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

[Richie Burnett]

I think it's a very good point.  Likewise at power-down.  One voltage 
regulator may drop out before the other as the power supply's reservoir 
capacitors gradually discharge.  This would be particularly true if there is 
a lot more current drawn from say the +ve compared to the -ve rail, as is 
often the case.

-Richie,



-----Original Message----- 
From: Michael Zacherl
Sent: Wednesday, February 04, 2015 11:09 PM
To: Synth DIY
Subject: [sdiy] OpAmp Overvoltage / Power Up (was: Re: Pots read 
bymicrocontroller and sourcing question)


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.

--
http://mz.klingt.org

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