Test peak current of a device using shunt resistor and oscilloscope

[Brown_Bog]

I try to test the start up current with oscilloscope voltage probe using a shunt resistor (0.1 ohm), see the figure for connection, i put CH1 and CH2 probe on both side of the shunt and leave the probes ground clip not connecting to anything. On the scope i set the display to MATH (CH1-CH2). But all I get is a huge noise.

Am I doing the correct way? Should I leave the ground clip open or connect to the same ground of the load?

 

[BobK]

Unless the circuit already has a common ground, you need some common connection between them. If the +24V supply is floating you can connect the ground clip to one side of the resistor and a single probe to the other.

Bob

 

[WHONOES]

Placing the resistor between the load and ground would be the sensible thing to do. Then, provided that the supply ground connection is not directly connected to mains earth, connection of the scope ground lead should present no problem.

 

[kellys_eye]

...or use a differential probe.

 

[WHONOES]

Yup, that's another way to do it. I just assumed one wasn't available.