Hi Sean,

Yes I have seen this before on Motorola brand only, many years ago. I
think it was with a 7815. I changed it with another brand and the
problem went away.

Cheerful regards,

Bob

On Tue, Jan 3, 2012, at 12:10 AM, Sean Breheny wrote:
> Hi all,
>=20
> 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.
>=20
> 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.
>=20
> 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.


--=20
http://www.fastmail.fm - Faster than the air-speed velocity of an
                          unladen european swallow

--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.