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Specify "shunt resistance" for current measurement

Discussion in 'General Electronics Discussion' started by wdariusw, Feb 22, 2015.

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  1. wdariusw

    wdariusw

    149
    7
    Nov 10, 2014
    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 !
     
  2. hevans1944

    hevans1944 Hop - AC8NS

    4,541
    2,116
    Jun 21, 2012
    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 and KrisBlueNZ like this.
  3. wdariusw

    wdariusw

    149
    7
    Nov 10, 2014
    Yes, i know how to do it with experiments, but sometimes is good to do less experiments , and here calculations helps :) Thanks.
     
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