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-   -   Capacitor Voltage Rating (http://www.physicsforums.com/showthread.php?t=165722)

j777 Apr14-07 04:33 PM

Capacitor Voltage Rating
 
Hi,

What is the rule of thumb for choosing the voltage rating of a capacitor? Should it be twice the operating voltage or is this overkill?

Thanks

Averagesupernova Apr14-07 05:30 PM

It really depends on the application. Double the voltage may be a good rule of thumb, but you may want to consider what the voltage MAY go to in case of a circuit malfunction. Also, a consideration may be the extra cost involved using a higher rated capacitor vs. the probability of an actual failure of the capacitor.

waht Apr14-07 05:37 PM

In most capacitors the plates are soaked up in various chemicals, and also are electroplated. So if you exceed the max voltage, a chemical reaction takes place that can eat away the plates, make them rough and punch holes.

So obviously the damage is proportional to voltage exceeded, and time. It's all different for every cap.

es1 Apr14-07 09:42 PM

This is a tough one to give a general answer to. Basically it is vendor and material specific so consult the datasheet of your part to be sure. In practice I usually use 2/3 during the design phase then check and optimize (for size and cost) later when doing placement and part consolidation.

To find out more do a google for "capacitor derating". I got some good hits, like this one
http://www.nemcocaps.com/PDF/TIPSDESIGNUSE.pdf

es1 Apr14-07 09:46 PM

Oh, by 2/3 I meant the derating value

Vmax = Vcap_rating * 2/3


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