Re: Maxwell AC vs. DC ratings

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

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 --