[ARC5] Cool New DC-DC Converter
Dennis Monticelli
dennis.monticelli at gmail.com
Sun Mar 18 12:31:41 EDT 2018
The failure of an electrolytic cap vs how close to its rating it is run is
a soft function. It's not like the sudden failure of a solid dielectric
cap. It takes a long time.
To get a decent lifetime a 'lytic should always be run under its rating,
just how much depends upon how much reliability you are after. HP was
conservative; their design guidelines were 60%. Most other makers were
less conservative in order to keep cost and size down. My vintage ham gear
seems to apply roughly 80% and sometimes more. They also don't respect
ripple heating enough, mounted high wattage bleeders close by and
increasing moved toward ESR hostile cap input filters with solid state
rectification.
The other factor is dry-out due to age and heating. The higher the ESR and
the higher the ripple, the more self heating. The more heating, the more
drying. The effect is a very slow runaway condition that eventually
manifests in cap failure.
I you buy a modern 105C-rated low-ESR cap for replacement and don't try to
squeeze the last volt out of it, it's going to reward you with long life.
They are built to survive in abusive PWM circuits. Our circuits are a
cakewalk by comparison.
Dennis AE6C
On Sun, Mar 18, 2018 at 9:12 AM, Bob kb8tq <kb8tq at n1k.org> wrote:
> Hi
>
> The real point is that a “250V” cap may or may not have a max rating of
> exactly 250.0V.
> It may indeed have an actual max voltage on that sample a bit higher than
> 250V. These
> days it’s very much a “who knows” sort of thing.
>
> Oddly enough the same rule that says “don’t use the last 10 %” also
> says “don’t use less
> than 60%” on an electrolytic cap. Learned that one in a design review ….
> on product headed
> for space no less …. gulp ….
>
> Bob
>
> > On Mar 18, 2018, at 12:06 PM, Scott Robinson <spr at earthlink.net> wrote:
> >
> > Well, a switching power supply designer who is even older than I am told
> me that if you avoid the using the last 10% or so of an electrolytic cap's
> voltage rating that it will last longer.
> >
> > FWIW,
> >
> > Scott Robinson
> >
> > On 3/18/18 7:33 AM, Bob kb8tq wrote:
> >> Hi
> >>
> >> In some cases, indeed a 250V cap is quite happy for a very long time at
> 300V. It’s not
> >> all “great balls of fire” sort of stuff ….
> >> Bob
>
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/arc5/attachments/20180318/0c56e5b2/attachment.html>
More information about the ARC5
mailing list