Re: Good & Bad Maxwell Caps, MMC questions?

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

Original poster: "Barton B. Anderson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <tesla123-at-pacbell-dot-net>

Hi All,

Bert's reply here is another excellent post that all should take note to. A
couple years ago, we
discussed this quite a bit (mostly during home brew cap discussions -
preMMC) and this became common
knowledge. But with so many new to the list, it's good Bert restated this
information.

I've been seeing the 30kV Maxwell cap threads and just assuming a few would
be mounted in series. I've
always liked the question: "how high should the cap voltage rating be?".
I use for minimum requirement  (Vrms * 1.414) * 2. This is "minimum"! You
would be safer changing the
last factor to 3. When I aquired my caps, I used a factor of 3 (60kV
rating). Higher is better (but the
cost goes up). 4 years of abuse, and still running strong.

Take care and thanks Bert,
Bart

Tesla list wrote:

> Original poster: "Bert Hickman by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <bert.hickman-at-aquila-dot-net>
>
> Bill,
>
> Sorry to hear about the premature death of your cap! I'll address your
> first set of questions, and will let others address the questions about
> MMC's. The particular Maxwell cap you have is indeed a "good" one - it's
> rated for 1000 pulses per second (PPS) with an expected lifetime of 200
> million pulses (90% survival), and is designed for 20% voltage reversal. It
> is an excellent choice for Tesla Coil use. However, you most likely
> overvolted it by driving it from a 15 kV NST bank. Why? The answer is a bit
> complex...
>
> Your NST's output voltage 15 kV RMS. This means that the peak voltage will
> be 1.414 times greater, or 21,200 volts assuming you don't aren't getting
> additional rise from resonance charging effects, and you're not purposely
> "boosting" the input voltage to 140 volts via your variac. Let's assume
> that the tank cap is charged to the peak voltage. When the main gap fires,
> the capacitor is discharged into the primary inductor, forming a "ringing"
> LC circuit with an effective Q of around 10-15 (assuming typical loading by
> the secondary). The circuit begins to oscillate, and one half cycle later
> the capacitor reverses polarity, hitting 85%-90% of its initial voltage,
> but with reversed polarity. The sudden voltage reversal severely stresses
> the capacitor's dielectric system - it's equivalent to applying the SUM of
> the initial voltage AND the reversed voltage - about 185% to 190% of the
> initial voltage!
>
> For your system, it's equivalent to applying 39 - 40 kV to your 30 kV cap.
> Even worse, these voltage reversals are quite rapid, occurring in tens of
> microseconds or less. The typical failure mode is corona breakdown, surface
> tracking, and eventual breakdown of the dielectric along the edges of the
> capacitor's plates (where the local E-fields are most intense). A useful
> rule of thumb is to take the RMS rating of the NST multiplied by 3 or 4
> times to arrive at the MINIMUM DC voltage rating for the commercial pulse
> capacitor. A 15 kV NST should have pulse caps rated for at least 45 kV and
> (better yet) 60 kV.
>
> Next time, try to get get two of these caps and connect them in series. Or
> construct an MMC... :^)
>
> Good luck!
>
> -- Bert --
> --
> Bert Hickman
> Stoneridge Engineering
> Email:    bert.hickman-at-aquila-dot-net
> Web Site: http://www.teslamania-dot-com
>
> Tesla list wrote:
> >
> > Original poster: "Bill Vanyo by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <vanyo-at-echoes-dot-net>
> >
> > I just blew my Maxwell cap (0.06uF, 30kV, catalog #37321 - type with
> > terminals on top).
> >
> > I was never sure whether this was a good cap for Tesla use.  I've seen
> > guidelines in the past that said those with the terminals on top
> > (instead of on opposite sides) generally didn't have a long life.  It
> > worked great for a while, but now it's dead (cap makes a sort of
> > fizzling sound when I first power up).
> >
> > Can anyone confirm that this is indeed not a good cap to use, or is this
> > what I could expect with any Maxwell cap?  And if mine was bad, which
> > are good?  The reason I'd like to get another Maxwell (if I can find a
> > more reliable one) is that it's "plug-and-play" - no assembly required,
> > and compact.
> >
> > Are MMC's really any better?  To what extent are they "self healing"?
> > To build a really sturdy MMC with the geek group caps (942c), for use
> > with 15/180 NST (three 15/60's), what is the minimum string length and
> > number of strings to handle the voltage and current they'll see?  I can
> > adjust upwards to get my desired capacitance, which is between 0.06uF
> > and 0.045uF --- 0.06 because that's what I was using and it worked,
> > 0.045 because that's the "right" LTR value for 15/180 with a static gap,
> > but I don't know (I haven't calculated yet) if my primary has enough
> > extra turns that I'd still be able to tap it in tune with 0.045 (I
> > currently tap around 7.5 with my biggest topload, and have 10 turns).  I
> > understand that the strings need to be of adequate length to handle the
> > voltage (though I'm not sure what that length is to get a long MMC life
> > with 15kV AC in tesla coil use).  Am I right that there is also some
> > minimum number of strings needed for adequate current handling (180ma,
> > in my case)?
> >
> > BTW (and I'm not sure, but I think I've heard this reported before), my
> > coil seemed to be performing better than it ever had shortly before it
> > died.
> >
> >         Thanks,
> >         -  Bill Vanyo