Hi there

I´m building a Arduino energy monitor using the emonlib library

All is good for now but I´m navigating in the inner workings and principles of what is happening in the background.

From the Open energy monitor page I got this:

"As the name suggests, AC Voltage and current continually alternate. If we draw a picture of the voltage and current waveform over time, it will look something like the image below. Depending on the type of load consuming power, the current waveform - blue in the diagram below - is what you get if you look at a typical laptop computer power supply. (There’s an incandescent light bulb present, as well).

The image was made by sampling the mains voltage and current at high frequency, which is exactly what we do on the emontx or Arduino. We make between 50 and 100 measurements every 20 milliseconds. 100 if sampling only current. 50, if sampling voltage and current. We’re limited by the Arduino analog read command and calculation speed."

“We’re limited by the Arduino analog read command and calculation speed.”

So looking in the library .cpp file i came across this

```
void EnergyMonitor::calcVI(unsigned int crossings, unsigned int timeout)
{
#if defined emonTxV3
int SupplyVoltage=3300;
#else
int SupplyVoltage = readVcc();
#endif
unsigned int crossCount = 0; //Used to measure number of times threshold is crossed.
unsigned int numberOfSamples = 0; //This is now incremented
//-------------------------------------------------------------------------------------------------------------------------
// 1) Waits for the waveform to be close to 'zero' (mid-scale adc) part in sin curve.
//-------------------------------------------------------------------------------------------------------------------------
boolean st=false; //an indicator to exit the while loop
unsigned long start = millis(); //millis()-start makes sure it doesnt get stuck in the loop if there is an error.
while(st==false) //the while loop...
{
startV = analogRead(inPinV); //using the voltage waveform
if ((startV < (ADC_COUNTS*0.55)) && (startV > (ADC_COUNTS*0.45))) st=true; //check its within range
if ((millis()-start)>timeout) st = true;
}
//-------------------------------------------------------------------------------------------------------------------------
// 2) Main measurement loop
//-------------------------------------------------------------------------------------------------------------------------
start = millis();
while ((crossCount < crossings) && ((millis()-start)<timeout))
{
numberOfSamples++; //Count number of times looped.
lastFilteredV = filteredV; //Used for delay/phase compensation
//-----------------------------------------------------------------------------
// A) Read in raw voltage and current samples
//-----------------------------------------------------------------------------
sampleV = analogRead(inPinV); //Read in raw voltage signal
sampleI = analogRead(inPinI); //Read in raw current signal
//-----------------------------------------------------------------------------
// B) Apply digital low pass filters to extract the 2.5 V or 1.65 V dc offset,
// then subtract this - signal is now centred on 0 counts.
//-----------------------------------------------------------------------------
offsetV = offsetV + ((sampleV-offsetV)/1024);
filteredV = sampleV - offsetV;
offsetI = offsetI + ((sampleI-offsetI)/1024);
filteredI = sampleI - offsetI;
//-----------------------------------------------------------------------------
// C) Root-mean-square method voltage
//-----------------------------------------------------------------------------
sqV= filteredV * filteredV; //1) square voltage values
sumV += sqV; //2) sum
//-----------------------------------------------------------------------------
// D) Root-mean-square method current
//-----------------------------------------------------------------------------
sqI = filteredI * filteredI; //1) square current values
sumI += sqI; //2) sum
//-----------------------------------------------------------------------------
// E) Phase calibration
//-----------------------------------------------------------------------------
phaseShiftedV = lastFilteredV + PHASECAL * (filteredV - lastFilteredV);
//-----------------------------------------------------------------------------
// F) Instantaneous power calc
//-----------------------------------------------------------------------------
instP = phaseShiftedV * filteredI; //Instantaneous Power
sumP +=instP; //Sum
//-----------------------------------------------------------------------------
// G) Find the number of times the voltage has crossed the initial voltage
// - every 2 crosses we will have sampled 1 wavelength
// - so this method allows us to sample an integer number of half wavelengths which increases accuracy
//-----------------------------------------------------------------------------
lastVCross = checkVCross;
if (sampleV > startV) checkVCross = true;
else checkVCross = false;
if (numberOfSamples==1) lastVCross = checkVCross;
if (lastVCross != checkVCross) crossCount++;
}
double EnergyMonitor::calcIrms(unsigned int Number_of_Samples)
{
#if defined emonTxV3
int SupplyVoltage=3300;
#else
int SupplyVoltage = readVcc();
#endif
for (unsigned int n = 0; n < Number_of_Samples; n++)
{
sampleI = analogRead(inPinI);
// Digital low pass filter extracts the 2.5 V or 1.65 V dc offset,
// then subtract this - signal is now centered on 0 counts.
offsetI = (offsetI + (sampleI-offsetI)/1024);
filteredI = sampleI - offsetI;
// Root-mean-square method current
// 1) square current values
sqI = filteredI * filteredI;
// 2) sum
sumI += sqI;
}
double I_RATIO = ICAL *((SupplyVoltage/1000.0) / (ADC_COUNTS));
Irms = I_RATIO * sqrt(sumI / Number_of_Samples);
//Reset accumulators
sumI = 0;
//--------------------------------------------------------------------------------------
return Irms;
}
```

Would it be of some benefit in order to enhance the number of samples the use of the analogReadFast library wich decreases the time for each analog reading from around 112uS to 17uS give or take?

My goal here is to use the faster analog read transitio to take more samples at a given interval and graph that RAW data into excel to see the sine waveform and see if I can catch some harmonic distortion

Bare in mind that I´m a newbie and enthusiastic about electronics so there should be a bunch here that I´m ignoring before hand

Link to analogReadFast Library

Link to Open Energy Documentation

Link to Open Energy Monitor Library

Regards