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From: dbc@tc.fluke.COM (Dan Carson)
Newsgroups: sci.electronics
Subject: Re: 7805 woes
Keywords: Latch off
Message-ID: <12236@fluke.COM>
Date: 6 Nov 89 19:23:29 GMT
Sender: news@tc.fluke.COM
Organization: John Fluke Mfg. Co., Inc., Everett, WA
Lines: 21


                                  ______
                                  |7805|
                       ~13Vdc     |    |   4.7Vdc
                         |        ------     |
                         v        | | |      v    _________
120V AC ---- AC adapter ----------| | |-----------|26 ohms|------
                                  V G V     |     ---------      |
                                  i n o     +-------|<-----------|
                                  n d u            K  A         Gnd
                                      t

As has been suggested, 10uF caps on the input and output of the 7805 might
fix your problem.  If that doesn't work, try a reverse-biased diode from
the output of the 7805 to ground.  I've had problems with various 3 terminal
regulators latching off.  It's usually caused by a glitch during power up
which pulls the ouput below ground.  The inductance of the relay coil might
provide just such a glitch.

Dan Carson
John Fluke Mfg.