Re: power supply design


"Bob Monsen" <rcsurname@xxxxxxxxxxx> wrote in message
news:pan.2006.01.05.05.59.05.345822@xxxxxxxxxxxxxx

Most of a very good post brutally snipped ;-)

> Using a cap that is bigger than you need is not really an issue, except
> that it'll store more energy (make sure you use a bleed resistor of like
> 100k to keep it from storing power over long periods). Also, the cost of
> caps goes up quicker than the rating.

Nit: More is Less!

The larger capacitors will discharge less over each 50 Hz cycle and thus
shorten the conduction time of the rectifier diodes. This causes the peak
current in the rectifiers/transformer to rise as the average current remains
constant. The rectifier- and conduction- losses in the transformer are
proportional to the current *squared* .... so the bigger the cap, the worse
the load on those components. Larger ripple will of course cause losses in
the regulator but here the peak current is much lower so it does not matter
so much.

So, the O.P. will want the *smallest* cap he/she/it can get away with; your
formulae answer the "how small is that?" question.

I once had some old papers, i.e. 50'ies, on the lost design equations for
linear power supples. Maybe they are on the net by the mercy of some
misconfiguration of a $$$PAY$$$ per view server where all research
litterature are now put to eternal sleep?

The radio amateurs handbook (ARRL??) - I Think it is - is strong on
practical stuff too.

But, good post.


.

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