Re: term understanding: voltage reversal.

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

Original poster: "Dr. Resonance" <resonance-at-jvlnet-dot-com> 



Bert has a great point here.  A nice neat equation.  If you use a storage
scope to see the primary discharge waveform you can find the nearly exact Q
factor of your primary circuit.  Knowing the % voltage reversal you can
solve the equation for effective Q factor.

Another method is to observe the decrement (decay) factor which is related
to circuit resistance.  You can use this information to determine effective
dynamic Q factor was well.

You fire the primary with and without the sec in place --- in a single shot
mode.  Do not try this at high rep rates or the excessive Q without the sec
will blow the cap.

In a single shot mode you can compare the loaded and unloaded Q factor and
thus determine the effective impedance of the primary looking into the
secondary load. This information can also be used to determine the peak
current in the primary and then compared to actual measurements with close
agreement.

Dr. Resonance

Resonance Research Corporation
E11870 Shadylane Rd.
Baraboo   WI   53913
----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Wednesday, February 18, 2004 6:04 PM
Subject: Re: term understanding: voltage reversal.


 > Original poster: Bert Hickman <bert.hickman-at-aquila-dot-net>
 >
 > Hi Christopher,
 >
 > This specification refers to the amount of voltage reversal during a
 > "ringing" discharge. Most commercial pulse caps are not designed for use
in
 > high Q ("ringing") circuits. This is why you'll find that they are
 > typically rated for only a 10% or 20% voltage reversal. This rating is
 > defined as the maximum rated reverse voltage swing expressed as a
 > percentage of the DC voltage rating for the capacitor. Rapid voltage
 > reversals create significant stress on the capacitor's dielectric system,
 > particularly at the edges of the metal plates.
 >
 > A sudden reversal of the capacitor's voltage is equivalent to the total
 > voltage swing between the initial and worst-case reverse voltage during
the
 > discharge. Excessive voltage reversals can induce partial breakdown of the
 > oil or the surface of the dielectric film, resulting in degradation of the
 > dielectric and premature failure. The estimated % voltage reversal can be
 > estimated if you know the "Q" of the circuit:
 >
 >     % Vrev = 1.57*Sqrt(1/(Q^2-1))
 >
 > A relatively low Q circuit will create a relatively high voltage reversal.
 > For example, a tank circuit with a  Q of only 15 (typical for a well
 > coupled TC primary) will apply a 90% voltage reversal across the tank cap.
 > If the initial voltage stored on the capacitor is Vo, the effective stress
 > "seen" by the dielectric will be  1.9*Vo.
 >
 > Because of voltage reversal limitations, commercial pulse caps used in
 > Tesla Coils are usually heavily derated. Knowledgeable coilers design
their
 > systems so that the peak DC voltage applied to the pulse cap is only ~50%
 > of the cap's DC voltage rating. A good rule of thumb is that a commercial
 > pulse cap used in a ringing tank circuit should have a DC rating that's at
 > least 3X the expected RMS voltage of the HV AC supply.
 >
 > This is why 35 kV Maxwell caps that are continually offered on eBay have
 > been observed to prematurely fail when used in 15 kV RMS systems:
 > http://cgi.ebay-dot-com/ws/eBayISAPI.dll?ViewItem&item=2595652751
 >
 > More robust 45 kV caps will survive in a similar environment:
 > http://cgi.ebay-dot-com/ws/eBayISAPI.dll?ViewItem&item=2596506003
 >
 > Since the problem is associated with oscillatory discharge in the tank
 > circuit, the method of charging the cap (AC or DC) is irrelevant.
 >
 > Best regards,
 >
 > -- Bert --
 > --
 > --------------------------------------------------------------------
 > We specialize in UNIQUE items! Coins shrunk by Ultrastrong Fields,
 > Lichtenberg Figures (electrical discharges in acrylic), & Scarce OOP
 > Technical Books. Stoneridge Engineering -- http://www.teslamania-dot-com
 > --------------------------------------------------------------------
 >
 > 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
 > >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.
 > >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.
 > >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
 > >
 > >.
 >
 >
 >
 >
 >
 >