Thanks to all who finally came forward with some data to look at. The bad
news is that the data is somewhat confusing. I am inherently suspicious of
data that does not 'compute'. This is the result of a life-long experience in
the electronics business, in which I have seen an enormous amount of voodoo
in mfrs. specs. The reasons are numerous and variable, but suffice it to say
that in many companies the documentation is prepared by people who have
little or no connection with the engineers who did the design.
I do not wish to step into yet another quagmire here. I would like to back up
a little and try to return to the essential point that I was originally
trying to address in this whole discussion of mica capacitors. I would ask
all of you to re-consider it from this viewpoint.
First of all, just exactly what does it mean for a capacitor to have a
current rating? I can already see signs of opinions lurking out there that if
you accidentally run 8.6A through your blocking cap rated at 8.5A, there will
be a meltdown of your beautiful 4cX00000000 amplifier.
My main focus, and it should be yours also, is on the Q, loss factor, power
factor, dissipation factor, or whatever term you want to put on it - they are
all the same thing. Dielectrics have losses due to molecular processes that
accompany the polarization that produces capacitance in the first place. This
loss shows up, from a circuit analaysis view, as a loss resistance in series
or parallel with the 'pure' capacitance. When your HF plate or antenna
current passes through this resistance, it will produce I-squared-R loss and
heat up just like a resistor. Our main goal in all of this capacitor voodoo
is to minimize this loss - plain and simple.
The only way that the manufacturers could REALLY help us would be to publish
some kind of a loss-factor vs. frequency data. From that we could calculate
the amount of loss that we have to justify and/or dispose of in our thermal
design.
I have seen a trend among contemporary manufacturers, especially in Europe,
to rate capacitors in KVAR, which is one step closer to what we want. But it
still doesn't give us the whole picture, especially any frequency
dependencies.
This is getting longer than I planned. My overall point in this posting is
that I would really like to hear from others what they are looking for (with
regard to mica capacitor ratings.) What does it mean to you when some mfr.
stamps 8.5A on the package?
What kind of data would make you feel confident in your component selection
process? What would you do if you had an unknown capacitor that you really
wanted to use but didn't know all of its specs?
Eric von Valtier K8LV
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