Last night I was testing a rocket launch circuit with a test load - shown below - the 4.7 Ohm resistor is the test load. I was measuring the voltage across it during test fires with my scope connected across it.

With my 0-30V 3A linear supply - no prob.

I have a 15A variable switchmode also, so thought I'd try it as well, but when I connect the scope ground to the ground side of the resistor, the PSU is delivering 2A - this is without the MOSFET being open.

I suspect this is due to one of these PSUs having a floating ground - would the scope be floating? so the switchmode must not be?? or vice versa??

I'm a bit confused about what is happening....

edit - I've just read the scope is most likely grounded.... still confused!

Can I work around this??

cheers
Lee

 

Sounds like the scope ground and PSU ground are connected via the power cord, you are effectively shorting the output to GND by connecting the scope across the resistor.

It's generally not good practice to connect the GND lead of your scope to anything other than the system GND, the workarounds are:

1) Use a scope with a true isolated floating GND. (rare)

2) Use two scope channels and setup the scope to subtract the signals, taking a differential measurement.

3) Connect the scope to the GND of your circuit and infer the voltage across the resistor by subtracting out the supply voltage, this is the simplest trick.

Sometimes you can get away with connecting the GND to other points in the circuit, but remember that the GND point of the scope is connected to the chassis of the instrument and the GND pin of the power cord too.

 

Thanks for that. Very useful - I'll do the differential measurements I think.