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Are variable frequency coils possible? If so, how do they opperate? Can they be tuned on the fly? Are they of spark gap, vacuum tube, solid state, etc design or would it be built into the coil itself?
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Do you mean resonant frequency, or BPS (on spark gap coils) ? Different resonant frequencies do not change the sound, and the sparks dont start changing until you change it by a few tens of kilohertz.
Resonant frequency can be changed by changing number of windings, length, gauge and width. Also, changing a topload is a very easy way of changing the resonant frequency. Remember, after these changes, retuning is necessary.
Changing BPS on a sgtc will change the spark appearance, length, and sound. With an AC tesla coil, i would not use anything but static spark gaps or syncronous (if it fires in the middle of the cycle peaks, it will lower efficiency and spark length, and has a greater possibility for spike that can kill transformers, which should not be a problem with a tank capacitor :P). If it is a DC coil though, you can use an asynchronous rotary spark gap, and change the speed (BPS) while it is running by changing voltage or PWM freq.
Yes, I wanted to be able to tune the resonant frequency of the coil itself. Is there any way of using a wiper arm type setup on the secondary to choose where on the coil it is being tuned from?
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
There are a number of ways you can tune a conventional coil as it is running:
1. Use two toroids electrically connected together with a wire, and raise or lower the top toroid using a nylon cord and pulley system. This alters the topload capacitance on the secondary coil shifting the resonant frequency. The combined capacitance of the two toroids varies with the spacing distance between them as they electrostatically shield one another when in close proximity.
2. Use a flexible wire extension to the primary winding. This flexible wire can either continue in the same rotationaly sense as the rest of the primary adding inductance, or it can be brought back along the outside of the primary winding cancelling out some of its inductance. This boosting or bucking of the primary inductance allows the primary resonant frequency to be adjusted on the fly.
3. Two primary windings can be used with an adjustable gap between them. Decreasing the spacing between the two windings increases the mutual inductance, whilst increasing the gap decreases the primary inductance.
4. A variometer can be made and connected in series with the primary circuit. This will allow primary inductance to be adjusted on the fly, but the off-axis inductance will also effect coupling coefficient slightly too.
There may be other methods, but those are the ones i'm aware of.
As far as "why" would I want to change the frequency on the coil, the reason is for different looking sparks and streamers. I wanted to be able to have lightning bolts one second, and flames the next. I guess pushing the limits on any technology is the only way to learn something new.
Would adding some sort of variable resistor in the secondary have an affect on tuning? Im guessing because of the frequency and voltage generated, the resistor would be smoked in about 1 nano second and full of lightning. What do you guys think? Or, wire taps coming off of the secondary to change where the pickup point is on the coil (bsaically by-passing some of the windings on the coil), assuming corona isn't an issue..
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Ryan wrote ...
As far as "why" would I want to change the frequency on the coil, the reason is for different looking sparks and streamers. I wanted to be able to have lightning bolts one second, and flames the next. I guess pushing the limits on any technology is the only way to learn something new.
Such dramatic change of resonant frequency is impossible.
You can experiment with a fixed frequency solid state driver, and maybe excite different resonant modes (and sub-harmonics), but I don't really know much about this...
Registered Member #1107
Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792
Ryan wrote ...
Thanks for your post Richie.
As far as "why" would I want to change the frequency on the coil, the reason is for different looking sparks and streamers. I wanted to be able to have lightning bolts one second, and flames the next. I guess pushing the limits on any technology is the only way to learn something new.
Would adding some sort of variable resistor in the secondary have an affect on tuning? Im guessing because of the frequency and voltage generated, the resistor would be smoked in about 1 nano second and full of lightning. What do you guys think? Or, wire taps coming off of the secondary to change where the pickup point is on the coil (bsaically by-passing some of the windings on the coil), assuming corona isn't an issue..
-Ryan
There is no real way to "have lightning bolts one second, and flames the next" using that method. To my understanding, there is no way to do that with a spark gap kind coil but it is possible using a solid state driver. It can be achieved by varying the duty cycle and frequency of a interrupter. running it in CW mode produces a short but very hot plasma flame and running it in interrupted mode produces more of a lightning bolt kind of discharge. Read up "solid state tesla coil' in the hvwiki!
Ok great, maybe I will run the SSTC board I have with an interuptor for sparks, and jump the interuptor for cw mode for the flames. Will there be a difference in power consumption if I did this?
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Variable Frequency Coil
