Current Sensing Circuit

Introduction: Current Sensing Circuit

In this simple tutorial, I am going to explain how to measure a current in your circuit. ALL that is here and more you can watch it on my channel VEGEtek on youtube (episode 003).

Step 1: Ways to Measure Current

Ways to measure current:

1- Indirect method: such as current transformers (in the figure) and Hall effect sensors, which relies on Faraday's law of induction to sense current in a circuit and convert it to a proportional voltage.

These methods are suitable more for high current systems.

2- Direct method: which relies on Ohm’s law which states that V = I x R.

This method is suitable for low currents and widely used in electronics devices.

Step 2: Measuring Current by Multimeter

Measuring current using a

1- cut the circuit.

2- use the “current” multimeter input jack, not the voltage one.

3- put the probes as the figure to close the circuit.

Step 3: High-side and Low-side Sensing

High-side and low-side sensing

Direct sensing has 2 methods: High-side and low-side sensing. It depends on the position of the shunt resistor with respect to the load.

This op-amp configuration is called “differential amplifier” which it amplifies the voltage difference between its inputs.

Step 4: Differential Amplifier

Differential amplifier

The op-amp will amplify the voltage difference between its two inputs according to this equation (also in the figure):

V_output = R3/R1 (V2-V1)

If all 4 resistors where the same value
(like 10k) this will be a unity gain differential amplifier which the output voltage is: [ V_out = V2 – V1 ] since R3/R1 = 1/1 = 1. This will give the voltage difference directly as it is.

However, it is common to have a gain of 10 or so in such practical circuits because the voltage difference may be so small, for example:

If shunt resistor is 0.1 Ohms (very common) and the current is 1A, this will result in 1 x 0.1 = 0.1v across shunt resistor, this will mean 0.1v output of the differential amplifier when unity gain is used, so it is 0.1v per 1 A. While using a gain of 10 will mean 1v per 1A which is a lot easy and practical.

Step 5: Video Tutorial With Example

This is the video tutorial that I made:

Please view the video and go apply what I explained in it. I also attached the slides for those who want them!

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    Question 1 year ago on Step 5

    Hi! Thanks for the great tutorial. It seems to work quite well. However, I see in LTspice some issues with low current consumption. When the R6 resistor in your LTspice schematics is around 200 ohms for instance (or more, leading to current consumption of around 60mA and lower), the LT1678 output starts to lose accuracy and then saturates around 56mv. Do you have any advice on how to deal with that? I need to be able to measure small currents.
    Thanks a lot!