Overview

The ZMPT101B module is an active single-phase AC voltage measurement sensor designed to safely step down high-voltage mains AC ($100\text{V}$ to $250\text{V}$ RMS) into a low-voltage analog sine wave suitable for microcontroller ADC inputs.

The module incorporates a precision micro current-type voltage transformer (ZMPT101B, 1:1 ratio) that provides galvanic isolation up to $4000\text{V}$ AC between the mains grid and low-voltage logic circuits. An onboard LM358 operational amplifier circuit adds a $\frac{V_{CC}}{2}$ DC offset bias ($2.5\text{V}$ at 5V supply) and amplification potentiometer adjustment, allowing microcontrollers to sample true AC waveforms and compute RMS voltage.

Quick reference

Terminals

High-Voltage Mains Terminals (2-pin Screw Terminal)

Low-Voltage Control Header (4-pin 0.1" Header)

Specifications

Sampling & RMS Calculation

The OUT pin produces an analog sine wave oscillating around $2.5\text{V}$. To calculate true Root-Mean-Square ($V_{RMS}$) voltage:

1. High-Frequency Sampling: Sample the OUT analog pin over 1–2 complete mains cycles ($20\text{ ms}$ for 50 Hz, $16.6\text{ ms}$ for 60 Hz), taking at least 100–200 ADC readings.
2. DC Bias Subtraction: Subtract the DC mid-rail offset ($ADC_{zero} \approx 512$ for 10-bit ADC at 5V) from each reading: $v[i] = ADC[i] - ADC_{zero}$.
3. Square & Average: Compute the mean of squared samples: $V_{mean\_sq} = \frac{1}{N} \sum v[i]^2$.
4. Scale Factor: Multiply $\sqrt{V_{mean\_sq}}$ by the calibrated gain constant ($K_{cal}$):

$V_{RMS} = K_{cal} \times \sqrt{ \frac{1}{N} \sum_{i=1}^{N} \left( ADC[i] - ADC_{zero} \right)^2 }$

Wiring

Example

const int sensorPin = A0;
const float calibrationFactor = 0.525; // Adjust based on multimeter reading

void setup() {
  Serial.begin(9600);
}

void loop() {
  long sumSquare = 0;
  int sampleCount = 0;
  unsigned long startTime = millis();

  // Sample over 200 ms (10 full 50 Hz AC cycles)
  while (millis() - startTime < 200) {
    int raw = analogRead(sensorPin);
    int zeroCentered = raw - 512; // Subtract 2.5V DC offset
    sumSquare += (long)zeroCentered * zeroCentered;
    sampleCount++;
  }

  float meanSquare = (float)sumSquare / sampleCount;
  float rmsADC = sqrt(meanSquare);
  float voltageRMS = rmsADC * calibrationFactor;

  Serial.print("Sampled: "); Serial.print(sampleCount);
  Serial.print(" | AC Voltage: "); Serial.print(voltageRMS); Serial.println(" V RMS");

  delay(1000);
}

Common mistakes

• Single-sample analogRead(): Taking a single instantaneous sample of an AC sine wave yields random values between 0V and 250V depending on where in the 50 Hz cycle the ADC triggers. Always compute RMS over complete periods.
• Forgetting potentiometer gain calibration: Fresh ZMPT101B modules ship with uncalibrated potentiometer positions. Use a multimeter to measure mains AC voltage and adjust the blue trimmer resistor until your code matches the multimeter reading.

Notes

• ZMPT101B vs Resistor Dividers: ZMPT101B provides $4000\text{V}$ magnetic isolation; direct resistor voltage dividers connect mains AC directly to microcontroller ground, creating severe shock and equipment hazards.