On Nov 25, 2010, at 4:47 AM, Jan-Erik Soderholm wrote:

>> As for power, there's not much to the circuit. A 12V battery
>> via a 1N4007, into a 78L05 with a 100uF on the input.
>
> The smoothing cap on the *input* side of a 78(L)05 is usualy
> there to "smooth" the supply from the power source. Now, a
> *battery* will probably not need to be smoothed at all.

Batteries can have really sucky impedence at high frequencies, and =20
bypassing is highly recommended even though there is no AC component =20
present.  I had some enlightening experiences with some battery-=20
powered audio circuits that just didn't work from a 9V battery without =20
a cap across the input power...

(Now, I don't know that modern voltage regulators care much about =20
input impedence at high frequencies, and much smaller caps may be all =20
that's required.  But it's not true that "smoothing" is the only =20
reason to have caps across your supply rails.)

BillW

--=20
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