> I have a heatsink to be mounted, but for the moment, there should be no
> power consumption, so I thought that the power to be dissipated is
> small.

Make sure that the 7805 is not oscillating at a very high frequency. When
this happens, as it can, many strange things can happen. A DC meter is not a
good enough check if this is happening - the mean DC level is probably still
5v. An oscilloscope is the best means of testing this.

Also check the datasheet for the 7805 and be sure to add input and output
capacitors of the values it says and WHERE it says. In problem cases, having
short lead lengths between capacitor and regulator may matter. On some newer
regulators the capacitors must not have an ESR (equivalent series
resistance) that is either too high OT too low. AFAIR the 7805 does not
specify this.

As a rule a large capacitor at input and output should settle it down if
stability is questionable. Normally a very large output cap should not be
needed and can cause damage at shut down under certain circumstances due to
back discharge through the regulator, unless suitable protection diodes are
used.

Who said 3 terminal regulators were simple ??? ;-)
(They are actually, compared to the alternatives, but there are still things
to be known).


        Russell McMahon

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