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Re: Capacitor calculation


TE>From SROYS-at-radiology.ab.umd.edu Tue Sep 26 09:15 MDT 1995

TE>> I have seen several postings recently regarding calculating the
TE>> correct capacitor size for a given transformer.  I assume these values
TE>> are coming from a program such as TESLADP.  I tried this for my pole

TE>I'm not familiar with the Tesla programs either , but I have seen
TE>capacitor selection described as a problem in impedence matching (i.e.
TE>-  for the most energy transfered, the impedence of the load should
TE>match the impedence of the source).

TE>In English, for a transformer with a given voltage (=V) and current (=I)
TE>rating, you first calculate the impedence that will load the transformer to

There are people better qualified than I to speak on this matter,
but I'll give it a go, and if I'm wrong then I too will learn
something. :-)

Neons "current" limit due to the internal magnetic shunting, they
operate in use with neon tubes at nearly a dead short across the
terminals so they have to current limit or they would blow up. A
pole pig on the other hand has no "built in" current limiting so
it can supply current up to the value of your circuit breakers.

So. With a pole pig the capacitor values are governed more by the
other tank circuit components.

With a neon it's current limiting means that with a HUGE
capacitor across the output it "sees" in it;s output a dead short
and can only supply it's rated current but at a much reduced
voltage. This rated current will only charge the given capacitor
up to a certain voltage in a certain time. If this time is
greater than one quarter mains cycle then it will never reach
maximum voltage output by the transformer. During the next
quarter cycle the capacitor will be discharging into the
transformer (if the gap hasn't fired) on it's way to the next
negative half cycle.

However if the capacitor value is smaller the voltage on the
capacitor will reach a nearly equal value to the voltage on the
output terminals of the transformer after a quarter cycle, thats
more desirable to get a good spark strike and max current from
the circuit through the gap and into the primary.

A pole pig (or microwave transformer) is designed to not current
limit at small values so the capacitor maybe as large as you like
(within reason) and the capacitor voltage will follow the
transformer output voltage, but the current will be initially
very high and fall off as the voltage reaches it's maximum at the
quarter cycle. You arrange the gaps to fire just at the top of
the quarter cycle (maximum voltage) and transfer the stored
energy in the capacitor through to the primary in a very short
time, thus getting a good pulse of current.

The formulae to calculate the values have been posted before and
are relatively simple to work with.

Jim Oliver <jim.oliver-at-welcom.gen.nz> (3:771/370)

 * SLMR 2.1a *