Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!iuvax!cica!tut.cis.ohio-state.edu!cs.utexas.edu!uwm.edu!lll-winken!tekbspa!optilink!elliott
From: elliott@optilink.UUCP (Paul Elliott x225)
Newsgroups: sci.electronics
Subject: Re: Need help to finalize my circuit
Summary: load current
Message-ID: <2618@optilink.UUCP>
Date: 9 Nov 89 17:20:00 GMT
References: <1508@clyde.Concordia.CA> <10040029@hprmokg.HP.COM> <1989Nov8.053847.9348@utzoo.uucp>
Organization: Optilink Corporation, Petaluma, CA
Lines: 43

In article <1989Nov8.053847.9348@utzoo.uucp>, henry@utzoo.uucp (Henry Spencer) writes:
> In article <10040029@hprmokg.HP.COM> barry@hprmokg.HP.COM (Barry Fowler) writes:
> >The problem may (or may not) be a result of this.  I would look at it
> >with a 'scope.  I have found that the regulators don't regulate running
> >open circuit.  Some type of resistive load could be substituted ...
> 
> In fact, if you look at the data sheet carefully, regulation is specified
> over a specific range of currents, and the low end is *not* zero.  When
> powering something like CMOS that may draw nearly zero current at times,
> one sometimes has to put a resistor across the output to guarantee the
> minimum load the regulator needs.

Switching regulators usually require a minimum load, as do some linear
types.  While it is correct that the 78XXX series regulators do not spec
regulation at zero load (and I do not recommend relying on un-spec'd
performance), in practice I have found that they will work reliably at
zero load.  If you look at the internal schematic (not guaranteed, I know)
you will see that the output is lightly loaded by the regulator feedback
path divider.

To re-hash the original problem:

  1) Low output voltage into a relay coil load (inductive).
  2) Correct output voltage with an equivalent (DC) resistive load.
  3) Input Voltage provided by a wall transformer DC supply of unknown
     characteristics.
  4) No capacitors shown on the schematic.

(2) suggests that the input supply has adequate current capacity and filtering,
although wierd interactions with the inductive load are possible.  Additional
bulk input capacitance is probably not needed, input bypass (0.1 to 10 uF typ)
may be useful.

I still think that output bypass caps are probably the key.

Has the original poster reported back yet?

Curiously,
-- Paul
-- 
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."