[Prev][Next][Index][Thread]

Re: xformer/cap - match

Hi Herwig,

At 09:45 AM 10/21/1999 +0100, you wrote:
>Hi all!
>
>Im not sure to understand the influence of the xformer/cap-match on the
>system performance. Therefore:

This issue can be extremely complex!!  I'll try my best to give my humble
opinions...

>
>For current-limited xformers like NSTs and OBITs it is recommended to use
>resonant charging of the main cap.		    1/(2 * pi * f * C) = V/I

That also applies to pig systems with current limiting (like a welder or
sliced variac) but pig systems are usually versatile enough to overcome the
need to worry about it much.

In this configuration, the neon and primary cap are "resonant".  If the gap
does not fire, the voltage can build to around 80kV!  Thus, one always
should use good safety gaps so that if anything goes wrong, the neon and/or
primary cap will not get blown up.  The gap may fire at many times the 120
Hz line frequency since this resonant condition can really pull a great
deal of energy from the neon.  It can charge to the rated voltage much
faster than the line frequency.  This allows a considerable higher energy
output from the neon.  For a 60mA transformer this is around 10.6nF.

>
>1. Capacitance is like calculated:
>The cap will reach nearly the peak voltage. The gap is adjusted to break
>down just below the peak voltage. 

This is were you charge the largest cap possible to the rated voltage and
rely on the extra energy the larger cap stores for your power transfer.
This works well too and has the advantage that it is much harder (almost
impossible) to over voltage the neon or cap.  The value is around 17nF for
a 60mA neon.  The firing rate will be 120 BPS (America's frequency).

>
>2. Capacitance is larger than calculated:
>The cap will not reach the peak voltage. Therefore the gap has to be closed
>down a bit in order to render possible discharges. The "bang" is smaller
>than optimal.

You can still get full voltage but the firing rate will be at 120 BPS...
See above.

>
>3. Capacitance is smaller than calculated:
>If the gap was adjusted to a breakdown voltage lower than peak voltage,
>the cap will be loaded several times during each halfcycle, resulting in
>several breaks. This higher breakrate causes longer discharges as the ions
>will not cool down between the breaks. So, if a correct capacitance isn't
>available, it
>is advisable, to use a *smaller* one.

You can charge a small cap fast and get a lot of pulses out of it.
However, the cap value gets so small that it either does not have enough
energy are the rest of the component values don't match well.



There is also the LTR (Larger Than Resonant) configuration that I like so
much.  This uses very large caps (27nF with 60mA).  In order to charge such
a large cap (120 BPS) from a 60mA transformer, we play timing tricks with a
synchronous rotary gap and need to do all kinds of calculation so that we
also get energy out of the inductive kick effects of the large inductance
of the neon's secondary windings.  Not too complex in practice, but the
theory and math behind it is nasty.  The gap actually fires after the peak
AC waveforms and other funny things.  It does work very well however... 

Cheers,

	Terry

>
>Are these assumptions correct? Any comments are welcome!
>
>Safe coiling to you!
>
>Herwig
>