Two rails on an Arduino Uno

// Two rails: an INA226 watches a 12 V rail and an INA3221 a 5 V one, each
// with its own load, both on the I2C bus, the way a bench supply or a robot's
// power board keeps an eye on what everything draws.
//
//   INA226 (0x40)    SDA -> SDA   SCL -> SCL   on the 12 V rail's load
//   INA3221 (0x41)   SDA -> A4    SCL -> A5    channel 1 on the 5 V load
//
// Each module carries a 0.1 ohm shunt in series with its load, and reads the
// small voltage across it: the current. The INA226 reads the rail's own
// voltage on VBUS too, and works out the power.
//
// Things to try. Swap a lamp for another model and watch its current follow.

#include <Wire.h>
#include "INA226.h"
#include "Adafruit_INA3221.h"

INA226 rail12(0x40);
Adafruit_INA3221 rail5;

void setup() {
  Serial.begin(9600);
  Wire.begin();
  rail12.begin();
  rail12.setMaxCurrentShunt(0.8, 0.1);
  rail5.begin(0x41, &Wire);
  rail5.setShuntResistance(0, 0.1);
  Serial.println("Two rails");
}

void loop() {
  delay(50);
  Serial.print("12V rail ");
  Serial.print(rail12.getBusVoltage(), 2);
  Serial.print(" V ");
  Serial.print(rail12.getCurrent_mA(), 0);
  Serial.print(" mA ");
  Serial.print(rail12.getPower_mW(), 0);
  Serial.print(" mW   5V rail ");
  Serial.print(rail5.getBusVoltage(0), 2);
  Serial.print(" V ");
  Serial.print(rail5.getCurrentAmps(0) * 1000, 0);
  Serial.println(" mA");
  delay(450);
}