Hal wrote:
Does anybody know anything about the temperature coefficients of large caps?
I'm not interested in the frequency shift of the filter as the temperature
but the voltage shift due to a fixed charge as the capacitance changes.
The common rule of thumb is "thousands of ppm per
degree C" through their normal operating range
(i.e., neglecting low temperatures where they are much worse).
Here is what Cornell-Dubilier says about the
temperature coefficient of capacitance of aluminum electrolytics :
The capacitance varies with temperature. This variation itself is
dependent to a small extent on the rated voltage and capacitor
size. Capacitance increases less than 5% from 25 ºC to the high
temperature limit. For devices rated 40 ºC capacitance declines
up to 20% at 40 ºC for low-voltage units and up to 40% for high
voltage units. Most of the decline is between 20 ºC and 40 ºC.
For devices rated 55 ºC capacitance typically declines less than
10% at 40 ºC and less than 20% at 55 ºC.
(see
<http://www.google.com/url?sa=t&rct=j&q=aluminum+electrolytic+%22temperature+coefficient%22&source=web&cd=1&ved=0CEwQFjAA&url=http%3A%2F%2Fwww.cde.com%2Fcatalogs%2FAEappGUIDE.pdf&ei=MwMDT728Jcfo0QGggpm4Ag&usg=AFQjCNEZwwiXQYXxoXbC_on3M5MlOh3ypg>
at p. 7)
5% is 50k ppm; an 85C cap would thus change 50k
ppm over 60 degrees C, or a bit less than 1k ppm
per degree C. Based on the timing circuits I've
seen implemented with aluminum electrolytics, I'd
say CDE is being optimistic here.
For integrating in an environment where stability
of parts in 10e10 or better is desired, you are
likely to find that all of the other
imperfections of electrolytic capacitors
(leakage, noise, dielectric absorption, etc.)
will stop you well short of the goal, never mind
the tempco of capacitance. You might get by with wet-slug tantalums.
Best regards,
Charles
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