Re: Maxwell AC vs. DC ratings

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

Original poster: "Crow Leader" <tesla-at-lists.symmetric-dot-net> 

They are much sturdier, and fully impregnated to prevent corona from eating
the dielectric up. Metallized dry caps are almost toys compared to heavy
duty film caps.

KEN


----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Monday, April 05, 2004 8:29 AM
Subject: Re: Maxwell AC vs. DC ratings


 > Original poster: "Gerry Reynolds" <gerryreynolds-at-earthlink-dot-net>
 >
 > Hi Bert,
 >
 > Thankyou for your in depth explanation.  I presume that the individual
caps
 > within the pulse cap are different from the CDE caps that we use in MMC's
or
 > are there just fewer number of them?
 >
 > Gerry R
 >
 >
 >  > Original poster: Bert Hickman <bert.hickman-at-aquila-dot-net>
 >  >
 >  > Gerry,
 >  >
 >  > I'm reminded of Mac vs PC or Linux vs Windows debates...  :^)
 >  >
 >  > It's actually very simple - it's up to the designer to understand the
 > specs
 >  > for the caps he uses in order to apply them properly. Most commercial
 > pulse
 >  > caps were initially designed and specified for use in Pulse Forming
 >  > Networks (PFN's) since that's where MOST of them ultimately end up.
 >  > Capacitors within a PFN normally see relatively little ringing.
Commercial
 >  > pulse caps have a maximum DC voltage spec and a Percent Voltage
Reversal
 >  > (%VR) spec. For most pulse caps the %VR is 10-20% of the rated DC
voltage.
 >  > This means that the peak-to-peak voltage swing for one of these caps in
 >  > your Tesla Coil tank circuit should not exceed 110-120% of the caps
 > maximum
 >  > DC voltage.
 >  >
 >  > You can find commercial caps that are specifically intended for high-Q
 >  > ringing and RF applications such as previously offered Condenser
Products
 >  > Tesla Caps and Plastic Capacitors' BNZ series. However, the dielectric
 >  > systems in these caps is proportionately thicker and the physical size
of
 >  > the caps is proportionately larger than pulse caps typically used in
 > PFN's.
 >  >
 >  > Since the overwhelming majority of surplus pulse caps came out of PFN's
 >  > used in lasers, radar, LINACs, or pulsed X-ray systems, it's quite
natural
 >  > for these caps to have low %VR ratings. Many inexperienced Tesla Coil
 >  > builders mistakenly assume that they can charge these caps to their
full
 >  > rated DC voltage, and they wonder why the caps prematurely fail.
 >  >
 >  > It is ultimately up to the Tesla Coil designer to appropriately derate
 >  > their capacitor so the dielectric and metals systems are kept within
their
 >  > design specifications. In the high-Q oscillatory environment of a Tesla
 >  > Coil this simply means that the maximum peak discharge voltage should
be
 >  > limited to ~50-60% of the pulse cap's rated DC voltage. This is not
new.
 >  > It's how capacitors have been specified for at least 70+ years. And,
this
 >  > derating also applies to new pulse caps that use "self healing"
 >  > metallization systems as well.
 >  >
 >  > Does this mean that commercial caps are "more fragile" than MMC's?
 >  >
 >  > No.
 >  >
 >  > While commercial pulse caps shouldn't be overvolted, most pulse caps
can
 >  > easily handle repetitive peak currents that would cause severe
heartburn
 > or
 >  > even vaporize the leads in an MMC array. Since MMC's can be cooked by
 >  > excessive current, does this mean that MMC's are "more fragile" than
 >  > commercial caps?
 >  >
 >  > No.
 >  >
 >  > It really affirms that ANY cap, including MMC's, must be used with a
clear
 >  > understanding of its voltage and current limitations so that, in your
 >  > intended application, you can obtain the desired design life.
 >  >
 >  > I suspect that one of the main differences between an MMC and a pulse
cap
 >  > is that snubber capacitor manufacturers know that their intended
 >  > applications can be fairly oscillatory. If this is the case, they'll
 > "build
 >  > in" more design margin by using a somewhat thicker dielectric system
than
 > a
 >  > comparably rated DC or PFN cap. By keeping the maximum voltage just
below
 >  > the point of corona inception (even under large voltage reversals), and
by
 >  > using self healing metal systems, they create a robust little cap that
 >  > doesn't require the degree of voltage derating necessary for a typical
 >  > pulse cap... as long as you don't cook it with excessive tank current.
:^)
 >  >
 >  > Best regards,
 >  >
 >  > -- Bert --
 >
 >
 >