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Spark gap sim, method revealed




John, Anotonio, Terry & ALL,

There has been some interest in my rotary gap and fixed gap simulations,
particularly regarding the method I used,  and accuracy.

Here is a brief summary:-

1. Firstly I took a real transformer.  This was an actual neon sign
   transformer for the static gap sims,  which has internal shunts.
   The transformer was connected to a 240V 50Hz supply, and I measured the
   open circuit HV secondary voltage,  and the short circuit secondary
   current.  (In this case V=6000, I=0.125)  I then turned off the power
   and measured the primary and secondary winding resistances with a DC
   meter.  These four parameters characterise the transformer pretty well.
   (The same was done with the Radar supply transformer,  it just had an
    external ballast instead.  The method is the same.)

2. I then formed a simplified Microsim PSpice model of the transformer.
   From the o/c sec voltage,  and s/c current,  I calculated the leakage
   inductance.  Both the leakage inductance and winding resistances were
   then referred to the HV secondary winding.  This gives me the following
   simplified model for the transformer.

           L=153H      R=2000             
    _______     ______/\  /\  /\______    
   !       UUUUU        \/  \/            The model was actualy checked by
  _!_                                     doing simulations with a shorted
 /   \                                    secondary.  (Note:  This model
is
 \___/ Vac = 6000Vrms                     very basic but quite accurate,
   !                                      however it does not take into
   !__________________________________    account possible core
saturation.)

3. I then modelled the rest of the charging circuit in a similar way,
using
   M'sims "SBreak" function as a voltage controlled switch as follows:-

              L=153H      R=2000                    ! !  C=?
    __________     ______/\  /\  /\_________________! !______________
   !  Irms    UUUUU        \/  \/        !          ! !              !
  _!_                                    O                           !
 /   \                                     / Sbreak was              !
 \___/ Vac = 6000Vrms                     /  triggered by a pulsed   !
   !                                     O   voltage source          !
   !_____________________________________!___________________________!

   Again this circuit is greatly simplified.  It does not model any of the
   RF resonant behaviour of the tank circuit,  only the slower LF charging
   and discharging of the cap.  This model assumes that the capacitor is
   fully discharged after each firing of the gap.
   (The static gap was modelled slightly differently,  using a system
    of voltage comparisions.)

4. The circuit was simulated for a wide range of possible capacitor
values.
   In each case the firing angle of the spark gap was adjusted for maximum
   voltage.  (Firing angle changes when cap is changed.)

5. After each simulation,  the following parameters were recorded:-
       i) Current draw Irms from the Vac source (far left,)
           This is directly proportional to what the supply cord current
           would be.
      ii) Peak capacitor voltage, just before firing of the gap,
     iii) Firing angle of the gap (for synch gap simulations)
      iv) Number of BPS (for static gap simulations)

6. For each simulation, the bang energy was calculated from E=0.5*C*V*V,
   the power throughput was calculated from P=E*BPS,  the supply VA was
   calculated from Vac*Irms,  and finally the power factor (pf) was
   obtained by dividing power by supply VA.  pf=P/VA

I should add that although these simulations are fairly simple,  they are
remarkably accurate where I have checked against the real world.  For
example I have run the 6/125 neon transformer with both a static gap,  and
a synch rotary,  and measured many parameters on a working coil.  Peak cap
voltage,  break rates,  firing angles,  and power consumption were all
within 10% of the model !

I would like to say if anyone is genuinely interested,  I can provide
more information by private Email (Off list,)  including Microsim
Schematic
files, Excel graphs etc.  Also check out Terrys excellent Web page,  for
plenty of simulation info.

Finally,  DO NOT try measuring S/C current of a HV transformer unless you
are ABSOLUTELY sure that the current limiting will be totally effective
under these conditions !

I would hate for someone to get hurt as a result of something I said.


				- More simulation stuff from

				   Richie in sunny Newcastle.