Specify "shunt resistance" for current measurement

[wdariusw]

Hi for all ! Measuring low current with multimeter now, but i want to do it with oscilloscope using shunt resistance voltage drop. How to calculate best resistance value for measuring ? It is 3V power supply, with multimeter i measured ~10mA current. So if i want that my circuit still works, i need to get voltage drop on shunt resistor about 500mV - 1V (circuit minimal voltage is 1.8V) . Then U=IR, R=U/I=500mV/10mA= 50 Ohm, yes ? And what if i do not know current (can't measure with multimeter) , how then i should calculate shunt resistor value? Maybe only experimentational way ? Thanks !

 

[hevans1944]

The "best" resistance value is the smallest value that will produce a measurable signal on your oscilloscope. Try using 0.1 ohms (or less) and set the vertical deflection sensitivity to 5 mv/div or less (if available). That will give you a "sensitivity" of 50 mA/div. If that isn't enough, increase to 1 ohm for 5 mA/div. Your 50Ω resistor will work, but it is probably larger than you need to monitor the circuit current with an oscilloscope of reasonable sensitivity. You could also insert a 10X to 100X gain op-amp amplifier between the shunt and the oscilloscope input to improve the sensitivity without using an excessively large resistance, but then you have to worry about op-amp drift and gain calibration if you want accurate rather than just qualitative measurements. Experiments are good. We all learn best from experiments.

 

[wdariusw]

Yes, i know how to do it with experiments, but sometimes is good to do less experiments , and here calculations helps Thanks.