[sdiy] OpAmp Overvoltage / Power Up (was: Re: Pots read by microcontroller and sourcing question)

[Michael Zacherl]

On 3.Feb 2015, at 10:11 , Oakley Sound <oakleylist at btinternet.com> wrote:

> One problem with an op-amp per pot is that you need some sort of over voltage protection on the ADC or the Mux. An op-amp on power up, or partial power up, can do some horrid things.

I tested OpAmp circuits with one rail disconnected and in most cases the output went up/down straight to the other rail.
That indeed can be way too much for a 3.3 or 5V µC in a 12 or 15V system.
For a good reason every now and then I can read warnings about making sure both voltages are present in a system otherwise parts can/would be damaged. 
I wonder how's that at power-up time? 
Due to tolerances etc. both supply voltages never would come up exactly at the same
time, but that's probably a different situation with capacitors charging, rails poperly connected etc?

Michael.

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