Electronic – Current measurement op-amp calculation

amplifiercurrent measurementoperational-amplifier

I am trying to measure the current using sense resistor method.

Low Side Current Sensing method:

  • I am using shunt resistor in the low side, there will be a voltage drop across the resistor.
  • Since the voltage drop is very small, using op-amp we can amplify to the required voltage level for microcontroller's ADC input.

The ratings are I = 30 & R = 0.001 (2 watts)

Voltage drop across the resistor,

Vrsense = IR
        = 30 * 0.001
        = 0.03 volt 

Power dissipation in resistor,

P = I * I * R
  = 30 * 30 * 0.001
  = 0.9 Watts

Op-amp gain,

Vout = Vrsense * (1+ Rf / R1)
     = 0.03 * (48)
     = 1.44

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My doubt is LM358 has a maximum power supply of 36 volts, but I am using 50 to 60 volt. I thought that the input channel will allow zero current, so by giving such a supply will not be a problem. Since I am new to op-amp correct me, friends, if I am wrong. Or I have to use some other op-amp.

Best Answer

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Figure 1. Low-side current measurement voltages.

  • Since (2) is connected to ground the voltage at that point is 0 V, not 49.97.
  • With 30 A flowing through the load the voltage at (1) will be + 0.030 V, not 50 to 60 V.
  • If you power the chip at (3) with 3 to 36 V you will be OK. I would be generous and use 5 to 30 V.