Current through a resistor

Current has to go through something

A voltage is a difference between two points, so a meter can read it by touching both. A current is a flow, and the only way to count a flow is to put something in its way. A multimeter's current range does exactly that: the red lead moves to another socket and the meter goes in series with the circuit.

The TK119 does the same job without the meter. Your supply's positive wire goes into the red POWER terminal, your load's positive wire comes out of the black LOAD terminal, and between them every electron the load draws passes through one resistor.

A tenth of an ohm

That resistor is 100 mΩ, a tenth of an ohm, and it is called a shunt: a resistor put in a circuit to be measured rather than to do anything. Ohm's law says the voltage across it is the current times its resistance, so 500 mA makes 50 mV and 1 A makes 100 mV.

A few millivolts is too small to disturb most circuits, and still large enough to measure. The INA219 beside the shunt reads it in steps of 10 µV, which on a tenth of an ohm is 0.1 mA.

What it costs

The shunt is not free. It takes its millivolts from the load, so a 5 V load drawing 1 A sees 4.9 V. And it turns current into heat: at 3.2 A, the most the chip can read, the shunt makes about 1 W. The part is a 2512 resistor rated 2 W, so it gets warm to the touch at the top of the range and stays within its rating.

The two things it reports

The chip measures two voltages: the one across the shunt, and the one from the LOAD + screw down to ground, which is what your load is actually getting. Current and power are worked out from those two, and Two measured, two worked out is about the difference that makes.

The rail can be anything from 0 to 26 V. The chip itself runs from its own supply of 3 to 5.5 V on the header, and talks to your microcontroller over I²C, so a 12 V or 24 V rail never comes near an ESP32's pins.