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Re: gEDA-user: Capacitors

Stuart Brorson wrote:
The only thing that bothers me about electrolytics is that they are only guaranteed to work for 2000 hours, typically. But if you choose higher temperature and voltage ratings than needed, then you can get much longer life. 


The lifetime is specified at the upper limit temp.  That is, an 85C
rated electrolytic is gauranteed to live for its rated lifetime when
run under bias at 85C.  It may live longer ...

The lifetime follows an Arrhenius activation law, so running the caps
at lower temps dramatically increases the lifetime. That is, the
lifetime increases by orders of magnitude as the operating temperature
decreases.

You can get different temp ratings for the caps. For cheap consumer
junk, 85C is a not uncommon rating. But for industrial equipment,
you're better off getting electrolytics rated 105C or better. Due to
the above activation law, the 105C cap run at room temperature will
have *much* longer lifetime than the 85C cap run at room temperature.

yep. You get something like a factor of 2 for every 10 deg. C so those 105C caps out of the gate have 4x the lifetime. You can also find electrolytics wih a rated lifetime of 5000 hours at 105 which if of course another 2.5x over the 2000 hour ones. Don't forget if you're using the electrolytics in a power supply filtering app that the ripple current is rather important. Ripple current * ESR = power = heat = lower life.

To the person who mentioned dV/dt on tantalums, that is true. In fact you usually want to limit the working voltage to 1/2 the rated voltage because of dV/dt problems. Basically defects in the dielectric cause an uneven internal E field which translates to stressing some portions of the dielectric more than other portions under transient conditions.

-Dan