The .1 cap is critical. I had experience with a design that worked fine
with a National regulator but failed with a Motorola regulator. The design
had no .1 caps and adding them corrected the problem. They need to be as
close as possible to the regulator to be effective.
Larry

At 03:15 PM 4/3/02 -0600, you wrote:
>That's pretty much what I have -- circuit diagram attached.
>
>So why would it oscillate?  The 12V supply should be fairly
>stable (at least any changes should not be high enough
>frequency to be called "oscillation"), and the output has a
>33uF cap on it.  So my guess is that the oscillation is being
>produced internally (within the 7805)?
>
>
>----- Original Message -----
>From: "michael brown" <n5qmg@AMSAT.ORG>
>To: <PICLIST@MITVMA.MIT.EDU>
>Sent: Wednesday, April 03, 2002 2:41 PM
>Subject: Re: [EE]: 7805 heating up...
>
>
> > > but it gets hot enough that I cannot leave my finger on it for more
> > > than 5-10 seconds.
> >
> > > Should it get this hot?
> >
> > Only when it's transmitting!  Sounds like it is oscillating to me.  Put a
> > .1uF cap on the input, near the pins on the regulator.  Others here can
> > offer much better suggestions I'm sure.
> >
> > michael brown
> >
> > --
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> >
> >
> >

Larry G. Nelson Sr.
mailto:L.Nelson@ieee.org
http://www.ultranet.com/~nr

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