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From: gene@cooper.UUCP (Gene from EK Enterprises)
Newsgroups: sci.electronics
Subject: Re: Choosing Cap Size in Power Supply
Message-ID: <981@cooper.UUCP>
Date: Wed, 29-Jul-87 15:18:00 EDT
Article-I.D.: cooper.981
Posted: Wed Jul 29 15:18:00 1987
Date-Received: Sat, 1-Aug-87 11:11:15 EDT
References: <935@sdcc7.ucsd.EDU>
Organization: The Cooper Union (NY, NY)
Lines: 39

in article <935@sdcc7.ucsd.EDU>, muller@sdcc7.ucsd.EDU (Keith Muller) says:
> The large capacitors are placed on the raw DC supply before the regulator
> circuit. Smaller ones are place on the other side. Although the capacitors
> do lower ripple, they have terrible response to dynamic load swings (which
> is the job of the active regulator).

In general, when using a 3-terminal regulator like the 7[89].. series (7805,
7915, 7812, etc.), I *always* use about 5000 uF/A between the bridge rectifier
and the regulator, and at least 10 uF/A at the output, and always in
conjunction with a 0.1 uF ceramic bypass cap. In addition, *ALWAYS* put a
blocking diode across the input and output terminals of the regulator; these
buggers just hate it when the output voltage exceeds the input voltage (for
example, on powerdown).


	                            --------|<---------
			   ______   |   __________    |
	----| ||| |--------+    +---*---+  7xxx  +----*----O  +
	    | ||| |        |~  +|   |   |________|    |
	    | ||| |        |    |  ===       |       ===
	    | ||| |        |~  -|   |        |        |
	----| ||| |--------+____+---*--------*--------*----O  -

					   diode
	      xfmr         bridge  cap      reg      cap


					Gene

					...!ihnp4!philabs!phri!cooper!gene


	"If you think I'll sit around as the world goes by,
	 You're thinkin' like a fool 'cause it's case of do or die.
	 Out there is a fortune waitin' to be had.
	 You think I'll let it go? You're mad!
	 You got another thing comin'!"

			- Robert John Aurthur Halford