Yeah, it was oscillating because of an incorrect ESR in your capacitors.
The LM series LDO regulators from National are *extremely* interesting
devices.  I'll leave the long version of the story out and just say that you
need to be careful when designing your circuit with those.  If either of
your capacitors fails, you will see *massive* voltage spikes on the outpout
as a result.  I can't remember which does what now, but the input cap is
there for voltage regulation and the output cap is there for cycle
regulation (or vice versa, don't remember exactly, but you get he gist).  So
if one of them breaks down because say it's rated for 16V and it
occasionally getting 25V spikes, then you will eventually be sending 40V
spikes in to your uC circuit.  Found this little nugget after reverse
engineering a previous years Senior Design project.


On 1/19/06, dr. Imre Bartfai <root@prof.pmmf.hu> wrote:
>
> Here is my story,
>
> I built a very straight 12V stabilized power supply, planned with 7812.
> Afterwards, I changed my mind and put a pin-compatible LM2940-12 which is
> a low-drop version of 7812. And it began to oscillate. Do you guess, why?
>
> The data sheet of the LM2940 tells to put definitely a 22uF capacitor
> paralel to the output. It is not required on the 7812. I learned again to
> read the data sheet even if the info seems to be
> unimportant/known/superfluous.
>
> Regards, and happy new year
> Imre
>
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--


Shawn Wilton (b9 Systems)
http://black9.com
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