power supply question

jhaible jhaible at debitel.net
Thu Mar 9 21:31:54 CET 2000

Don't do it.
Voltage regulators can die from their output being more positive than
their input. In a good design, there are reverse diodes to prevent
damage when the input voltage falls faster than the output voltage
(switch the unit off, capacitors storing energy ...), but often these diodes
are not used in order to keep costs low.
You may be lucky if it works, but I wouldn't rely on it.


> >Does it hurt if you connect an external power supply to a device, after
> >own voltage regulator circuit?
> >I want to use an external power supply on Atmel's STK200, is it okay to
> >connect the ground and 5v of the external power supply to some of the
> >STK200's gnd and 5V out?
> >I does work, but does it hurt the internal power circuit?
> I tried this with the standard 7805 and it works fine.
> It is not a "clean" method but no problems if the output voltages of the
> two circuits are nearly the same.

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