Read one rail on an ESP32

// TK119 INA219 wiring for this sketch.
//
//   ESP32 GND  -> GND       (first pin on the board)
//   ESP32 3V3  -> 3V3       (second pin; 3 to 5.5 V, and it sets the bus level)
//   ESP32 SCL  -> SCL       (THIRD pin - the clock comes before the data here)
//   ESP32 SDA  -> SDA       (fourth pin)
//
//   supply + -> POWER +     supply - -> POWER -
//   load   + -> LOAD  +     load   - -> LOAD  -
//
// Connect the rail last, so nothing is live while a screwdriver is in a
// terminal. The supply's negative and the ESP32's ground are the same net
// through this board, which is what makes the voltage reading possible.
//
// Arduino IDE: any ESP32 board. Sketch > Include Library > Manage
// Libraries, then install "Adafruit INA219". No other Tools settings.

#include <Wire.h>
#include <Adafruit_INA219.h>

// 0x40 with neither pad soldered. 0x41 with A0, 0x44 with A1, 0x45 with both.
Adafruit_INA219 ina219(0x40);

// Read with nothing in the LOAD terminal and put the number here, in
// milliamps. It is the chip's fixed zero-point error and it does not scale,
// so subtracting it is the one correction worth making by hand.
const float ZERO_MA = 0.0;

void setup() {
  Serial.begin(115200);
  delay(500);
  Wire.begin();

  if (!ina219.begin()) {
    Serial.println("no INA219 at 0x40 - SCL is the third pin on this board, SDA the fourth");
    while (1) delay(10);
  }

  // Three ranges to choose from in this library. The widest reaches 3.2 A;
  // the narrow one halves the zero-point error, so use it if you know the
  // current is small. Nothing here changes the size of one count: that is
  // always 0.1 mA on a 100 mohm shunt.
  ina219.setCalibration_32V_2A();      // +-320 mV, up to 3.2 A
  // ina219.setCalibration_32V_1A();   // +-320 mV, scaled for 1 A
  // ina219.setCalibration_16V_400mA(); // +-40 mV, the accurate one

  Serial.println("shunt mV\tbus V\tmA\tmW\tsupply V");
}

void loop() {
  // Measured.
  float shunt_mv = ina219.getShuntVoltage_mV();
  float bus_v    = ina219.getBusVoltage_V();
  // Worked out, from a shunt resistance the calibration call set.
  float ma       = ina219.getCurrent_mA() - ZERO_MA;
  float mw       = ina219.getPower_mW();

  // The bus voltage is the load's, not the supply's: it is measured on the
  // far side of the shunt. Add the shunt drop back on for the supply's.
  float supply_v = bus_v + shunt_mv / 1000.0;

  Serial.printf("%.2f\t%.3f\t%.1f\t%.1f\t%.3f\n", shunt_mv, bus_v, ma, mw, supply_v);
  delay(500);
}