Path: utzoo!attcan!uunet!husc6!bloom-beacon!mit-eddie!uw-beaver!teknowledge-vaxc!sri-unix!hplabs!hpda!hpcuhb!hp-sde!hpfcdc!hpfclm!myers
From: myers@hpfclm.HP.COM (Bob Myers)
Newsgroups: sci.electronics
Subject: Re: Lowering power-supply impedances (Re: Homebrew audio equipment)
Message-ID: <1320009@hpfclm.HP.COM>
Date: 5 Aug 88 19:03:05 GMT
References: <4740@pasteur.Berkeley.EDU>
Organization: Hewlett Packard -- Fort Collins, CO
Lines: 59


My stuff:

>>While this will work, there are better ways to handle the inrush
current
>>than overly-overrating your diodes...
>>Two common means of taking care of this problem: ...thermistor...
power
>> resistor [shorted by relay]

 Henry's stuff:

>I  think  you've  misunderstood;  the problem is not  the  inrush 
>current at powerup (although that is something to watch) but the 
>current drawn on each half-cycle of the AC afterward.... Hence 
>the need for bigger diodes, 
>-- 
>Henry Spencer at U of Toronto Zoology

Yep - I blew it.  As Henry correctly pointed out (as have several others
who sent E-mail), I read the original posting too quickly, and went off
on a tangent about *turn-on* inrush.  Which IS, by the by, something to
be concerned about, and I'll refer you back to my original mistaken
response for further details.  Forgive me - I've been working recently on
some power-supply-blow-fuse-at-turn-on complaints, and my mindset was
in the wrong direction (and due to the sluggish response of my mind, loathe
to change direction once set).

Having said that, I note that there have in the interim been some excellent
responses on the problem which is actually at hand, but will go ahead and
add my (course-corrected) $0.02 to them:

One of the mailed comments I received (and I'm sorry I can't recall the
author right now, because it was a very well-thought-out response) pointed
out the actual problem being discussed in terms of the capacitor's
charge/discharge action on alternate half-cycles.  While this is certainly
one correct way to look at it, I want to point out that for analysis purposes,
it may be more helpful to consider the output of a power supply as being the
superposition of two sources - a DC source (the desired DC output) plus an
AC source (the ripple).  Your mission is to design a circuit which minimizes
the AC voltage across the (assumed) resistive load.  This can be done through
a combination of BMF (a technical term, the meaning of which can be found
in various electronics dictionaries :-)) capacitors and inductors (the 
filter).  The filter circuit is amenable to simple AC analysis.  (The DC
analysis is even simpler, of course, except for some possible problems shown
in the following....).  As a previous poster pointed out, choke (inductor)-
input filters hold down the repetitive inrush to the filter caps.  The
problem is that they're big, heavy, and can be more difficult to locate
than big caps.  Fortunately, they're not too hard to BUILD if you get
desperate.

When performing this analysis, though, don't forget that there ain't no such
thing as a "perfect" capacitor or inductor.  One important characteristic
(which should be readily available from the capacitor catalogs) is the
ESR of your BMF electrolytics.

And you ARE going to put a fuse on this sucker, aren't you?

Bob M.