Re: Re: Re: Power supply question.....

On 24 Apr, CAD_CAM_EDM_DRO@yahoogroups.com wrote:

> >
> > 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.

Jon;

I agree with you about designers using higher voltage, and hence
physically larger caps, for greater heat dissipation. The question is,
did they also use higher capacity, in expectation of an eventual
decrease in capacity, such as I described.

The effect I mentioned is documented in a Panasonic ? ( I think ) cap
design guide. I'll see if I can dig it out .. it's been a few years.

( The "short period of time" is, as I dimly recall, about a year )

> 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.

I still, with any cap old or new over 10,000 uf or so, connect it
to a power supply with a 100k - 1meg or so series resistor. Lets it
reform slowly and gives me an idea of leakage.

An old habit, perhaps, but not a bad one.

Alan

--

Alan Rothenbush | The Spartans do not ask the number of the
Academic Computing Services | enemy, only where they are.
Simon Fraser University |
Burnaby, B.C., Canada | Agix of Sparta