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Test peak current of a device using shunt resistor and oscilloscope

Brown_Bog

Aug 27, 2018
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Aug 27, 2018
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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?
 

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BobK

Jan 5, 2010
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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

May 20, 2017
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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.
 

WHONOES

May 20, 2017
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Yup, that's another way to do it. I just assumed one wasn't available.
 
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