# Re: Question?

```At 11:49 AM 12/9/98 -0700, you wrote:
>Original Poster: Scott MacGregor <smacgregor-at-aasp-dot-net>
>
>To the list,
>
>Does anyone know the formula for calculating the reverse voltage and
>current levels delivered from a MULTI-LAYER coil after a steady state DC
>voltage applied to the coil is suddenly interrupted? (if the coil is
>able to discharge through a spark gap) I hope I'm stating this properly.
>Example:12VDC battery across an air core coil consisting of 20 layers of
>#20 copper 2" D (average) and 6" tall. Break the battery connection
>suddenly and provide a .025" gap in its place.
>--
>Regards,
>
>Scott MacGregor

Well in theorey, if you instantaineously remove the current from a perfect
inductor the output voltage would become infinite. Inductors try to maintain
a constant current through themselves. So as you break the gap with the
inductor the voltage will rise at a rate limited by the parasitic
capacitance of the coil. The maximum voltage could be calculated by

Energy stored in inductor(Joules) =.5LI^2

Energy stored in cap(Joules)=.5CV^2

LI^2=CV^2

where L is in heneries, C is in Farads I in Amps, and V in Volts.  That
stated you will have to move the terminals of your gap apart very quickly to
prevent the voltage developed from arcing across before peak voltage is
developed. As for the current you could obtain out of this arrangement, well
this is a little more complicated. In the simplest case when you short the
leads of the inductor immediatly after charging the current flow will be
equal to that with wich it was charged. This current will decay at a rate
detirmined by the resistance of the coil. If you wait untill peak voltage is
obtained by this arrangement to draw your current it will deliver quite a
bit since it is like a charged capacitor.

Eddie Burwell

```