Re: [CAD_CAM_EDM_DRO] Re: Re: Power supply question.....

beer@... wrote:

> On 22 Apr, CAD_CAM_EDM_DRO@yahoogroups.com wrote:
> >
> > have seen several pwr supply questions about caps....and it seems to me
> > that if they are being used as brute filter caps people are using way to
> > low a working voltage....in this case i would feel much better with a
> > 80-100v rating....and yes you can put them in parallel
>
> This not good advice for electrolytics.
>
> There is some charge thing that goes on related to the chemistry of an
> electrolytic cap, such that a cap used below its rated voltage loses
> capacitance, almost in direct proportion to the rated voltage / applied
> voltage.
>
> That is, a 100 volt electrolytic cap used at 50 volts will, within a
> short period of time, have only half the capacitance.

No, this is not true. In fact, many switching power supplies deliberately use
caps with way over the needed voltage, because higher voltage caps are
less compact, and thereby dissipate heat better.

There WAS an effect, in the early days, where electrolytic caps would lose
the ability to take full voltage if operated at less than full voltage for a long
time. If you had a need to operate them at higher voltage than they were
used to, you would need to ramp the voltage up slowly or they might explode.
They are built with much better aluminum and dielectrics today, and these
problems are mostly of the past. I haven't seen more than one or two
electro. cap used within its ratings blow in many years, even powering up
gear that has been in storage for a decade.

Now, if they are run in very hot conditions, the electrolyte boils away, and
then the cap defilitely DOES lose capacitance, and may also suffer an
explosive end of life.

Jon