As I mentioned previously , I had a need to double check that the 5v USB supply from my laptop was floating with respect to my lab power supply.
I measured a no load difference on the 0v lines of around 120v – which raised the obvious concern that if I connected these two 0v lines together (to allow me to demonstrate one Arduino on one power source communicating fine down the RS485 to another Arduino on the PC USB power) – that this might be backed up with a rather large current to boot and effectively ‘fry’ everything in its path. This is a known risk when using multiple power supplies or Earthed devices, if you provide a potential difference either to Earth or 0v greater than what your devices are rated for – things go pop very quickly.
So, how do you test for the presence of this dangerous condition without actually destroying anything or putting yourself in harms way? The first step is to find yourself a high value resistor, somewhere around 50k ohms will do. What we want to do is see if in the presence of such a resistance between the two 0v lines the voltage difference will still remain (i.e. there is enough ‘juice’ in the system to maintain the voltage difference given the draw).
Now carefully set this up away from anything else and put the resistor in place, then measure the voltage across the resistor. if it remains the same or a significant percentage of the original unloaded difference – you have a problem. Simple V over R will tell you the current being driven and you won’t need much imagination to realize a lower resistance will ruin your day. If on the other hand the voltage goes down to something very near to zero across the resistor – your good to go, as this indicates the voltage difference you measured without load has no current behind it..
To repeat if the voltage across the resistor is not near zero, do not connect the two 0v lines – nasty things will happen.
Luckily for me, both power supplies are floating and not directly connected to the mains or Earth lines in anyway – I reckon this was just a ‘feature’ of the fact I have a dual positive/negative bench supply, so the 0v when no load is ‘shifted’ half mains wrt to 0v coming from the laptop USB… Whatever the exact cause – I know now these two power sources will play nicely together.