Hi Brent,

> Original Poster: Brent L Caldwell <stretchmonster-at-juno-dot-com> 
> Norm, and anyone else interested:
> Norm, you wanted to know if your 25kV DC capacitor would be okay:
> Well, I can't really speak for the DC problem, but I would like to let
> you in on a private little secret amongst us coilers:
> ******************
> Even though your power supply may only be 10kV, the voltages across
> the capacitor may reach as much as 3 times that value, or more, on a
> 60Hz Line.  Here's why:
> ******************
> The secret:
>  Well, I'm not positive about all this, but I think this is the way it
> works:  The current through a capacitor is equal to the capacitance
> time the rate at which the voltage across it drops (or rises,
> whatever).  Now just because the max voltage produced by your power
> source is only 10kV (or 15kV, whatever), that doesn't mean that the
> value of capacitance times voltage change won't be huge.  Also
> remember that the voltage across a capacitor is its reactance times
> current.  
>  This means that at any particular time, huge Capacitive Reactance
>  times
> huge current (current is huge because of reasons stated above) = huge
> voltage across your capacitor (possibly much larger than whatever your
> power supply is).  Most of the experinced guys I have worked with say
> you should use a capacitor rated for at least 3 times the voltage of
> your power supply.  

Agree if significant leakage inductance in the charging circuit 
makes the circuit resonant at Fmains. But then of course it takes 
more than one cycle to ring up to that value. A cap hung across a 
stiff voltage source (ideal transformer with no ballasting) can't go 
higher than the voltage source will allow.  

> >
> > . . . I have seen pictures of the primary coil wound  vertical, 
> >horizontal  and conical...what are the differences in results . . .
> >
>  Well, first I want you to know another secret that most otherwise
>  well
> informed people don't know:  You don't necessarily want a tight
> magnetic field coupling between your primary and secondary coils. 
> These different coil forms offer different degrees of loose coupling. 
> You'll have to wait for an answer from the really experienced guys
> like FutureT, and Dr. Resonance, and those guys, to tell you exactly
> which one is best.  The differences are that the vertical primary coil
> is probably the easiest to build.  The flat spiral and conical give a
> little looser coupling, and are generally better.  
You can achieve much the same degree of coupling with any type 
of primary simply by altering the height of the bottom turn of the 
secondary with respect to the primary.

>  I suppose the flat pancake spiral is a little easier to build that
>  the
> conical.  Most of the best coils I see are built with flat spirals,
> though.  But like I said, you have to wait for one of these big dogs
> to tell you exactly when and where a conical beats a spiral and vice
> versa.
> >3. I have a small dc motor that I can control the speed ....I am
> >thinking of using it to power a Rotary spark gap.  Cant I tune the
> >primary by changing the speed of the motor??
> >
>  Well, again a big dog on this list might disagree with me, and if
>  they
> do, then believe THEM.  But anyway, I don't know of any way to tune a
> coil other than to change the inductance of the primary, or change the
> capacitance of the tank circuit.  

You can also alter the size of the top terminal. It is possible to 
increase or decrease the capacitance while maintaining the same 
radius of curvature (breakout voltage) within limits. Even altering the 
height the terminal is mounted above the secondary will affect its 
resonant frequency.

>  Now, once the circuit is in tune, yeah, there might be an optimum
>  motor
> speed so that the spokes of the rotor line up at some angle every time
> there is a pulse from the tank capacitor.  But really, I think that's
> the idea of a Synchronous Rotary Spark Gap, which is another topic you
> might want to ask the list about.
>        Safe Coiling:
>        Brent
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