[R-390] Capacitor replacement C609

[Bob Camp]

Hi

I got nailed on that one during a design review. 

It turns out that electrolytic caps are odd beasts. The do strange things when you run them well below their rated voltage. The chemistry that creates the insulation is the problem. It can "re grow' to the new voltage level. As it does this things can happen. Because of this, their reliability vs voltage looks more like a bathtub curve than the normal activation energy curve. 

Bottom line - you don't get any benefit from running an electrolytic below 1/2 it's rated voltage. You should not run them below 1/4 their rated voltage. 

Yes indeed I was more than a bit surprised when that was pointed out by the NASA guys. Turns out that it is true.

Bob
 
On Mar 4, 2013, at 6:48 AM, Charles P. Steinmetz <charles_steinmetz at lavabit.com> wrote:

> Drew wrote:
> 
>> Note that the voltage rating of C-609 need not be this high.  In a cathode-anode short condition in the associated tube, C-609 would only see a bit less than 4V. Normal operating voltage would be much less.  A 6V part would be more than adequate.
> 
> When you are considering only the voltage rating, that is true.  However, when you consider reliability, you find that there is very good reason to use a capacitor with a significantly higher voltage rating.
> 
> At elevated temperatures such as C609 experiences, electrolytics fail at much higher rates than they do at room temperature.  One must always choose a cap that is rated for the temperature it will experience; however, capacitor life is still radically reduced at high temperature even if the capacitor's temperature rating is not exceeded.  Using a capacitor with a significantly higher voltage rating than the actual voltage on the cap helps to mitigates this.
> 
> Best regards,
> 
> Charles
> 
> 
> 
> 
> 
> 
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