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Re: Possible MMC?

Hi Andy,

There is one thing I would worry about.  Pulse current ratings.  Since
these caps seem to have low dissipation, the RMS rating is probably fine.
However, the caps need to withstand very high current (~200amp) pulses.
Pulse rated caps have very low inductance and the internal foil layers are
all continuously attached to a heavy end lead connection.  These heavy
connections allow very high current pulses to pass without burning out.
Many caps that have otherwise good sounding specifications my have thin or
weak lead connections that will quickly fail during high current pulses. 

You might try to find a dV/dT, dI/dT, or repetitive pulse current rating
for these caps.  If you could get a few, you could also tear one apart to
see if it has the needed heavy metal and low inductance connections.  They
may be able to tell you these things and the internal inductance and series
if you call them.

Judging by the pictures on the web site, I would worry a lot about the lead
connections and other design issues associated with these audio poly caps.
These caps seem to be rather low volume custom devices for the high end
audio industry.  I wonder if they have the knowledge and quality control of
a Panasonic, Phillips, or WIMA when it comes to the really heavy duty cap
applications.  A modern EMMC may put 20kV across only 4 mil of "perfect"
polypropylene.  That is not easy to do without really knowing many tricks.
That is also what separates the places that make thousands of capacitors a
year from the places that make hundreds of millions...  I also wonder if
they have the complex "self-healing" properties of the high voltage pulse
caps...

Best to be cautious.  High energy poly caps or a very specialized niche.
There are a lot of manufacturers that have it figured out.  However, there
are many that do not...

Cheers,

	Terry



At , you wrote:
>	I am planning on building an MMC capacitor for my new Tesla Coil, and I
>came across the following on the capacitors-dot-com web site.  It seems pretty
>good to me, with fairly high voltages and larger capacitance values.  The
>following is a quote directly from their web site.  Since I am not too
>knowledgeable about the specific ratings, and what is best for coiling, I
>was hoping someone could give me a good run-down on these.
>
>Series PPMT
>AudioCap Metalized High Voltage Capacitor PPMT
>
>The PPMT AudioCap capacitors are metalized polypropylene, available in
>voltages high enough to be ideal for all high-voltage applications, such as
>power supplies for tube electronics, high-voltage bypassing, high-voltage
>coupling and electrostatic loudspeaker crossovers. They exhibit excellent
>electrical and environmental characteristics, and will provide high quality
>performance and long life at a reasonable price. The gold-plated OFHC leads
>provide long-term high reliability, superior to silver wires, owing to gold’
>s non-oxidizing properties.
>Capacitance Values:  .10 to 2.0 mfd
>Tolerances:  +/- 10%. Also: 5%, 2%, 1%
>Working Voltages:  1000, 1500 V: 2400V; call for information on lower
>voltages
>Test Volts:  2.0 x rated voltage for one minute.
>Dissipation Factor:  < 0.05% 1.0 - 10.0 mfd -at-1kHz -at- 25o C
>< 0.05% > 10.0 mfd -at- 120 Hz -at- 25o C
>< 0.03% .1 - 1.0 mfd -at- 1kHz -at- 25o C
>< 0.02% .001 - .1 mfd -at- 1kHz -at- 25o C
>Insulation Resistance: >300,000 megohm/mfd
>Operating Temperature Range: - 55o C to 105o C
>Temperature Coefficient: See Capacitance Change vs. Temperature Chart
>Life Tests: Rated voltage -at-85 degrees C for 1000 hrs.
>Vibration: Will Meet Mil-C 19978B
>Lead Material: Gold-plated OFHC
>
>Thank You,
>
>Andy Cleary
>