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From: paulf@Jessica.stanford.edu (Paul Flaherty)
Newsgroups: comp.sys.mac
Subject: Re: crash and burn!!!  Why no fuse?
Message-ID: <3341@Portia.Stanford.EDU>
Date: 17 Aug 88 05:14:51 GMT
References: <561@calvin.EE.CORNELL.EDU> <27429@oliveb.olivetti.com>
Sender: news@Portia.Stanford.EDU
Reply-To: paulf@Jessica.stanford.edu (Paul Flaherty)
Organization: The Three Packeteers
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In article <27429@oliveb.olivetti.com> gnome@olivey.olivetti.com (Gary) writes:
>Fuses are very good in preventing certain types of failures from
>becoming fire hazards, but in general, they won't really protect
>a hell of a lot in a computer system.  The semiconductors will
>blow much faster than a fuse.

That's what crowbars are for.  The firing time of a modern SCR is such that 
most overvoltage conditions can be clamped off before they damage the 
semiconductor stuff.

A quick explaination for those who want to know:

	A crowbar circuit is designed to protect equipment from a failed
	power supply.  It works by detecting an overvoltage condition
	and then shorting out the power supply; the short draws enough current
	to blow a fuse.  The advantage of this technique is that an
	overvoltage condition lasts only for the trigger time of an SCR,
	which is small enough to protect most semicondutor circuitry.

A power supply without a crowbar and fuse is a risky proposition.

-=Paul Flaherty, N9FZX      | "There is no distinctly American criminal class
->paulf@shasta.Stanford.EDU | except Congress." -- Mark Twain