I can't say for certain, but I am almost certain I've used 78xx series=20
regulators with such a low load that it may as well be zero - I.E. a=20
couple of 74HC series or a single opamp or similar, with absolutely no=20
ill effects.

I HAVE though had more than one case of the regulators ringing without=20
sufficient, close, input capacitors.  I seem to recall at least one=20
datasheet which is rather misleading about what is really needed.   Like=20
stating it isn't necessary if there's another one close.  Are they=20
well-bypassed on the input side?

-forrest

On 1/4/2012 5:04 AM, Sean Breheny 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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