Rather than do all this guessing and wondering and trying to re-invent the
wheel, why don't you pose your questions and theories to the folks at CDE
or HEC or ATC: they're the ones that stamp the current ratings on their
parts, not us. Asking us what an "8.5 amp rating means to us" is
pointless. It means 8.5 amps. What does a resistor marked 1.2 K really
mean ? What does a fuse marked 3 A really mean ? Jeez.
Phil
At 10:12 AM 5/25/2002 -0400, you wrote:
>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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