Re: Theory - primary cap duty -transformer output current

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Original poster: "Jason Petrou by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jasonp-at-btinternet-dot-com>

Pot Luck? (plz use your name on the list :)

> My inquiry focuses on transformer current and the primary cap.
> My understanding is that once the primary cap is charged and the firing
spark
> occurs that the transformer's current during the HV pulse and "ringing"
are no
> longer a factor, that the energy stored in the primary cap becomes the
sole
> provider of current in the tank circuit. Once that arc is quenched then
the
> primary cap is again charged.
Yes. The primary capacitor charges, and then discharges through the spark
gap, as you rightly say. The current produced in this pulse can be in the
tens of hundred of amps, depending on the capacitor. The output current of a
transformer is at most 1A, and is completely insignificant to the actual
magnetic induction in the TC system. the 50 (or 60) Hz line is also not part
of the resonant system so there could be no effect on the resonant rise of
the system. The transformer is shorted out through the spark gap (hence the
need for current limiting and quenching).

> So how does transformer current fit into the
> scheme of things?
Simply - it doesnt. Its sole purpose is to charge the primary cap and to
fire a static gap.

> If that cap is allowed to charge within 90% of the voltage
> available before discharge then where does transformer current fit in?
Does
> higher current allow quicker charge time?
It's not a case of the cap charging to the voltage (which it obviously does)
but a case of storing the energy that the transfomer puts out. As the cap
reaches its peak capacity the voltage across the transformer increases as
the circuit becomes more and more open. Finally the voltage that occurs at
full charge will cause the air in the gap to conduct and then the resonant
rise process will begin. Higher current does allow a quicker charge time,
but the capacitors need to reach their peak charge as the AC line reaches
its peak. In this way, the maximum energy is transferred into the primary
inductor. Therefore your capacitors need to be matched to your power supply,
to get the correct charging rate. If you have a variable ballast and you
turn the curent up, then the TC performance will rapidly fall, because the
current supply is no longer matched to the capacitor. THIS IS ASSUMING A
SELF TRIGGERENG STATIC GAP. Practically any cap value can be used if an
async adjustable rotary gap is used. Generally if you are going to use an
async rotary, then the bigger the capacitor, the faster a break rate you
will need. For a 50z line you cannot use a break rate lower than 100BPS, and
for a 60Hz line, 120BPS, logically.

> I can see where firing at zero-crossing is also advantageous, allowing the
cap
> to "see" the full transformer output voltage and eliminates shorting the
> transformer secondary during discharge.
Kinda. Firing at zero allows the cap to store the full amount of ENERGY from
the transfomer, not voltage. However you need to use a sync. rotary to
achieve this, as a standard static gap will fire at the peaks, not at zero.
Also your cap needs to be double the size (i think) to accomodate the extra
half of the AC sine wave.

If you are building your first coil, I would suggest that you go for a
static gap, or if you decide to use a rotary, then have a static gap across
the rotary, so that any misfirings caused by bad calculations/logic will not
damage your transfomer or caps.

Regards,
Jason

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