ZMPT101B AC Voltage Sensor Transformer

Working Principle

The ZMPT101B is a miniature voltage transformer (VT) that provides galvanic isolation between mains AC voltage and the MCU. The primary winding connects across the AC line (via a high-value resistor to limit current). The secondary winding produces a proportional low-voltage AC output centred around a DC bias (typically VCC/2 = 2.5 V), safe for ADC input.

Analogy: Imagine a pair of coupled tuning forks. Striking one (mains voltage) causes the other (secondary) to vibrate at the same frequency but at a much smaller amplitude. The transformer couples energy magnetically without any electrical contact.

Electrical Characteristics

Parameter	Value
Input Voltage	Up to 250 V AC
Output	Analog AC waveform centred on VCC/2
Turns Ratio	1:1 (with external resistor for ratio scaling)
Isolation	Galvanic (primary-secondary)
Phase Shift	<0.2° at 50/60 Hz
Module Supply	5 V DC
Current Draw	<10 mA

Interfacing with an MCU

The ZMPT101B module has 4 pins: VCC (5 V), GND, signal (analog out), and a potentiometer to adjust output amplitude. The signal output is an AC waveform riding on a ~2.5 V DC offset — connect directly to an ADC pin.

Calibration

Calibration requires a reference multimeter:

• Adjust gain: Turn the module potentiometer until ADC peak-to-peak range is within 0–1023 without clipping.
• Find DC offset: With no AC input, read the ADC — this is your zero reference (should be ~512 on Arduino).
• Determine scale factor: Apply known voltage (e.g., 230 V), compute RMS from ADC samples, then: calibration_factor = actual_Vrms / measured_Vrms.

Code Example

/*
 * ZMPT101B AC Voltage RMS — Arduino
 * Wiring: Signal→A0, VCC→5V, GND→GND
 * WARNING: Primary connects to mains AC — observe safety
 */
#define SENSOR_PIN A0
#define SAMPLES    1000
#define VREF       5.0
#define ADC_MAX    1023.0
#define CAL_FACTOR 234.0  /* Calibrate with multimeter */

void setup() {
    Serial.begin(9600);
    Serial.println("ZMPT101B AC Voltage Sensor Ready");
}

float readVoltageRMS() {
    long sumSq = 0;
    int offset = 512;  /* DC bias ≈ VCC/2 */
    for (int i = 0; i < SAMPLES; i++) {
        int raw = analogRead(SENSOR_PIN) - offset;
        sumSq += (long)raw * raw;
        delayMicroseconds(100);  /* ~10 kHz sample rate */
    }
    float rmsADC = sqrt((float)sumSq / SAMPLES);
    float voltage = (rmsADC / ADC_MAX) * VREF;
    return voltage * CAL_FACTOR;
}

void loop() {
    float vrms = readVoltageRMS();
    Serial.print("AC Voltage: ");
    Serial.print(vrms, 1);
    Serial.println(" V RMS");
    delay(2000);
}

Real-World Applications

Limitations

• Mains safety: Improper installation can cause electric shock or fire — only qualified persons should wire mains connections.
• Frequency dependency: Optimised for 50/60 Hz; accuracy drops at higher frequencies (harmonics).
• ADC resolution: Arduino's 10-bit ADC limits precision to ~0.5% — use an external 12/16-bit ADC for energy metering.
• Temperature drift: The transformer and resistor divider introduce small gain changes with temperature.