[time-nuts] OCXO and fluctuations after EFC adjustment

[jimlux]

On 4/11/20 8:22 PM, Ben Bradley wrote:
=
> More recently, I saw this Kemet presentation on Digikey about tantalum
> capacitors. Certainly for aluminum electrolytic capacitors, the rated
> voltage is "the rated voltage" and as long as the capacitor never goes
> ABOVE that voltage (and has no overcurrent that would heat it up,
> etc.), the cap is good for its combination of temperature and lifetime
> rating. I (and as far as I know, everone I've known) assumed this was
> the same for tantalums, but it appears that's not the case (this
> presentation mentions several failure causes and shows how they are
> multiplicative). As you go from 1/2 rated voltage to full rated
> voltage, the chances of a tantalum failing goes up substantially. The
> implied rule seems to be for maximum reliability, don't operate a
> tantalum above HALF the rated voltage. I'd heard a lot of anecdotal
> things about tantalums suddenly shorting out for this or that reason,
> but hadn't heard of this, and here it is straight from the
> manufacturer.
> https://www.digikey.com/en/ptm/k/kemet/derating-guidelines-for-surface-mount-tantalum-capacitors/tutorial
> 

reading the data sheet and ap notes very carefully is important. A lot 
of times, the ratings are for a situation which actually doesn't occur 
in the application.  That is, you have some current, which leads to some 
heating, which puts the internal temperature at some number other than 
25C, at which the rating is specified.

When it comes to pulse capacitors with polypropylene dielectric, the 
ratings go as V^7.5  - that is, doubling the voltage reduces the life by 
a factor of 181.

tungsten filaments go as something like the 12th power.