Shunt and Current-to-Voltage Converter
Shunt is a resistor that carrying a very small value of resistance, normally 1 Ohm (or less than that) and shunt has important role in current measurement. In equation,
V = I * R
where R = 1 ohm, voltage across R must equivalent to current value (ideally, in practical there’s always small tolerance in resistance). For example, if probed voltage got 1.5V, it must be 1.5A across that resistor as well. Therefore, from here we can build a circuit to convert current value into voltage value to ease control for current limiter circuit.
[Update and Revise 23/10/2018]
Two-stage operational amplifier are implemented in the circuit. The first stage op-amp is being used as line follower or unity gain amplifier due to its high impedance characteristic avoid drawing too much power from the source. The second stage amplifier is a differential amplifier. Its Vout can be determined as:
Vout = (R5/R4) * (V2 – V1)
The resistance of both R5 and R4 is 18K ohm which bring the gain of 1. Therefore we can have output voltage by deducting V2 and V1. But rail-to-rail op-amp like LM6142 must take place of LM358 because LM6142 able to swing its output voltage very close to 0V (or supply voltage). The lowest point output voltage of LM358 is around 600mV, so LM358 is not good enough to convert small amount of current like 50mA that equivalent to 50mV.