[sdiy] filter cap needed?

[Colin Hinz]

On Mon, 23 Mar 2009, db wrote:

> I'm sure it's a good idea to have the cap...
> But, I already fully built my device and closed it up without a cap, and 
> don't want to get back into it unless it's semi-important..  :)

One of my first tasks as a rookie EE (lo those many years ago) was
to troubleshoot a microcontroller board which had strangely gone
insane. It turned out that the 7805 regulator (lacking the recommended
bypass caps) was oscillating between 3V and 7V. Looked OK when checked
with a DVM, too -- I had to get wise with a scope, instead.
"But it worked fine on the bench," the designer wailed.

YMMV, of course. If it's gonna fail, though, it'll probably fail under
the least desireable circumstances. You'll never say to yourself, "Boy,
I'm glad I opened it up and put the cap in," but you sure might be
saying, "Argh! I should have corrected that before I went on tour."

- Colin