capacitor value

I use a simple rule that if I use capacitor in main supply filter
circuit of say
24V DC i use it at atleast twice the rated voltage(48V).

I use the same rule for the bypass capacitors. Does this make sense ?

 

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.

 

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.

 

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.

 

Shouldn't that be half the rated voltage?

 

Maybe he likes lots of electrolyte on his boards?

 

Here is a guide for derating of capacitors and components:
http://www.interfacebus.com/Component_Derating_Guide_line.html

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

 

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.

 

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.