Re: gEDA-user: Alarm clocks and switching regulators

[Dan McMahill <dan@xxxxxxxxxxxx>]

DJ Delorie wrote:
>> My bulk capacitors are Al electrolytics.  Speaking of board real
>> estate -- yikes!  But how else am I going to get ~700uF of capacitance
>> to deal with the 120Hz ripple?  (Without spending $30 each on
>> high-tech capacitors, that is.)
> 
> Panasonic FC series caps.  680uF 63v is available in 12.5mm diameter
> (35 mm tall!)  size.  Rated to 105C, they're only $1.35 at Digikey.
> 
> I've got four of their 820uF/63v caps on my furnace board.  It's like
> a 3 second UPS :-)
> 

The 105C is a fairly big deal.  Is that a 5,000 hour lifetime part?  The 
lifetime of a electrolytic capacitor is typically given for the rated 
temperature and doubles for every 10 degree C reduction in temperature. 
  so a 105C cap with 5,000 hour lifetime actually has 4x the lifetime of 
an 85C cap with 5,000 hour lifetime.  Lets not even go near 2,000 hour 
85C caps please.

Note that 5,000 hours is only 208 days or 0.57 years.  To get to a 10 
year life, you've got to drop the temperature by a little over 40 
degrees.  So a 105C cap should only run in the low 60 degree C range and 
a 85C cap in the low 40 degree C range.  And if you decided to use an 
85C 2k hour part, you're down at a miserable 30 degrees C.  i.e. room 
temperature for the capacitor ambient.  Not terrible, but marginal even 
for an alarm clock if you ask me.  Of course I like to keep my 
electronics for a long time...

-Dan



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