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Debugging a Tesla coil

  1. Jun 14, 2009 #1
    I'm trying to build a Tesla coil, for the first time, and have some questions.
    Whatever sources i came across on the internet only tells how to build one out of household items, trash, radio shack components, etc, but not why and which way the thing works.

    The thing:
    The sparking coil is a A3 sheet of paper rolled to 7cm in diameter, with wiring on the first 18 cm of it, roughly 250 loops.

    The main coil is on the bottom, 14 loops of the slightly bigger diameter as the spark one.

    20KV 680pf capacitor in the circuit.

    Roughly 0.6cm spark gap parallel to the circuit.

    And a pocket taser as a power source, which stops sparking at about 2cm distance between terminals.

    Now, the questions are:
    1. The spark coil produce only at most 1-1.5cm streamer between lower and upper wires, which is less than what i expected. Is that normal, or should it be better on such a setup?
    What determines the spark length - power, tuning, frequencies, geometry, proportion between all of the said things?
    Is anything obviously wrong in the design?

    2. What the spark gap distance responsible for? Does it only control the frequency, or something else?

    3. On the sparking coil, the wiring insulation seems perfectly conductive for the voltage being generated in it, if the lower wire is brought near the wiring, sparks form. Is that normal, or should the wires be better insulated?

    4. The capacitor is rated for 20Kv, while the taser gives up to 2cm arc, which i translate into 60Kv. Now, the spark gap being set to less than 0.5 cm should limit the voltage going into the capacitor well below 20Kv, right? If not, what will, and what is the impact on the capacitor? It's not easy to get a new one if that one will burn.

    5. How dangerous a charged 20Kv 680pf capacitor is? Are we talking about shocking, lethal, or explosive? And, how do i calculate that for a different capacitor?

    6. What could be used for grounding the lower wire of the sparking coil? Central heating radiator doesn't seems right, and the earth is far below and well paved.
  2. jcsd
  3. Jun 14, 2009 #2
    Energy in a cap = 1/2 c v^2 (had to look it up lol.)
    .5 * 6.8* 10^-10 * (20000)^2 = 0.136 joules

    Possibly lethal, but most likely a bad shock.
  4. Jun 16, 2009 #3


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    You need to tune it. The coil must be in resonance while operating otherwise your efficiency (and hence your spark-forming ability) will be severely impaired. Normally, tuning is done by adjusting the electrical length of the primary coil; typically bare wire is used for this coil, since it's easy to move the feedpoint around until you find the primary resonance--which will be the point where you produce the longest arcs.

    Good luck.
  5. Jun 16, 2009 #4


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    If your coils are like the ones in the pictures, the primary one seems a long way from the secondary one. If you have the grounded end of the secondary at the top (in your case) you should be able to come closer with the primary coil. This would give you better coupling at some risk of sparking.

    Maybe wind a spare coil and try this rather than move the existing one?

    Are you in Russia? Which city?

    What voltage are you applying to the input?
  6. Jun 19, 2009 #5
    I did some math, thanks to the http://deepfriedneon.com/tesla_frame0.html site, and some re-design, getting approximately correct values, then hand-tuned it for maximum performance i could get, but if it is on maximum performance or not is still a bit of an if.

    Finished (sort of) model:

    And in action:

    I put a spacing between the primary and the secondary just for that reason - the secondary arced on the primary, even thru the paper.

    The transformer gives about 2.5 cm sparks when fired open, and the spark gap is about a millimeter wide, not sure which one is more important.

    The power supply is a 12V battery pack, and an ampermeter gives over 10A in the circuit, which feels kind of weird.
    If that is right, then the transformer is running on less than 1% efficiency, or the coil is quite badly made - the open-air sparks it gives is roughly 2 cm in the dark, and that corresponds to 1w of power, if the site above is to be believed.

    Still, considering the heating of the transformer and the batteries, i guess it's at least a few A's in the low-voltage part, so there should be something more than 1w in the high-voltage part, no?

    Home-made plasma ball is nice too, but i expected some sparks.

    Yes, Moscow.
  7. Jun 19, 2009 #6


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    Sounds like you have about 60000 volts coming from the step up transformer. (2.5 cm sparks.)

    So, having a 1 mm gap is probably far too close. This would discharge when the voltage got to 3000 volts and waste a lot of your input voltage.

    I would wind this out to at least 3 mm and see if this improved the spark. That should arc at about 9000 volts and still protect your capacitor with its 20 KV rating.
    The spark gap may be arcing over at a lower voltage because of the ends being points. If they were flat surfaces, the arc-over voltage would be higher.
    The capacitor is being charged to opposite polarity at mains frequency, so you want it to be discharged by the spark gap when it is fully charged at its maximum rating.

    It is the discharge of the capacitor through the spark gap via the Tesla primary that is causing the high voltage secondary pulses. In your case, the primary coil will resonate at about 1.4 MHz with about 19 uH inductance in series with 680 pF. So it will produce an initial pulse of about 0.7 uSec. The output coil should ideally be resonant at the same frequency for a big boost in efficency.
    You could check this by feeding a signal into the primary from a signal generator and watching for maximum output on the secondary with an oscilloscope.
    You can also check the input resonance by including the capacitor and looking for a big increase in voltage across the primary coil.

    It seems like that capacitor might be a limiting factor because of its voltage rating and relatively low capacitance. Have you considered making one?
    You could get a glass jar or bottle and put metal foil on the inside and outside of it.
    You could get some plastic sheeting and put flat metal plates on either side of it.
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