Path: utzoo!attcan!uunet!samsung!usc!ucsd!helios.ee.lbl.gov!ncis.tis.llnl.gov!lll-winken!tekbspa!optilink!elliott
From: elliott@optilink.UUCP (Paul Elliott x225)
Newsgroups: sci.electronics
Subject: Re: Need help to finalize my circuit
Summary: inductive load?
Message-ID: <2584@optilink.UUCP>
Date: 3 Nov 89 17:00:39 GMT
References: <1508@clyde.Concordia.CA>
Organization: Optilink Corporation, Petaluma, CA
Lines: 39

> Here's the funny part.  When I power the circuit with my bench power supply,
> it's fine.  Then when I try the AC adapter instead, the output of the 7805
> drops to only about 1V, instead of 5V!  Immediately, I thought that my AC
> adapter was the culprit, but when I go to measure the input to the 7805, it's
> a healthy 13V.  Nevertheless I tried another '9V' AC adapter that I happened
> 
>                                   ______
>                                   |7805|
>                        ~13Vdc     |    |   4.7Vdc
>                          |        ------     |
>                          v        | | |      v    _________
> 120V AC ---- AC adapter ----------| | |-----------|26 ohms|------
>                                   V G V           ---------      |
>                                   i n o                          |
>                                   n d u                         Gnd
>                                       t
> 

(with 26 Ohm load regulator works ok, with relay load it doesn't)

Just a quick suggestion:
  Yes, it may be the inductive load of the relays causing the regulator to
oscillate.  Try some output bypass caps (input bypass wouldn't hurt either).
These don't need to be huge; if they are too big you have problems with
transient fault conditions.  Try a 0.1uF ceramic cap in parallel with a
10uF tantalum cap at the output and input of the regulator (caps to ground, 
of course).  Please re-post when you find the solution; I am curious.

Good Hunting!

p.s.  I tried to e-mail, but couldn't find the path...

--- Paul Elliott


-- 
Paul M. Elliott      Optilink Corporation     (707) 795-9444
         {pyramid,pixar,tekbspa}!optilink!elliott
"I used to think I was indecisive, but now I'm not so sure."