As a side note, the Texas Instruments UA78Mxx doesn't need any input
or output cap, and is stable under any and every condition. It doesn't
even cost a lot, although it's not even the cheapest in town.

Cheers,
Mario


At 06.10 2012.01.03, you wrote:
>Hi all,
>
>I just encountered something that I have not seen before and I am
>interested to see if others have encountered this. I have a PCB with
>several 78xx series regulators on it which were working fine. I then
>made some modifications which meant that the loads which were
>connected to the 7815 and 7812 regulators were not needed, so I
>disconnected them. I then began to have problems with a periodic
>signal occurring on the main 24V rail. I eventually discovered that
>both the 7815 and 7812 were oscillating and because they have large
>output capacitors, this was drawing significant AC current from the
>24V rail.
>
>I searched two 78xx series datasheets, as well as a quick Google
>search, and found no mention of a minimum load for 78xx series
>regulators, although I do notice that in all over the specifications,
>they never mention conditions with an output current of less than 5mA.
>
>Anyone seen this problem before? It makes sense given the design of
>the regulator but I am surprised that I've never seen it mentioned
>before since other kinds of regulators, such as switchers, often have
>a listed minimum load current.
>
>Sean
>--=20
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