Measuring Instantaneous and RMS Voltage, Current, and Frequency with Arduino

Measuring the intricate parameters of alternating current (AC) with an Arduino can be a challenging but rewarding task. This guide will walk you through the necessary components, methods, and safety precautions to accurately measure instantaneous and RMS values of 120V and 240V AC voltage, as well as current and frequency. By following these steps, you can set up a reliable and versatile measurement system.

Components Needed

Arduino Board: For example, an Arduino Uno. Voltage Sensor Module: Such as the ZMPT101B for accurate AC voltage measurements. Current Sensor Module: Like the ACS712 for lower currents or Current Transformers (CT) for higher currents. Resistors and capacitors: For signal conditioning if needed. Breadboard and jumper wires. Optional: LCD or OLED display for real-time readings. Safety equipment: Fuses, circuit breakers, etc.

Safety Precautions

Working with 120V and 240V AC is inherently dangerous. Always ensure you have a basic understanding of safety principles and use appropriate electrical insulation and protective equipment.

High Voltage Warning

Always work with power turned off until you are certain of the circuit connections. Unplug the power source when testing with lower voltages.

Isolation

Use opto-isolators or transformer-based sensors to isolate the Arduino from high voltages, ensuring the safety of your system and data integrity.

Measuring Voltage

Using a Voltage Sensor

Connect the voltage sensor to the AC line. The output of the voltage sensor will be a scaled-down AC voltage, typically between 0-5V, which can be used by an Arduino. Use the conversion formula to convert the analog reading back to voltage: V_rms (V_out/ADC_max) × V_scale × √2 V_scale is the scaling factor of the voltage divider or sensor.

Instantaneous Voltage

Read the analog value from the sensor using analogRead(). Convert this value to voltage using the formula above. For real-time readings, plot the raw analog readings.

Measuring Current

Using a Current Sensor

If using the ACS712, connect it in series with the load. Read the analog output using analogRead(). Convert the reading to current using the formula: I_rms (V_out - V_offset) / Sensitivity V_offset is the output voltage at 0A, typically 2.5V for ACS712. Sensitivity is the sensor's sensitivity, e.g., 185mV/A for ACS712 5A.

Measuring Frequency

Using Zero-Crossing Detection

Connect an interrupt pin to the AC line through a resistor divider or opto-isolator. Use an interrupt service routine (ISR) to count the number of zero crossings in a specific time interval, e.g., 1 second. Calculate frequency as: Frequency (Number of zero crossings / 2) for AC, count both positive and negative crossings.

Calculating RMS Values

RMS Voltage and Current

To calculate RMS from instantaneous voltage and current readings, sample the waveform multiple times:

float calculateRMS(int readings[], int numSamples) {    float sum  0.0;    for (int i  0; i  numSamples; i  ) {        sum  pow(readings[i], 2);    }    return sqrt(sum / numSamples);}

Store multiple readings in an array and pass it to the calculateRMS function.

Sample Code

Here is a simplified example of how your Arduino code might look:

const int voltagePin A0; const int currentPin A1; volatile int zeroCrossings 0; void setup() {
(9600);
attachInterrupt(digitalPinToInterrupt(2), zeroCrossingISR, RISING);
} void loop() {
int voltageReading analogRead(voltagePin);
float voltage voltageReading / 1023.0 * 5.0 * scalingFactor * sqrt(2);
int currentReading analogRead(currentPin);
float current currentReading - 512 / 1024.0 / sensitivity;

delay(1000);
} void zeroCrossingISR() {
zeroCrossings ;
}

Conclusion

Ensure to test your setup with lower voltages before applying 120V or 240V. Always prioritize safety and consider using protective components to prevent damage to your Arduino and ensure safe operation. If you are not experienced with high-voltage systems, consult with a professional.