Re: term understanding: voltage reversal.

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Malcolm Watts" <m.j.watts-at-massey.ac.nz> 

Hi Chris,

On 18 Feb 2004, at 8:06, Tesla list wrote:

 > Original poster: "Christoph Bohr" <cb-at-luebke-lands.de>
 >
 > Hello All.
 >
 > I came  across something I am no longer sure I understood that right:
 >
 > In pulse cap data sheets there is usually a point called "voltage
 > reversal". To say it less technical I understand that voltage reversal
 > causes stress on the cap and is undesireable if you like longelivity.
 > But what exactly is this voltage revesal in a AC, sync gap, TC? is it:
 >
 > 1.: The changing polarity of the carging current, i.e. the fact that I
 > once carge the cap with the one polarity and during the next half sine
 > wave to the other

yes, but... (read on)

 > or
 >
 > 2.: The changing polarity during the HF-"ringing". As the changes
 > happen more often and more rapidly here I feel that this is the main
 > voltage reversal relatet Stress on the cap.

this is the one that will cause the most stress. Voltage reversal
includes any voltage reversal at whatever frequency, the highest
obviously being the most stressful since it causes the most rapid
torsion in the dielectric medium.

 > I fear this relates to point 1 and 2 as well. I only hoped that if it
 > was point 1, one could reduce the stress on the cap by full-wave
 > rectifying the xformer output to charge the C1. Would this work? I am
 > not talking about a real DC supply with smoothing and charging
 > inductor, only charging with pulsed dc to avoid the changing in
 > polarity of the charging current.

It will not help much if at all. Also, the higher the amplitude of
the reversal, the more stressful it will be. For the average TC,
expect a reversal in the 80 - 90% region (or higher if you run the
primary with no secondary present). In summary, high voltages and low
pri-sec coupling together with high frequencies give capacitors what
is possibly the ultimate workout.

Malcolm

 > Probably I am missing something but the longer I think about it the
 > more unsure I get...
 >
 > Maybe some of you guys can explain that to me.
 >
 > Best regards
 >
 > Christoph Bohr
 >
 >
 >