On many regulators the ESR of the vreg output cap must be within a certain
range, if it's lower or higher it will oscillate.


At 13.20 2012.01.04, you wrote:
>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
>
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