Measuring AC Voltage Using Arduino: Maximum Voltage Method

A voltage sensor is used to measure AC and DC voltage, however the use of measurement algorithms are different. The DC voltage is constant so that the measurement is relatively easy, in contrast with AC voltage that varies according to a sine wave form and has a voltage magnitude in the positive and negative quadrant. AC effective voltage magnitude can be determined if the maximum voltage / peak known. By using the right algorithms and mathematical equations, the maximum value and the effective value of the AC voltage can be found.

Problems and significance

A voltage sensor is a device that is commonly used in electronic equipment. basically, a voltage sensor can be obtained through the design of the voltage divider circuit as shown in Figure 1a, and also can be designed using the transformer as shown in Figure 1b. Sensor voltage with the voltage divider technique can be used on AC and DC voltage, while the voltage sensor which uses a transformer can only be used for reading AC voltage. 

Figure 1. AC voltage sensor. a). using a voltage divider technique
b. using a transformer
(source)

Regardless the type of voltage sensor is used, in the microprocessor-based applications the algorithms of AC and DC voltage reading is very different. DC voltage as shown in Figure 2 has a relatively constant value properties, the DC voltage is generally always be in one quadrant (only positive or only negative). By the voltage characteristic as has been described, DC voltage reading becomes easy to apply. Unlike the AC voltage, the voltage is not in constant shape but continued to follow the shape of a sine wave. In addition, the AC voltage is in two quadrants, positive or negative, so it can not be given directly to the input pin of the microprocessor, which can only receive a voltage between 0 to 5 VDC.

Figure 2. The shape of the DC and AC voltages (Source)
(source)

This tutorial is focus to describes the AC voltage reading technique using a voltage sensor with transformer-based as shown in Figure 1b.

Technique and Algorithm

This experiment uses an analog voltage sensor as shown in Figure 1b. The generated signal will be used as an analog input (A0 pin) of ATmega328P microprocessor which integrated in the Arduino Uno development board . To be able to read the AC voltage, such program script as the following can be used:

double sensorValue=0;

double sensorValue1=0;

int crosscount=0;

int climbhill=0;

double VmaxD=0;

double VeffD;

double Veff;

void setup() {

Serial.begin(115200);

}

void loop() {

sensorValue1=sensorValue;

delay(100);

sensorValue = analogRead(A0);

if (sensorValue>sensorValue1 && sensorValue>511){

  climbhill=1;

  VmaxD=sensorValue;

  }

if (sensorValue<sensorValue1 && climbhill==1){

  climbhill=0;

  VmaxD=sensorValue1;

  VeffD=VmaxD/sqrt(2);

  Veff=(((VeffD-420.76)/-90.24)*-210.2)+210.2;

  Serial.println(Veff);

  VmaxD=0;

}

}

The principle of the script being used is to find the maximum voltage Vm of the measured sine wave. Maximum voltage Vm can be found by continuously observing the sinusoidal voltage, if the maximum voltage Vm is smaller than the previous maximum voltage Vm-1, then Vm-1 is the maximum voltage expected, (theoretically it happened only on the peak of AC signal). In script above Vm-1 is "sensorValue1" and Vm is "SensorValue".

sensorValue1=sensorValue;

delay(100);

sensorValue = analogRead(A0);

 The maximum voltage is converted into a effective voltage using equation (1)

 In a program script it written as follow

     VeffD=VmaxD/sqrt(2);

Where VeffD is the effective value in digital number. To get the actual effective voltage a calibration process is required, it is by comparing the digital values to the actual voltage value and integrate it into a linear equation as follows:

     Veff=(((VeffD-420.76)/-90.24)*-210.2)+210.2;

if completed, do not forget to compile and upload the program to the Arduino board, and plug the voltage sensor cables according to the rules on the datasheet. If there is no error (should not be) just click on the serial monitor in the top right corner of the Arduino IDE, the voltage value readout will appear. You can compare it with the reading of the voltmeter. 

Calibration may be better and appear accurately when not using the linear equation but rather uses the n-order polynomial equation. I will discuss this polynomial technique next time.

Potential Applications

• Power quality data logger
• Digital Power meter (KWh Meter)
• Control of electricity protection system

Conclusion

AC voltage has a sinusoidal pattern whose value keeps changing as a function of time and had a magnitude of the voltage in positive and negative territory. If the maximum voltage of the AC signal is known, the effective voltage also can be known. The maximum voltage is determined by comparing the actual value of the voltage (Vm) to the previous voltage value (Vm-1), if Vm-1 is greater than Vm,  the maximum voltage is equal to Vm-1 value.

AC voltage can also be searched by measuring the area of sinus signal, once again I will discuss-developed-design this measurement technique later, be patient :D

Reference
[1] http://images.tutorvista.com/content/electricity/ac-dc-voltage-time-graph.gif
[2] https://fisikakontekstual.wordpress.com/materi-arus-dan-tegangan-listrik-bolak-balik/