In my bicycle project there was the need to measure battery voltage, around 40 Volts, and the current, up to 14 Ampere.
I was worried to burn some valuable parts and therefore made very simple test circuits an a breadboard together with one Arduino nano connected to a pc. The whole test in a living room on a stand.
To measure voltage there was a voltage divider with 20k resistor connected to 40 Volts, middle point voltage to analog input A3 in Arduino nano, and lower side of the voltage divider being 1k and connected to ground.
The analog conversion value was about level 410 ( out of 1023) , when the results were viewed at Arduino monitor window in a windows pc. The connection was with the usual usb-cable.
The current was measured on the same nano, the same breadboard with Acs712T (hall effect sensor ). Its 5 V logic power was fed to it from the 5 V pin of the same arduino nano, and the analog signal was connected to analog pin A2.
The analog conversion value made sense.
The measurements above were made so that first the usb cable was connected to nano, and thereafter the 40 volt battery.
But then I left the 40V battery connected to nano and disconnected the usb cable. The nano was supposed to be without power, in my understanding.
I reconnected the usb cable between my pc and nano (the battery 40 V was still connected). Windows XP complained that the usb connection was invalid or the device defective and asked to disconnect/reconnect or replace the device.
Disconnectin/connecting the usb-cable did not help.
The next day I connected the usb cable first, then the battery, and the Arduino monitor showed good values for current and voltage. Sigh of relief.
But in the next test session there was faint smell of something hot: the cpu of the nanoboard was too hot to touch and pc could not communicate correctly with it. Even without any breadboard wires the nano became very hot in 10-20 seconds.
To make the circuit simple I had omitted all the customary(?) protection diodes and all the capacitors. The only protection for the analog inputs of A2 and A3 was a 10 k resistor in series.
Is it possible or probable that in the above circuit the 40 V battery causes damaging voltages or currents, when Arduino nano is without power? (provided the components ACS712T and resistors are not at fault).
How would you recommend to continue?
3 pictures should be visible at flickr, here