ACS712 Overview

The ACS712 IC operates from a 4.5V to 5.5V supply and is normally powered from 5V on this style of module. With no current flowing, the analogue output is nominally half the supply voltage, or about 2.5V when powered from 5V. Current in one direction moves the output above this midpoint and current in the opposite direction moves it below the midpoint.

5A, 20A and 30A Variants

The three versions use different factory-set sensitivities. The sensitivity value used in your software must match the ACS712 variant fitted to the module.

At IC level, the ACS712 has an 80kHz nominal bandwidth and approximately 1.2mΩ primary conductor resistance. Components fitted to a particular breakout board can reduce its effective bandwidth.

Module Connections

The pictured module style has a three-pin signal header marked VCC, OUT and GND, plus a two-position screw terminal carrying the measured current.

Arduino UNO Wiring

The Arduino UNO is convenient for this module because both use a nominal 5V supply and the UNO provides a 10-bit analogue input.

Low-Voltage DC Current Path

Switch the load supply off before changing the current-path wiring. A typical high-side connection is:

Power supply positive → ACS712 screw terminal → ACS712 screw terminal → load positive

Connect the load negative back to the power supply negative in the normal way. The two screw terminals form a series current path; neither screw terminal is the module's signal GND connection.

Arduino DC Current Test

No external Arduino library is required. The sketch below measures the zero-current offset during startup, averages the analogue input and converts the result to amperes.

Set SENSITIVITY_V_PER_A to 0.185 for the 5A version, 0.100 for the 20A version or 0.066 for the 30A version.

const int SENSOR_PIN = A0;

// Select the value for your ACS712 variant:
// 5A  = 0.185 V/A
// 20A = 0.100 V/A
// 30A = 0.066 V/A
const float SENSITIVITY_V_PER_A = 0.185;

const int ZERO_SAMPLES = 1000;
const int READ_SAMPLES = 200;

float zeroCounts = 0.0;

void setup() {
  Serial.begin(115200);
  delay(500);

  Serial.println("ACS712 current sensor");
  Serial.println("Keep current at 0 A while calibrating...");

  long sum = 0;

  for (int i = 0; i < ZERO_SAMPLES; i++) {
    sum += analogRead(SENSOR_PIN);
    delay(2);
  }

  zeroCounts = sum / (float)ZERO_SAMPLES;

  Serial.print("Zero ADC reading: ");
  Serial.println(zeroCounts, 2);
  Serial.println("Calibration complete.");
}

void loop() {
  long sum = 0;

  for (int i = 0; i < READ_SAMPLES; i++) {
    sum += analogRead(SENSOR_PIN);
  }

  float averageCounts = sum / (float)READ_SAMPLES;

  // The ACS712 and UNO ADC are both referenced to the same 5 V rail.
  // Sensitivity values above are specified at a nominal 5 V supply.
  float voltsFromZero =
      (averageCounts - zeroCounts) * (5.0 / 1023.0);

  float currentA =
      voltsFromZero / SENSITIVITY_V_PER_A;

  Serial.print("Current: ");
  Serial.print(currentA, 3);
  Serial.println(" A");

  delay(500);
}

Expected Result

With no current flowing, the serial output should remain close to 0A, although a small amount of variation is normal. Apply a known low-voltage DC load and the displayed current should change accordingly.

If the current magnitude looks reasonable but its sign is reversed, the current is simply flowing through the ACS712 in the opposite direction to the software convention. Reverse the screw-terminal current-path connections or invert the result in software.

Low-Voltage AC RMS Measurement

The ACS712 can also measure alternating current. For a 50Hz or 60Hz low-voltage AC signal, the following example repeatedly samples the waveform and calculates RMS current over a 200ms measurement window.

const int SENSOR_PIN = A0;

// Select the value for your ACS712 variant:
// 5A  = 0.185 V/A
// 20A = 0.100 V/A
// 30A = 0.066 V/A
const float SENSITIVITY_V_PER_A = 0.185;

const int ZERO_SAMPLES = 1000;
const unsigned long WINDOW_US = 200000UL;

float zeroCounts = 0.0;

void setup() {
  Serial.begin(115200);
  delay(500);

  Serial.println("ACS712 AC RMS measurement");
  Serial.println("Keep current at 0 A while calibrating...");

  long sum = 0;

  for (int i = 0; i < ZERO_SAMPLES; i++) {
    sum += analogRead(SENSOR_PIN);
    delay(2);
  }

  zeroCounts = sum / (float)ZERO_SAMPLES;

  Serial.print("Zero ADC reading: ");
  Serial.println(zeroCounts, 2);
  Serial.println("Calibration complete.");
}

void loop() {
  unsigned long startTime = micros();
  unsigned long sampleCount = 0;
  float sumSquares = 0.0;

  while ((unsigned long)(micros() - startTime) < WINDOW_US) {
    int raw = analogRead(SENSOR_PIN);

    float voltsFromZero =
        (raw - zeroCounts) * (5.0 / 1023.0);

    float currentA =
        voltsFromZero / SENSITIVITY_V_PER_A;

    sumSquares += currentA * currentA;
    sampleCount++;
  }

  float rmsCurrent = 0.0;

  if (sampleCount > 0) {
    rmsCurrent = sqrt(sumSquares / sampleCount);
  }

  Serial.print("RMS current: ");
  Serial.print(rmsCurrent, 3);
  Serial.println(" A");

  delay(300);
}

Expected Result

With no load current, the calculated RMS value will normally be slightly above zero because sensor and ADC noise are also included in the RMS calculation. With a steady 50Hz or 60Hz AC load, the displayed value should settle near the actual RMS current after calibration.

Accuracy, ADC Resolution and Calibration

The ACS712 output is analogue, so measurement quality depends on the sensor variant, ADC resolution, electrical noise, zero-current offset, temperature and calibration.

These figures use the UNO's nominal 5V, 10-bit ADC. Averaging many readings reduces random variation but does not remove sensor offset, sensitivity tolerance or temperature drift.

Zero-Current Calibration

Measure the OUT voltage or ADC value while exactly 0A is flowing and use that measured value as the zero point rather than assuming it is precisely 2.500V. The example sketches perform this automatically at startup.

Known-Current Calibration

For better absolute accuracy, pass a stable known current through the sensor and compare the ACS712 result with a suitable reference meter. The effective sensitivity can then be calculated from the change in output voltage divided by the reference current.

Effective sensitivity = |loaded output voltage − zero-current voltage| ÷ reference current

Use the resulting value in place of the nominal 0.185, 0.100 or 0.066V/A constant.

Troubleshooting