capacitor value

For mass-produced consumer gear, no. For sattelites and A-bombs, yes.

Something tells me you are doing something else, so I cannot tell whether your rule results in over- design or not.

I mean in general for a reliable system, capacitor working at twice the rated voltage seems a good idea or not ?

This is a circuit for a water purification instrument for water plants.

For aluminum electrolytics, reliability goes down as the 5th power of the applied stress voltage, but reliability starts to be significantly degraded (~10%) only after an applied stress voltage of 0.35 of the rated value.

For tantalum electrolytics, reliability goes down as the 17th power of the applied stress voltage, but he reliabilty starts to be degraded significantly only after an applied stress voltage of more than 0.5 of the rated value (and gets much worse much faster after that compared to aluminum electrolytics).

(source: MIL-HDBK-217F notice 2)

I build avionics for launch vehicles and satellites and we typically derate tantalum electrolytics by a factor of 0.5 . Aluminum electrolytics in general, are not allowed for space flight except in special circumstances.

You are being very conservative, unless you need a capacitor to approach either its ripple current rating or its case temperature rating. I usually choose electrolytic caps rated one or two ranges above actual. E.g.. For 5 volts, 6.3 volt rated. For 15 volts, either 16 or 25 volt rated.

I have sometimes gone more conservative than this if I needed lowest possible internal resistance, or got too close to the ripple current rating. If I am expecting the cap to spend long periods of time near its maximum rated temperature, I may go as far as double the actual voltage, especially for very low voltages.

As to bypass capacitors, I often use ceramic surface mount caps that are rated for 2 or more times the actual voltage, because the large sizes are sometimes cheaper (you have to check) or because I want a bit lower ESR. I never use Z5U or Y5V types, because they lose a lot of their capacitance as you approach their rated voltage, and as they get hot. I like X5R and X7R.

Here is a guide for derating of capacitors and components:

No you are not over-designing, by derating your components ~ it's good design practive....

On 25 Feb 2005 12:31:38 -0800 in sci.electronics.basics, "aman" wrote msg :

Not a bad idea to do; if it's in error, it's in error on the safe side. It isn't going to cost a lot to implement. I'm currently working on a circuit using 3W resistors to handle a nominal 400mW. Curious, but not all that unusual. Thanks to "tlbs" for that informative post.

Shouldn't that be half the rated voltage?

Maybe he likes lots of electrolyte on his boards? ;)

What's this "boards"?

Usually shirt and glasses with me :-(

That's why I still bring linear supplies up with a variac! ;)

At least the glasses keep it out of your eyes. Buy a piece of 1/4" (or thicker) plexiglass to lay over something you don't trust before you fire it up.