Re: Maxwell AC vs. DC ratings

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

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 --
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Tesla list wrote:
>Original poster: "Gerry Reynolds" <gerryreynolds-at-earthlink-dot-net>
>
>What's the deal with the commercial caps?  Aren't they essentially just a
>packaged string caps like we use in MMC's.  Is the VDC spec to include
>voltage reversal or is there a separate voltage reversal spec?
>Gerry R
>  > Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>
>  >
>  > I never realized these caps were so fragile!!  I sold those two I have
>from
>  > that Lichtenberg figure experiment (wouldn't have worked...) to Kreso and
>  > we were wondering about this.  I have always played with MMCs, not
>  > commercial caps ;-)
>  >
>  > Cheers,
>  >
>  >          Terry
>  >
>  >
>
>.