An important additional aspect - you didn't ask this but it's vital.
Electrolytic Capacitor lifetime drops by approx a factor of 2 times
(pessimistic rule of thumb) for every 10 degree Celsius temperature
rise. If you are consistently running at 80C you MUST use 105 degree
C capacitors or your lifetime will be minimal. Specs vary but
typically caps are specified for eg 1000 hours at rated temperature
on the assumption that you will run them MUCH lower than this. eg an
85C part run at 45C is 40 degrees lower so lives 2^4 = 16 times
longer = 16000 hours. Actually it would be somewhat better than this.
Pseudo Randomly opening my Philips cap book (well thumbed page) gives
specs for their 037 series which are common.
85C    2000 hours
40C    70000 hours
.
Another CRITICAL fact is that if the capacitor has no voltage applied
or low voltage compared to its rating it will die much quicker at
elevated temperature. The above 037 caps are rated for 500 hours at
85C when not powered !!!!. this may not be intuitive!




-----Original Message-----
From: Lawrence Lile <lilel@toastmaster.com>
To: PICLIST@MITVMA.MIT.EDU <PICLIST@MITVMA.MIT.EDU>
Date: Wednesday, September 16, 1998 5:20 AM
Subject: Electrolytic Capacitors and Heat [OT]


>Ok, so this time it doesn't have a PIC in it.  But my prototype IS
on
>the same table as my ICEPIC, so it's remotely related.....
>
>
>I'm having trouble with an electrolytic capacitor (100uF, 85 C,
>50WVDC) that drops in  value as it gets hot.  It is in an RC
>timing circuit, so a change in capacitance is a problem.  The
circuit
>has to work in an environment that starts at room temperature, and
>climbs to 80C.  I don't know how mush the capacitor drops, but the
>slope of the voltage on the capacitor gets steeper the hotter it
>gets.
>
>The question is:  Do all electrolytic capacitors change capacitance
>as they get hot?  If so, how much and what direction?  Is there a
>type that is more stable at higher temperatures?
>-- Lawrence Lile
>
>     "The ideal design has zero parts"  -
>            (attributed to Harold Hallikainen)
>
>Download AutoCad blocks for electrical drafting at:
>http://home1.gte.net/llile/index.htm
>