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From: chris@umcp-cs.UUCP (Chris Torek)
Newsgroups: net.analog
Subject: Re: Big Capacitors and now power supplies
Message-ID: <1701@umcp-cs.UUCP>
Date: Fri, 7-Dec-84 19:10:54 EST
Article-I.D.: umcp-cs.1701
Posted: Fri Dec  7 19:10:54 1984
Date-Received: Mon, 10-Dec-84 02:34:16 EST
References: <1812@sun.uucp> <1215@hou4b.UUCP> <6171@brl-tgr.ARPA> <10102@watmath.UUCP> <259@umd5.UUCP> <549@voder.UUCP>
Organization: U of Maryland, Computer Science Dept., College Park, MD
Lines: 24

Yes, but....

7805s are only rated at 1 amp.  Now I know you can pull about 1.5 amps
through most of these suckers before they melt (have *you* ever seen
the thermal overload protect *work*?), and that you can get 3 and 5 amp
versions of essentially the same thing, but you are still current
limited by the things.

Also, just having a 7805 doesn't do a whole lot of good.  You still
need quite a bit of capacitance, somewhere.  In fact, with most real
computers you need lots of caps sprinkled about the boards near each of
the ICs, 'cause when those suckers switch, they want CURRENT.  (Not
much actual power, but over a very short time period, so lotsa
current.)  If you have lots of switching, you also need lots of
capacitance BEHIND the regulator (that is on the rippled DC input side)
so that it can recharge all the little caps even if the AC cycle is
well below 5 volts (actually, with 7805s, you need about 6.5V min
before regulation, but that's beside the point).
-- 
(This line accidently left nonblank.)

In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (301) 454-7690
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