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

beer@... wrote:

> 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 )

Well, I'll have to look for this info. I do know there is a negative voltage
coefficient on some ceramic capacitors, as the voltage INCREASES the
capacitance goes down. Depending on heat (both internal and external)
electrolytic caps dry out and lose capacitance over time, but if you use
them conservatively (I use a lot of 105C rated caps at VASTLY lower
temperatures) they should last a long time. We even used 105C
aluminum electrolytics in vacuum without failure. I wasn't too sure about
that until we tried it.

> 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.

I have built gear with over 10,000 22 uF 16 V Panasonic mocrominiature caps,
running on either 5 or 12 V, and had no trouble with them whatsoever.

Jon