1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Tesla Coil - Capacitor necessary?

  1. Jan 18, 2014 #1
    Hi to all,

    It may be a low-level question, but nevertheless, one to which I'm looking for an answer.

    The main question is as follows:
    - Within a Tesla coil, what is the main purpose of the capacitor and spark gap?

    Other such questions arising from the principle question:
    - What would be the outcome if you were to connect and A.C circuit directly to the primary coil, to then induce the emf through the secondary coil?
    - Would the same thing happen or would it be potentially more dangerous?
    - Are those two components truly necessary to achieve the breakdown voltage of the air and to create the visible sparks.

    Some may say I've asked numerous questions in this thread, but, in fact, the response to the main question should cover the other questions mentioned.

    Thank you for all help in advance,
  2. jcsd
  3. Jan 19, 2014 #2
    Spark gap makes the current pulsating (constant current can not induce any voltage)
    The charge builds up, then there is a breakdown and then again, charge builds up and so on.
    The capacitor is needed to adjust the resonant frequency, so that the primary and secondary circuit to have the same resonant frequency.
  4. Jan 19, 2014 #3
    Thank you, first of all, for your speedy response.

    I'm not actually sure what you mean here. If you passed the A.C current through the step-up transformer, then connected that to the primary coil, the constantly changing direction of the charges means a constantly changing flux due to the changing of the direction of the magnetic field; thus, adhering to Faraday's law, inducing an emf in the secondary coil? Please do correct me if my logic is incorrect.

    Why is the same resonant frequency needed in both circuits?

    Thank you again.
  5. Jan 19, 2014 #4
    your logic there is correct.It's just that tesla coils usually operate on much higher frequencies.
    the capacitor determines the spark gap frequency because , a bigger capacitance rated capacitor will take more time to charge up , so more charging time means lower discharge frequency , as each cycle would take longer to charge the cap back to the breakdown voltage of the gap.
    yet each cycle would be stronger due to a larger charge, so more current.
    with a smaller cap you would increase the frequency yet each cycle would have less current but the frequency would rise because it would take less to charge the capacitor up to the same voltage.

    the resonant frequency is different in each circuit , it depends on the wire diameter , coild winding count etc etc , in other words each setup has its own frequency and you can only tell by calculating your specific parameters to find out.

    the reason why tesla coils normally operate on much higher frequencies is because if you would want to operate from a lower frequency like the mains frequency you would need the whole device to be much bigger, much much bigger.
    remember the higher the frequency the lower the turn count of the same wire to achieve the same voltage level.this is the basic law in all switch mode power supplies which is the reasin why the transfomer is so small.
    also a tesla coil even though being called a resonant transformer sometimes, is not exactly a typica ransformer like the one you would find in household devices, with an iron core etc.
    it ofcourse works on the same principles , induction, but it has it's own specifics and that's why it works better with pulsed bigh frequency, not sine waves of low frequency.
    It's very similar to a cars ignition coil.
    Last edited: Jan 19, 2014
  6. Jan 19, 2014 #5
    Brilliant! I'm following the logic now you've explained it so well.

    Thank you for all the help.
  7. Jan 19, 2014 #6
    are you planning on building one?
    if you are , be careful , it involves high voltages. easy to get zapp zapp in the fingers and then they are stiff for the rest of the day. :D
  8. Jan 19, 2014 #7
    Stiff fingers? I am afraid being careless with Tesla coils can get your entire body stiff for the rest of its life (which won't be long).
  9. Jan 19, 2014 #8


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    Modern tesla transformers tend to use an oscillator as the source of RF and not a spark gap. That method is far more effective. Both primary and secondary circuits can be tuned better - although the secondary resonance is more or less 'what you get' from the dimensions of the coil.
  10. Jan 19, 2014 #9

    I'm not planning on building one, no. Perhaps in the future but at the minute, I'll stick to understanding the basic theories and concepts of physics in general.

    I don't think I'm ready to take the risk!
  11. Jan 19, 2014 #10


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    It was the primary volts from the red hot transmitting valve that scared me. The secondary sparks were impressive but pretty harmless (apart from burning bare skin - so hold a metal rod)
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook