Newsgroups: sci.electronics
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: Lowering power-supply impedances (Re: Homebrew audio equipment)
Message-ID: <1988Aug3.162042.10476@utzoo.uucp>
Organization: U of Toronto Zoology
References: <1075@gethen.UUCP> <1320005@hpfclm.HP.COM>
Date: Wed, 3 Aug 88 16:20:42 GMT

In article <1320005@hpfclm.HP.COM> myers@hpfclm.HP.COM (Bob Myers) writes:
>>	As the ripple gets smaller the 'conducting-time' (forward bias)
>>	of the rectifying diodes gets smaller. If the power to the 
>>	load stays the same then the current during foward bias gets bigger.
>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]

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.  Current flows through the rectifiers
only when the voltage behind them exceeds that on the filter capacitor
(ignoring their forward voltage requirement).  With big capacitors that
don't drop much in voltage between half-cycles, flow can occur only at
the very peak of the wave, so all the current needed by the load
throughout the half-cycle has to come through the rectifiers during a
brief period at peak.  Hence the need for bigger diodes, or else some
more sophisticated regulator downstream so that the capacitors can be
made smaller.
-- 
MSDOS is not dead, it just     |     Henry Spencer at U of Toronto Zoology
smells that way.               | uunet!mnetor!utzoo!henry henry@zoo.toronto.edu