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From: tell@oscar.cs.unc.edu (Stephen Tell)
Newsgroups: sci.electronics
Subject: Re: Need help to finalize my circuit
Summary: Do you have filter & bypass capacitors around the 7805?
Message-ID: <10351@thorin.cs.unc.edu>
Date: 2 Nov 89 22:10:23 GMT
References: <1508@clyde.Concordia.CA>
Sender: news@thorin.cs.unc.edu
Reply-To: tell@oscar.cs.unc.edu (Stephen Tell)
Organization: University Of North Carolina, Chapel Hill
Lines: 54

In article <1508@clyde.Concordia.CA> jean@maxwell.Concordia.Ca ( JEAN GOULET ) writes:
>I've been working on this circuit for a while now, and I thought I was almost
>finished, but it turns out that what I thought would be the easiest part of
>the design has  become one of the strangest problems I've had.  Here's what
>it's about.
Power supplies are never as easy as they seem.

>                                  ______
>                                  |7805|
>                       ~13Vdc     |    |   4.7Vdc
>                         |        ------     |
>                         v        | | |      v    _________
>120V AC ---- AC adapter ----------| | |-----------|26 ohms|------
>                                  V G V           ---------      |
>                                  i n o                          |
>                                  n d u                         Gnd
>                                      t
>
>Now if you replace the 26 ohms with the relay coil terminals, Vout drops
>to about 1V.  What's wrong here?  Does it have anything to do with the 
>inductive nature of the coil?  Is the coil resistance that I measure with my
>multimeter a bogus value?
>                                             Jean Goulet
>                                             Electrical Engineering
>                                             Class of '89
>                                             Concordia University
>                                             Montreal, Canada

These 3-terminal regulators usually are much happier if you put
capacitors to ground at the input and output terminals, physicaly close
to the 7805 itself.
Try somthing like 1 to 10 microfarads, preferably tantalum-type,
at the output and a few hundred uF at the input.  A long time ago
I noticed this kind of behavior, and now always use said capacitors
in conjunction with a 78xx regulators.  I think the things can even
oscillate without the capacitors.
Likely it is related to the inductance of the coil, then.

Add to this the fact that your "9v" ac adaptor probably has the smallest
(read cheapest) filter capacitor across its output that they could get
away with.
Just for kicks, try looking at the input and output with an oscilliscope.
I'll bet its not the pure DC you expect to see.

I decided to post this in hopes that someone could follow up with
the real theory on this; I only know it works.
They don't teach you things like this in EE classes....

Steve
--------------------------------------------------------------------
Steve Tell					tell@cs.unc.edu
CS Grad Student, UNC Chapel Hill.
EE, Duke University school of Engineering, class of 1989.
Former video guy, Duke Union Community Television, Durham, NC.