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Registered Member #50
Joined: Thu Feb 09 2006, 04:07AM
Location: Vernon, B.C, Canada
Posts: 324
So, Dilemma. I have a 4.5" X 22.5" Coil and a 3" X 18" coil all varnished up and ready to use. I'm planning on using one of these with a 900W NST and an MMC, the question is if I choose the larger coil, will that give me larger arcs or am I better to stick with the smaller coil? and on a side note I already made the primary to suit the 3" coil but theres a lot of extra turns, will the tuning position change a lot outwards if I use the 4" coil or should I be good? Its alot of questions but I'm itching to get building....
Registered Member #79
Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673
You'll probably get the same length with both secondaries depending on the constuction, # of turns, etc. The sparks will look bigger on the smaller one. The last question, I'm not sure about with the information you gave us, plug your numbers into TeslaCAD or something. That should tell you.
Registered Member #50
Joined: Thu Feb 09 2006, 04:07AM
Location: Vernon, B.C, Canada
Posts: 324
Hi Marko! Well the resonant frequency of the Larger coil is 254.63Khz and the other one is 495.16Khz. I did want to use a larger coil so it would balance out the larger base I made for the whole thing but if they're both going to produce the same arc length then I might as well use the smaller coil...
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Most importantly, larger secondary with lower frequency will allow to use much bigger primary for same bang energy, reducing peak current and losses.
Frequency of the smaller one here looks a bit high, but I guess topload can fix it. I think you'l be able to push him well enough to start flashing over anyway..
Registered Member #79
Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673
Firkragg wrote ...
Most importantly, larger secondary with lower frequency will allow to use much bigger primary for same bang energy, reducing peak current and losses.
Frequency of the smaller one here looks a bit high, but I guess topload can fix it. I think you'l be able to push him well enough to start flashing over anyway..
What's wrong with high peak currents? That's what you want! The goal is to get the current as high as possible as quickly as possible without flashover or exploding something, and have the spark gap quench so that all that power is quickly delivered to the coil and then trapped. Lower inductance (therefore higher capacitance) should get you bigger sparks. BUT, like Marko said you will waste more in the primary with higher current, and more inductance will have better coupling. Trick is to find the middle, I generally don't worry about it that much, I usually just give my power supply the capacitance it can handle, then build the primary accordingly. About the topload, I think I'd put one on that drops the frequency about 1/2.
About the secondary, *technically* you should get longer sparks with it. Because it has a lower resonant frequency, it will probably be more efficient. I honestly don't know how much, but output will be comparable. Personally, I would go with the smaller coil, because even though you may gain an inch with the bigger one, it won't look bigger.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
What's wrong with high peak currents? That's what you want! The goal is to get the current as high as possible as quickly as possible without flashover or exploding something, and have the spark gap quench so that all that power is quickly delivered to the coil and then trapped. Lower inductance (therefore higher capacitance) should get you bigger sparks. BUT, like Marko said you will waste more in the primary with higher current, and more inductance will have better coupling. Trick is to find the middle, I generally don't worry about it that much, I usually just give my power supply the capacitance it can handle, then build the primary accordingly. About the topload, I think I'd put one on that drops the frequency about 1/2.
Well, Q factor of the primary alone, not counting the secondary, is 1/R sqrt(L/C), y?
Your surge impedance is sqrt(L/C), so your peak willl be proportional to ratio of your inductance and capacitance!
You could easily get humongous peak currents in your primary by charging a huge capacitor at low voltage and discharging into a single turn primary, wich intuitively doesn't look like a well designed tesla coil! Massive primary current will not do anything except creating huge losses on spark gap and require us to increase coupling to transfer most energy to secondary before it's lost. High coupling often leads to flashovers, and, more importantly, current will heterodyne much faster, making it very difficult to quench properly with a gap that is already torched with massive current.
So, you just want as big as possible surge impedance. Secondary itself seems to have little importance about losses.
It was all well explained on richie burnett's site
And some questions are answered here
Before I baffle everything with hardcore TC theory, just design your cap after your power supply and your primary after your secondary... don't be afraid to use larger topload and more primary turns.
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Coil decision
