Multiple 9v batteries

[billsship]

Hi everyone,
I've been testing all of my Lunetta designs with a bench power supply running approximately 9v, but constant. No regulators. My plan is to build modules into separate enclosures and run each box on separate 9v batteries.

But then I thought: what happens when a battery slowly runs out of juice and sags under 9v and its CMOS chip is then powered by, let's say, 8.5v? Would an incoming voltage of 9v on an input pin damage the IC because that signal is higher than the Vcc? Or is the difference negligible? I really don't want to regulate all of the batteries down to 5v, or use pesky 12v adapters with a 9v regulator.

Thanks for your help!
Bill

[PHOBoS]

That's a great question!
Never really thought about it but yeah, if the voltage difference is too large it could damage the inputs. I have connected
circuits that were powered from different wallwart supplies to eachother without any problems but I do have 1K resistors
on the outputs which will limit the current a bit I guess. I also use transistors on some inputs to be able to use a larger
voltage range so that helps too. You could of course add some extra protection with diodes and a resistor to be sure.

(just an example I found, you can ignore the voltages)

hmm very interesting.