Typically we use the 80% rule of thumb, where the actual working voltage of
the circuit is 80% of the voltage rating of the capacitor.  For
electrolytics this results in longer life expectancy.  This is why you can
use a 6.3V cap on a 5V power supply, a 16V cap on 12V, 4V cap on 3.3V, etc.
(see a pattern?).  For circuits that see repetitive peaks above the working
voltage, the peak voltage is used to determine the cap voltage rating.
If you're not sure, Russ' solution is a good one to use.

Roger M. Stoops, C.I.D., PCB Designer


Spectra Precision Inc./Trimble Navigation Ltd.
5475 Kellenburger Rd.
Dayton, OH 45424-1099 USA
Ph: 937.233.8921 or 937.233.4574 ext 288
Fax: 937.233.7511



                    David Cary
                    <dcary@BRUNSWICKOU        To:     [log in to unmask]
                    TDOOR.COM>                cc:
                    Sent by:                  Subject:     [DC] cap voltage rating
                    DesignerCouncil
                    <DesignerCouncil@I
                    PC.ORG>


                    10/23/00 08:47 PM
                    Please respond to
                    "DesignerCouncil
                    E-Mail Forum.";
                    Please respond to
                    David Cary





Dear designers,

We're switching from tantalum to multi-layer ceramic (MLC) capactors, and
we're
having a little debate over capacitor voltage ratings on MLC caps. (The
answer
to the same question about aluminum electrolytic would also be handy).

Some people say the "6 V" rating is for normal working conditions -- a "6
V" cap
works fine on the output of a 5V regulator; higher voltage ratings there
would
be overkill.

Some people say that the "6 V" rating is for marketing purposes -- it can
hold
that absolute maximum voltage for a little while, but long-term reliability
is
shot -- for example, the cap on the output of a 5 V regulator really needs
to
have a minimum rating of at least 10 V.

Anyone have any pointers to good reliability studies ?

--
David Cary

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