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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