when Arduino nano is without power
Quotewhen Arduino nano is without powerNever ever connect any voltage to the input of ANY chip when that chip is not powered. As you have found out this is a great way of killing a chip.I would try and power everything off the same source, even if it means several stages of step down.Another option is to connect the input through to the arduino through a reed relay and have the arduino power the relay, so that when the relay drops out the input is disconnected.
I would have used 200k and 10k for the voltage divider, ten times your values, since then the max currentfrom the divider would be 200uA rather than 2mA.With the supply disconected that 2mA would flow through one of the input protection diodeswhich might be enough to put the chip into CMOS latch-up (to be avoided since the whole chip thenconducts between Vcc and ground).Of course some other failure mode might have been involved.
Is there any safe level of current or energy, which Arduino Mega2560 would tolerate in its pin when not powered?
QuoteIs there any safe level of current or energy, which Arduino Mega2560 would tolerate in its pin when not powered?Any chip will not be able to take more than 0.5V without power. This is because an unpowered chip just looks like a bunch of diodes.One way of getting round this is to have a transistor buffer in front of the input. But this would only be any good for a digital signal not an analogue one.You can also use an opto isolator but again it is not good at transferring analogue voltages.
Should there be a low limit for power (or current) at the same time to be on the safe side?
I assume "a transistor buffer" is like this:
Grumpy_Mike, where do the Base and Signal connect? Can you post a schematic?