Arduino AC measurements

Sometimes there is need to measure power going to different devices. Measuring DC power consumption is pretty easy, but when you try to do the same for AC circuit, things start to become complicated. How to Measure Electrical Power article will discuss best practices for making electrical power measurements, starting with power measurement basics and proceeding to the types of instruments and associated components typically used to make measurements. DC power measurement is relatively simple as the equation is simply watts = volts x amps. For AC power measurement, the power factor (PF) introduces complexity as watts = volts x amps x PF. This measurement of AC power is referred to as active power, true power or real power. Power is typically measured with a digital power analyzer or a DSO (digital storage oscilloscope) with power-analysis firmware.

What if you want to implement power mesurement yourself? Power consumption in AC circuit is correctly measured by calculating volts x amps = volt-amps (apparent power) over time, using at least one complete cycle. Using digitizing techniques, the instantaneous voltage is multiplied by the instantaneous current then accumulated and integrated over a specific time period to provide a measurement. This method provides a true power measurement and true RMS measurements for any waveform up to the bandwidth of the instrument.

In addition to math, you will need to provide some sensors to measure current and voltage. For AC current measurements potential sensor types are shunt resistor, current transformer, hall current sensor and rogowski coil. Usually current transformer and hall current sensor are the most suitable types to use with Arduino (they both provide isolation from measurent circuit). For AC voltage measurements the most suitable sensor types are voltage transformer,  capacitive voltage divider and resistive voltage divider. From those alternatives only voltage transformer can provide galvanic isolation from circit being measured.

Can this measurement of AC power made using Arduino? The answer is yes, ans there are several ways it can be done. AC Power Theory – Arduino maths web page provides inntroduction how this can be implemented with Arduino (check also Advanced maths). There is EmonLib library that provides you a set of ready made AC measurement calculation routines that you can use easily.

Arduino sketch – voltage and current from OpenEnergyMonitor project shows how you can do the AC power measurements with Arduino.

#include "EmonLib.h"              // Include Emon Library
EnergyMonitor emon1;              // Create an instance

void setup()
{  
  Serial.begin(9600);
  
  emon1.voltage(2, 234.26, 1.7);  // Voltage: input pin, calibration, phase_shift
  emon1.current(1, 111.1);        // Current: input pin, calibration.
}

void loop()
{
  emon1.calcVI(20,2000);          // Calculate all. No.of crossings, time-out
  emon1.serialprint();            // Print out all variables
}

To us this simple looking source code, you need to have EmonLib installed (it does all the complex calculations so you don’t need to worry about them). The simplest way is to download EmonLib zip packet and extract it to arduino libraries folder. It will give the needed library and project examples.

Here is example code I used “voltage_and_current” (my modified version):

// EmonLibrary examples openenergymonitor.org, Licence GNU GPL V3

#include “EmonLib.h” // Include Emon Library
EnergyMonitor emon1; // Create an instance

void setup()
{
Serial.begin(9600);

emon1.voltage(2, 11.7 /* 234.26 */, 1 /* 1.7 */); // Voltage: input pin, calibration, phase_shift
emon1.current(1, 5.5 /* 111.1 */); // Current: input pin, calibration.
}

void loop()
{
emon1.calcVI(20,2000); // Calculate all. No.of half wavelengths (crossings), time-out
emon1.serialprint(); // Print out all variables (realpower, apparent power, Vrms, Irms, power factor)

float realPower = emon1.realPower; //extract Real Power into variable
float apparentPower = emon1.apparentPower; //extract Apparent Power into variable
float powerFActor = emon1.powerFactor; //extract Power Factor into Variable
float supplyVoltage = emon1.Vrms; //extract Vrms into Variable
float Irms = emon1.Irms; //extract Irms into Variable
}

I got the following output when I measured 20W halogen lamp system:

Here is picture of the Arduino Nano + Nano Sensor Shield based test circuit I used:

In this test circuit I measured 12V halogen lamp system current and voltage. For current mesurement I used ACS712 sensor based current measurement sensor module with +-5A current measurement range. For voltage measurement I used a simple voltage divider module (22k ohm from input pin to +5V, 22 kohms from input pin to ground, 120 kohms from AC voltage source to input pin).

One note in plannng to use this power measurements: If you plan to measure mains power, you need to understand all the safety details related to mains power. When testing the circuit, I highly recommend that you have a low voltage test system that you can sefely test and debug your designs (hardware and software). I have used 12V halogen lamp system for this: a traditional 12V transformer gives out safe 12V AC output usually at several amperes, you can control the load easily by turning 12V lamps on/off as needed. When you have debugged your design at safe voltages, you can start thinking of working with higher dangerlous voltages!

Related project pages:

OpenEnergyMonitor building blocks

Emoncms

Home Energy Monitoring System

Watt Meter build walks you through Power Measurement basics

DIY Digital AC Watt Meter

Digital Data from a Cheap Power Meter