A smart plug with energy monitoring is an electrical outlet adapter that connects to your standard power socket and allows you to control and monitor any device plugged into it via a smartphone app or voice assistant (compatible with Google Assistant, Alexa, etc.). What sets these smart plugs apart is their built-in energy monitoring feature, which tracks and displays real-time and historical energy consumption data for the connected device.

Detailed Component Breakdown and Signal Flow

Power Supply Section

The smart plug begins with AC mains input (typically 110-240V AC) that feeds into two parallel paths:

  • Energy monitoring path: Voltage divider network reduces 250V to 0-5V for safe measurement by the ADC
  • Power conversion path: AC-to-DC converter (often using flyback topology with controllers like VIPer16) converts mains voltage to regulated DC voltages

The power supply generates multiple voltage rails:

  • 5V DC: Powers the relay and higher current circuits
  • 3.3V DC: Powers the microcontroller, WiFi module, and digital circuits (converted from 5V using LDO regulator)
  • -12V DC: Used for relay and TRIAC gate drive (in some designs)

Energy Monitoring Circuit

The energy monitoring section consists of specialized components for precise measurement:

Voltage Sensing:

  • High-precision resistive voltage divider network (typically 25 ppm accuracy)
  • Signal conditioning to scale 0-250V AC to 0-5V for ADC input
  • 24-bit Sigma-Delta ADC for high-resolution voltage measurement

Current Sensing:

  • Current transformer (CT) or precision shunt resistor (typically 5mΩ, 25 ppm)
  • For CT: Burden resistor converts current to voltage signal
  • For shunt: Direct voltage measurement across the resistor
  • 24-bit Sigma-Delta ADC for current measurement

Energy Monitoring IC:

Popular ICs include STPM10, HLW8012, ADE7753, or BL0937. These specialized chips:

  • Calculate real-time power (P = V × I × cos φ)
  • Measure both active and reactive power
  • Perform energy integration over time
  • Provide calibrated output with <0.1% accuracy

Microcontroller and Processing

The heart of the system is typically a WiFi-enabled microcontroller:

  • ESP8266/ESP32: Most common, integrated WiFi capability
  • STM32W108CB: With separate WiFi module
  • MKM14Z64: ARM Cortex-M0+ based solution

Functions performed:

  • Read energy monitoring data via SPI/I2C
  • Process and calculate power/energy consumption
  • Handle WiFi communication protocols (HTTP/MQTT)
  • Control relay switching logic
  • Manage scheduling and automation

Control and Switching Circuit

The switching mechanism uses:

  • Electromechanical relay: Primary switching element (typically 10-16A rating)
  • TRIAC (optional): For soft-start/stop or dimming functions (T2035H in some designs)
  • Relay driver: Protection and isolation (ULN2003 or similar)
  • Optocouplers: Electrical isolation between control and high-voltage circuits

Communication Interface

WiFi Module Components:

  • Integrated or external WiFi transceiver (2.4GHz band)
  • Antenna (PCB trace or external wire antenna)
  • Communication protocols: TCP/IP, HTTP, MQTT
  • Security: WPA2/WPA3 encryption, AES-128 in some implementations

Data Transmission Flow:

  1. Sensor data → Microcontroller → WiFi module
  2. WiFi module → Home network → Internet/Cloud
  3. Cloud service → Mobile app
  4. Mobile app → Cloud → WiFi module → Microcontroller → Relay control

Safety and Protection Circuits

Critical safety features include:

  • Fuse protection: Primary overcurrent protection
  • MOV (Metal Oxide Varistor): Surge protection
  • Optocoupler isolation: Between low-voltage control and high-voltage switching
  • Thermal protection: Automatic shutdown on overheating
  • Current limiting: Prevents damage from overcurrent conditions

User Interface Elements

  • Status LEDs: Power, WiFi connection, relay state indicators
  • Push buttons: Reset, WiFi pairing, manual control
  • Mobile app interface: Real-time monitoring, control, scheduling, energy reports

Data Processing and Analytics

The microcontroller performs continuous calculations:

  • Instantaneous power: P = V × I × cos(φ)
  • Energy consumption: Integration of power over time (kWh)
  • Power factor calculation: cos(φ) for reactive power analysis
  • Historical data logging: Stored locally or in cloud
  • Threshold monitoring: Automatic alerts for abnormal consumption

This technical implementation allows the smart plug to provide precise energy monitoring while maintaining safe, reliable remote control capabilities through a user-friendly smartphone interface.