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    how to select capacitor for ruhmkorff coil?

    i am planning to build a ruhmkorff coil ruhmkorff coils use a quenching capacitor how do i determine the amount of capacitance of the capacitor to use? is there a way to calculate the amount of capacitance to use?

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    Re: how to select capacitor for ruhmkorff coil?

    It isn't necessary to use one at all but I would suggest something in the region of 0.22uF if you insist on using one.

    These are not user friendly or efficient devices, you can get better results using a car ignition coil. The only difference is the contact breaker is operated by the magnetic field instead of the rotary action of the car engine. All the capacitor does is reduce sparking across the contacts, it will work without one.

    Note that Ruhmkorff coils, like any open ended inductor will wreak havoc with nearby radio and TV reception - you were warned!

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    Re: how to select capacitor for ruhmkorff coil?

    but according to wikipedia the quenching capacitor eliminates arcing across the switch contacts and the arcing across the switch contacts reduces voltage output



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    Re: how to select capacitor for ruhmkorff coil?

    I would argue the opposite. It helps to preserve the contacts although there is some additional loss as the capacitor is shorted out and discharges into the contacts. The coil is essentially a high ratio step-up transformer with a mechanically pulsed current in the primary winding. Resonance comes into play to some extent but in general, the faster you pulse the primary, the more output it will produce. An arc at the input could well increase the output rather than reduce it.

    A good MOSFET driven with a square wave instead of the contacts will give much higher output.

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    Re: how to select capacitor for ruhmkorff coil?

    Quote Originally Posted by betwixt View Post
    I would argue the opposite. It helps to preserve the contacts although there is some additional loss as the capacitor is shorted out and discharges into the contacts. The coil is essentially a high ratio step-up transformer with a mechanically pulsed current in the primary winding. Resonance comes into play to some extent but in general, the faster you pulse the primary, the more output it will produce. An arc at the input could well increase the output rather than reduce it.

    A good MOSFET driven with a square wave instead of the contacts will give much higher output.

    Brian.
    may i ask if you are an engineer?



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    Re: how to select capacitor for ruhmkorff coil?

    but according to wikipedia the quenching capacitor eliminates arcing across the switch contacts and the arcing across the switch contacts reduces voltage output
    It does not eliminate arcing across the switch contacts; but it reduces.

    Think in terms of the total energy: when the current collapses, a part of the energy may be stored into the capacitor (a significant part may be sent to the secondary coils). The stored energy in the capacitor will be fed into the coil when the contact is made again.

    The problem is that when the contact is broken, the current must change very rapidly and the induced voltage must be very large (a singular process). So you need a very high voltage capacitor.

    The ideal capacitor value will be determined by the LC time const matching the spring time const (for the make and break switch). Tricky to figure out but not impossible.



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    Re: how to select capacitor for ruhmkorff coil?

    Hi,

    So you need a very high voltage capacitor.
    The voltage of the capacitor is also limited by the available energy.
    E = 0.5 × V × V × C --> V = sqrt( 2 E / C)

    When the energy comes from a coil, then E = 0.5 × I × I × L

    Klaus
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    Re: how to select capacitor for ruhmkorff coil?

    may i ask if you are an engineer?
    Yes, with 50+ years of experience, much of that designing test equipment!

    Brian.
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    Re: how to select capacitor for ruhmkorff coil?

    Quote Originally Posted by c_mitra View Post
    It does not eliminate arcing across the switch contacts; but it reduces.

    Think in terms of the total energy: when the current collapses, a part of the energy may be stored into the capacitor (a significant part may be sent to the secondary coils). The stored energy in the capacitor will be fed into the coil when the contact is made again.

    The problem is that when the contact is broken, the current must change very rapidly and the induced voltage must be very large (a singular process). So you need a very high voltage capacitor.

    The ideal capacitor value will be determined by the LC time const matching the spring time const (for the make and break switch). Tricky to figure out but not impossible.
    any way to measure the spring time const at home?



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    Re: how to select capacitor for ruhmkorff coil?

    any way to measure the spring time const at home?
    If you haven't got an oscilloscope or frequency counter, try using a mobile phone app that measures audio frequencies (there are many!) and let it hear the output of a nearby AM radio. The 'buzz' will be at contact operating frequency.
    Note that the frequency will not be particularly stable.

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    Re: how to select capacitor for ruhmkorff coil?

    Quote Originally Posted by betwixt View Post
    An arc at the input could well increase the output rather than reduce it.
    Brian.
    I don't agree with you. Arcing between contacts means they are shorted by the arc for a while. Due to this the commutation has to be slower with respect to a commutation arc-free and then the output voltage is lower.



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    Re: how to select capacitor for ruhmkorff coil?

    I don't agree with you. Arcing between contacts means they are shorted by the arc for a while.
    You are obviously correct.

    But a capacitor also shorts the coil for a while.

    But the arc wastes energy while the capacitor stores the energy for the future.

    But it is not possible to say a-priori that the "commutation has to be slower with respect to a commutation arc-free and then the output voltage is lower" because a lot depends on how you plan to get a commutation that is arc free.



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