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
Multimeter

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.