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Registered Member #3859
Joined: Sun May 01 2011, 03:47PM
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Posts: 179
As I understand that this relationship is not linear. What is the voltage amplitude on the secondary of a Tesla Coil that I am goint to get for an X amount of current in the primary?
It sounds like a mathematical function.
How many turns should I use on a primary coil? If we set this theory of weight matching between primary and secondary aside, how many turns do I make? What value am I looking for? Is there any use in testing out the primary independantly of the secondary?
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
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It is linear, at least if we neglect the various non-linear things going on like varying spark conductivity with current, time, and contamination envirinment around the electrodes. If we consider the spark as a perfect switch, then it's linear.
Neglecting losses, the energy stored in the primary gets transferred to the secondary. Ideally, the spark quenches at that point, and the energy stays in the secondary. If it fails to quench, then the energy gets tranasferred back again. Then over again, then back again, for all the time that the spark is conducting. Hence rotary spark gaps or blown gaps are used to encourage the spark to quench as soon as the first energy transfer has taken place.
The energy is stored in each resonant circuit as a mix of current in the inductance, and voltage across the capacitance, oscillating between all in one, then all in the other, and back again. The sqrt(ratio) of the L and the C gives the impedance of the resonator, in other words the ratio of peak voltage to peak current. Obviously with its few turns and nF-sized MMC, the primary resonant circuit will have a much lower impedance than the secondary, with its 1000 turns and a few pFs to ground.
By equating the energy in the two resonant circuits, we can write that the transfer impedance, that is the secondary voltage to primary current ratio that you asked for, is sqrt(primary_L / secondary_C). Obviously it will be a little less than that due to the various losses.
How many primary turns? Most people use a couple of handfuls, give or take a factor of two. The main thing is to make the number of effective turns you use tunable. The reason is that primary and secondary should be tuned to roughly the same resonant frequency, and adjusting a tap point on the primary is usually far easier than adjusting the primary MMC, or anything to do with the secondary. A fuse-holder terminal and 1/4" copper pipe make a nice convenient combination for the primary and adjustable tap. Aim for 10 when you design your coil, and build 15 so you can tap down.
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Tesla Coil primary current -> secondary voltage.
