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How to best normal voltages to inputs
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author How to best normal voltages to inputs
jakobprogsch
Say I have an input jack that is connected through a pot (attenuator) to provide a control voltage. And I want the pot to do something even with nothing connected, so I connect 10V to the switch/normalling pin on the connector. What is the safe way to do that? The 10V will be connected to the tip of an incoming patch cable while plugging it in for a moment. But I don't want to fry the module on the other end of the patch cable.

I figured something like limiting the current by connecting the 10V via a resistor (10k sufficient?) and then use a 1M pot or buffer before the pot so it doesn't "upset" the voltage divider too much. But maybe there is a more elegant or better solution?
euromorcego
i would definitely put a current limiting resistor.

As I see it (and people may correct me): 10k should be sufficient. If you connect to 10V then according to I = U/R = 10V/10kO = 1mA current. That should do no harm.

Then another question is: how stable do you want the voltage to be. If precision is important, then a buffer or something similar is probably a good idea. There are plenty of schematics, i.e. from mutable instruments, how to create a reference voltage from 12V.
jakobprogsch
Thanks. The precision in a magnitude sense depends on the application I guess. This came up for me in the context of normaling control voltages for an envelope. So I guess the absolute precision isn't that important since this tends to be a value you dial in by feel anyway. I was wondering how problematic it is to use the power rails or voltages divided from them for these kinds of things. It seems that is what most of the modules do that I looked at (mostly MI and Befaco schematics).
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