Current, voltage and power

ACS712 current sensor with ESP32

The ACS712 is a 5 V analog part, and that makes it awkward on an ESP32. Its output sits at half the supply, about 2.5 V at zero current, and swings up or down with current direction. That can reach beyond what an ESP32 pin accepts, so a voltage divider is required. With 10 kΩ from the output to the pin and 18 kΩ from the pin to ground, the ratio is about 0.643: a zero-current output of 2.5 V becomes about 1.61 V, and the 30 A version stays within about 0.33 to 2.88 V.

Three versions look alike, so read the marking on the chip: 185 mV per amp for 5 A, 100 mV per amp for 20 A, and 66 mV per amp for 30 A. Using the wrong value in code gives a neat, wrong number. The output is ratiometric, so any change in the 5 V supply moves the reading too. Measure the zero point with no load instead of assuming 2.5 V. The example does that at start-up, so start it with the load off.

Precision is limited. At 66 mV per amp, a small current is lost in ADC noise, and the ESP32 ADC is not linear. Averaging helps. For small DC currents, an INA219 or INA226 does better. Where the ACS712 fits is higher current and isolation: the sensing path is separate from the output side. On AC, the output is a wave around the zero point, so a single sample means nothing and you need to sample a whole cycle. LCD Ultra Pro can read an analog pin and scale it for a bar without code.

Power and logic levels

5 V signal: needs a divider or level shifter

Supply	5 V
Signal level	Output up to about 5 V
On an ESP32	Use a divider on OUT. Never connect it straight to a GPIO.

ESP32 pins, including those on a display board's connectors, work at 3.3 V and are not 5 V tolerant. Modules differ, so if in doubt, measure the signal before you connect it.

const int OUT_PIN = 4;
const float RATIO = 18.0 / (10.0 + 18.0);
const float MV_PER_AMP = 185.0;       // 100 or 66 on other versions
float zeroMv;

float readMv() {                      // average to reduce ADC noise
  long sum = 0;
  for (int i = 0; i < 200; i++) sum += analogReadMilliVolts(OUT_PIN);
  return sum / 200.0 / RATIO;
}

void setup() {
  Serial.begin(115200);
  zeroMv = readMv();                  // load must be off at boot
}

void loop() {
  Serial.printf("%.2f A\n", (readMv() - zeroMv) / MV_PER_AMP);
  delay(500);
}

Use this information at your own risk. It is for general guidance only: the example code compiles, but it has not been tested on every part, and modules differ between sellers. Check the manufacturer's data before you build. Product names belong to their owners, and LCD Ultra Pro is not affiliated with them. See our Terms.