Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!mailrus!ames!ucsd!ucsdhub!hp-sdd!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: <1320005@hpfclm.HP.COM>
Date: 28 Jul 88 18:00:36 GMT
References: <1075@gethen.UUCP>
Organization: Hewlett Packard -- Fort Collins, CO
Lines: 42

>	I've got one comment on power-amps when rolling-your-own.
>	In the power supply, the bigger the caps you use the better
>	the ripple filtering (60Hz stuff) BUT ALSO the bigger the
>	current surge through your rectifying diodes.
>	
>	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.
>	(lots bigger, the average current [over time] must be constant 
>	for a given load.)
>	
>	So, for mongo(big) use mongo rectifiers.
>	
>       just a thought. :)
>	
>
>	dsm
>	


While this will work, there are better ways to handle the inrush current
than overly-overrating your diodes (although a healthy bit of margin in the
current-handling is always a good idea!).  The biggest problem is that,
while the diodes may now handle the current, you're still gonna need a
MUCH larger fuse than your power supply really should have - and you ARE
going to put a fuse on it, aren't you?

Two common means of taking care of this problem: first and simplest, place
an approriately-selected thermistor in the path of the inrush current.  The
initial current is limited, but then the resistance drops off so that you
can get enough current through.  It works, but there's a better - and more
complex - method.

The second method is to simply place a power resistor in the path of the
current, again to keep the inrush current down to some reasonable value.
Add a relay on a time-delay (switch the relay via a transistor with some
suitable R-C delay on it) so that the relay will short the inrush limiting
resistor after the main caps have charged up.

Bob M.

hplabs!hpfcla!myers