Tuning and spark length
IamSmooth, Mon Mar 05 2007, 12:52AMI 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.
Re: Tuning and spark length
Hazmatt_(The Underdog), Mon Mar 05 2007, 01:07AM
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.
Hazmatt_(The Underdog), Mon Mar 05 2007, 01:07AM
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.
Re: Tuning and spark length
IamSmooth, Mon Mar 05 2007, 02:49AM
Thanks for the reply. I had used this site (I believe)
The formula for LTR sych cap = 0.83*I/BPS*V
In my case: 0.83*(0.12)/(120*15kv)
and came up with a value of 0.055uf. I didn't think the 0.005uf would make a difference. How did you get 0.065uf?
IamSmooth, Mon Mar 05 2007, 02:49AM
Thanks for the reply. I had used this site (I believe)
The formula for LTR sych cap = 0.83*I/BPS*V
In my case: 0.83*(0.12)/(120*15kv)
and came up with a value of 0.055uf. I didn't think the 0.005uf would make a difference. How did you get 0.065uf?
Re: Tuning and spark length
..., Mon Mar 05 2007, 03:31AM
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
..., Mon Mar 05 2007, 03:31AM
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
Re: Tuning and spark length
IamSmooth, Mon Mar 05 2007, 04:02AM
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.
IamSmooth, Mon Mar 05 2007, 04:02AM
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 ofVp = 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.
Re: Tuning and spark length
sparky, Mon Mar 05 2007, 05:55AM
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.
sparky, Mon Mar 05 2007, 05:55AM
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.
Re: Tuning and spark length
Hazmatt_(The Underdog), Mon Mar 05 2007, 07:49AM
I already mentioned the thread where I've gone over this exhaustivily in my research.
its all pretty much there.
Hazmatt_(The Underdog), Mon Mar 05 2007, 07:49AM
I already mentioned the thread where I've gone over this exhaustivily in my research.
its all pretty much there.
Print this page