Re: SRSG break rate

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "R.E.Burnett by way of Terry Fritz <twftesla-at-qwest-dot-net>" <R.E.Burnett-at-newcastle.ac.uk>


<<< Terry,  I posted this last week but it doesn't seem to have
    surfaced yet.    If this copy goes missing also,  I'll know
    you've axed it ;-)     >>>

Hi Malcolm, Chuck, Steve, all,

There have been several posts discussing resonant charging and
different types of rotary gaps recently.  Here's my thoughts...

I almost totally agree with Malcolm's explanation of resonant
charging ;-)  The only difference is that I think the ballast
is not so much "charged" by the shorting action of the spark gap,
(this event is fairly brief and does not contribute much to
stored energy in the ballast.)  Instead  I believe that the
ballast is mostly charged as current flows through it to reach
the tank capacitor.

Resonant charging seems like one of the harder things to
understand in this hobby.  This is how I get my head around
what is going on,  (My explanation of resonant charging...)

Consider the circuit below with a voltage source V,  a ballast
inductor L,  and our tank capacitor C.  The voltage source can
be a fixed DC level as in DC resonant charging (Electrum style)
or an AC voltage source. Operation is the same either way.
      ____     _____
     !    UUUUU     !
    _!_     L      _!_
 V /   \           ___ C
   \___/            !
     !              !
     !______________!

1. Initially assume that the voltage source is zero,  there is
   no current through the inductor,  and the capacitor has no
   voltage across it.

2. The voltage source starts to rise from zero when the supply
   is turned on or as the mains sinewave rises from zero. This
   causes a voltage to  develop across the ballast inductor L
   since the capacitor voltage cannot change instantaneously.
   (The capacitor must charge gradually.)

3. The voltage across the ballast means that current starts to
   flow through the ballast L and charges the capacitor.  (The
   current actually ramps-up at a rate determined by the
   inductance.)

4. This current flowing through the ballast implies that ENERGY
   IS STORED IN THE BALLAST.  Remember that energy stored in an
   inductor is equal to  0.5 x L x I * I   (Although we readily
   accept that the current causes charge to be stored in the
   capacitor,   it is easy to forget that the mere action of
   current flowing through the inductor implies that energy is
   already stored in its magnetic field !)

5. Eventually the capacitor charges up to a point where its
   terminal voltage equals that of the voltage source.
   Therefore there is no voltage across the ballast inductor at
   this instant.

6. Although there is no voltage across the inductor,  the
   current through the inductor cannot change instantaneously.
   The inductance of the ballast acts to keep current flowing
   in the same direction.

7. As the magnetic field in the ballast collapses,  the ballast
   inductor releases its stored energy and effectively becomes
   another "power source".

8. Since the ballast inductor is in series with the supply
   voltage, its inductive kick  (or EMF)  adds to that of the
   supply,   and charges the tank capacitor to a VOLTAGE HIGHER
   THAN THE SUPPLY ALONE COULD ACHIEVE.

9. Eventually the magnetic field in the inductor has decayed
   completely and all of its stored energy has been passed to
   the tank capacitor.  Now is the time to fire the spark gap,
   since the tank capacitor is at its max voltage.

10. If the spark gap is fired at this instant, the tank
    capacitor is emptied of its stored charge in a split second.
    Then there is a voltage across the ballast and the whole
    charging cycle starts again at step 3.

11. The whole trick is firing the spark gap at the right time.
    If the spark gap is not fired at the right instant this is
    what happens...

12. The tank capacitor has been charged to a higher voltage than
    the source so there is a voltage across the ballast inductor.

13. Current begins to flow from the tank capacitor back through
    the ballast inductor to the source !

14. Essentially the capacitor is transferring some of its stored
    energy back to the ballast.

15. This resonant exchange of energy between the ballast inductor
    and the tank capacitor continues until the spark gap fires
    and takes some energy out of the system.

16. Given that the system is continually fuelled by the voltage
    source, it is easy to see how a run-away resonance condition
    can occur if the spark gap is set too wide to fire.

I didn't try to draw any "ASCII waveforms",  but there are many
examples of resonant charging waveforms on my web page if they
are hard to visualise from this desciption.

By appropriate choice of the resonant charging frequency it is
possible to make the tank capacitor charge and fire many times
during each half-cycle of the supply.   Although many of the
firings are not at the maximum voltage,  this larger number of
firings still represents more power throughput for a given
capacitor size than with just one firing at the peak of each
half-cycle.

I hope this information is of some help in de-mystifying the
headache that is resonant charging !  My appologies if this
explanation is still confusing to anyone.  I really did try.

                                        Cheers,

                                        -Richie Burnett,