50kv Primary: approx 9 turns refrigerator tubing, inverted cone @ 5-10 deg secondary: 6" acrylic, 29.5" 22g Topload: duct tubing 6 x 20 vs 4 x 12 Tuning: done with an oscilloscope and frequency generator to achieve double-peak with largest center drop.
What I am getting is about 4' sparks on a windy night. I see sparks start flying off of the toroid when my input voltage hits 70v. If there was no wind I guess they would be bigger. I am also getting multiple streamers. I have read people saying that they are getting close to this spark length with 12kv/60ma, so I feel that I am underperforming. I have moved the primary tap a little to the right and left with no change so I think my oscilloscope tuning was done correctly.
Now, just to be clear, my toroid is not perfectly smooth. I am sure that when I get a finished aluminum spun toroid my breakout voltage will be higher, yielding longer sparks. I am also not sure if I have tuned my sych gap properly so I have some stuff to work on for this point.
So, my question is: will the spun toroid make a big difference? Should my sparks be bigger for the parameters I have given? If so, what should I do? I also noticed that as I adjust the phase for my spark gap the input current changes. Will my input current be the lowest when I am firing at the peak of the input voltage?
Thanks in advance to everyone who wishes to contribute.
Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
Well, I would point out that your cap value needs to be changed. You can find a lot more info on "Calculating LTR" thread. My calculated for my 9kv/120 is .052uF, so your value should be quite different, perhaps .065 or something. So look into your cap value.
I'm also getting 4' with the 12/60, and you could guestimate about 200W/foot for SGTC. Just go through your numbers and triple check the calculations and measurements and you'll get where you want to be.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
For rerfrence, max spark length (for a given type of coil and bps) is proportional to the square root of the power. For small/mid sized SGTC's you multiply the square root of the input power by 1.7 to get the max inches. So you should be able to get 6 feet
Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I figure that I should...the question I am asking is what am I doing wrong? Is my cap too small?
I am thinking that Hazmat is right about Cp not being right although I am not sure about the value. This site suggests for my parameters of Vp = 120v BPS = 120 Vs = 15kv Is = 0.12A I should have a Cres (for a synch gap) of 0.026uf
What is the right value to use? I guess I am just going to try different Cap values, retune an d see what yields the biggest value without blowing my NST.
Registered Member #530
Joined: Sat Feb 17 2007, 07:56AM
Location: Victoria BC, Canada
Posts: 178
well hold on... on spark gap operation your NST is going to be pulling more current than what the rating of the transformer states. So having a larger capacitor may not be such a bad idea. My 12kV 120mA NST is pulling 160mA during spark gap operation- and the capacitor is fully charging a 32nF cap. So maybe add a larger top load and add a few more turns on the primary and tap the outer turns and see what happens.
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Tuning and spark length
