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Re: xformer/cap - match

Hi Mike,

At 09:39 AM 10/23/1999 -0400, you wrote:
>In a message dated 10/23/1999 5:48:40 AM Mountain Daylight Time, 
>tesla-at-pupman-dot-com writes:
>
><snip>

>    It seems to me that Terry F. has been using a what he terms, a  
>"Larger-Than-Resonance" capacitor with good success.  He can correct me if I 
>am wrong, but I think he has been using a cap with a value about twice that 
>of the calculated impedance match.  

The LTR coil uses 27nF with a 15kV/60mA neon.  A little more than twice the
10.6nF from the equation.

>    On the other hand, NST's (at least Allanson brand) are meant to be 
>operated at a voltage of about 1/2 of the voltage shown on the nameplate.  
>The 15kV is need to get the neon light started, but then the load of the tube 
>is supposed to cause the RMS voltage to be 7500V during continuous operation 
>for the transformer to last longer.   I read this on Allanson's web site - 
>probably in their online catalog (but don't hold me to that). 
>    So if one equates the two conditions above, it seems to me that really 
>one should use a capacitor value of   0.5*maxV/I = 1/(2*pi*f*C)  or if this 
>is rearranged, C = 1/(pi*f*V) where V is that nameplate voltage.  This would 
>suggest that the primary capacitance should be twice what we have been using. 

There is a problem.  If your reduce the voltage to half by bringing in the
gap distance, you charge the cap to 1/4 of it's previous energy.  That
makes the sparks shorter.  If you turn down the variac, the current
available from the neon also decreases.  The load current is proportional
to the input voltage.  So the transformer may last forever at 7500 volts
but the coils performance will be about half.  

Personally, I think with safety gaps and a little care, a neon will last
fine in our occasional room temperature service.  Used neons have had a lot
of abuse long before we get them (especially here in Colorado) so if they
die for no reason, they were probably 99.9% dead to begin with.  I "seems"
to me since we all started using safety gaps and protection filters, that
the number of dead neons has decreased dramatically??

My neon (a new Transco) is rated for 900VA.  I run it at about 860VA and
the voltage and current I pull are at it's ratings.  If the name plate
means anything, it should run just fine for many years.  However, since It
only runs a few times a month for short duration at room temperature and
lives the rest of the time at ~70F in the basement, It "should" last
forever I would think.  We I run it, it seems to get only slightly warm and
shows no signs of distress.

> 
>    I do remember reading a note by Tesla that the primary cap should be able 
>to take ALL of the energy provided by the transformer and it would seem that 
>in the case of NST's, doubling of the capacitance we have been calculating 
>would certainly do that.  It would also be simple to adjust the spark gap to 
>fire at one half voltage by simply adjusting it to fire when the variac is at 
>50%.  It would save a lot of NSTs from early death too. 

I charge the larges cap size possible with the LTR coil.  The larger the
cap value the more energy.  *It is possible to use a smaller cap and fire
more than 120BPS to get close to the same output power but it is tricky.
Often it is easier just to add another transformer rather than fiddling
with the complexities.  In my case, I was trying to do all I could with a
single neon.

>      This would not be the case for pole pigs, MOT's or other transformers 
>that are not current limited.  The caps for these should be designed in the 
>normal manner - based on the power they are designed to be used at.

Pig and MOT system really ARE current limited.  They are just controlled
externally where a neon has the limiting built in.  I really like neons for
TCs.  However, when the power required gets really high, one has to go to
something else.

>    I would like to hear comments on this as I haven't heard of anyone 
>mentioning this before.  It is just a thought that recently occured to me and 
>since Herwig brought up the subject, I figured I would present this idea.
>Mike
>

Cheers,

	Terry