Industrial-Grade Isolation MOSFET Power Switch Module (LR7843 Compatible)

Pinout & Interface Guide

The module is clearly divided into a low-voltage control side and a high-voltage power side.

Control Side (2-Pin Header)

• PWM / SIG (Signal): The control input. Apply a HIGH logic signal (2.5V-20V) to turn the MOSFET ON. Apply LOW (0V) to turn it OFF . Sending a PWM signal enables speed or brightness control .

• GND (Ground): Common ground for the control side. Connect to a GND pin on your microcontroller. This ground is electrically isolated from the load ground .

Power Side (3-Pin Screw Terminal)

• + (Positive Input): The positive input for your external DC power supply (6-28V). Connect the positive lead from your battery or power supply here .

• LOAD (Switched Output): The switched output. Connect the negative wire of your load (motor, LED strip, etc.) to this terminal. The MOSFET acts as a switch between this terminal and the power supply ground .

• – (Negative Input): The negative input for your external DC power supply. Connect the negative (ground) lead from your power supply here .

Important Wiring Note: This module is designed for low-side switching. The positive wire of your load must be connected directly to the + terminal of your external power supply. The module switches the ground (negative) side of the circuit .

Theory of Operation

Understanding how this module works helps in troubleshooting and optimizing its use.

1. Control Signal Input: Your microcontroller sends a signal to the PWM/SIG pin. This pin has a 1kΩ series resistor to limit current to the internal optocoupler LED .

2. Optocoupler Activation: When the signal is HIGH, it illuminates the LED inside the PC817 optocoupler. This light activates the phototransistor on the other side of the isolation barrier .

3. Gate Biasing: The activated phototransistor completes a circuit, creating a voltage divider (typically using two 4.7kΩ resistors) that biases the MOSFET’s gate to approximately 50% of the load power supply voltage . For a 12V supply, the gate gets about 6V, which is sufficient to fully turn on the logic-level MOSFET.

4. Load Switching: When the MOSFET turns on, it connects the LOAD terminal to the power supply ground (– terminal), completing the circuit and powering your device .

5. Visual Indication: The completed circuit also provides a ground path for the onboard status LED, which lights up to show that the MOSFET is conducting .

6. Deactivation: When the control signal goes LOW, the optocoupler LED turns off. The phototransistor opens, and a pull-down resistor (4.7kΩ) quickly drains any remaining charge from the MOSFET’s gate, turning it off and disconnecting the load .

Usage Guide

Important Safety Warnings

• DC ONLY: This module is designed strictly for DC circuits . Never connect AC mains power (e.g., household 110V/220V AC) to this module. It will be destroyed instantly and poses a serious safety hazard .

• Inductive Loads Require a Flyback Diode: When controlling inductive loads like motors, solenoids, or relays, you MUST add an external flyback (snubber) diode across the load’s terminals (cathode/banded end to load positive, anode to load negative). The module does not include this diode, and switching inductive loads without it will eventually damage the MOSFET .

• Thermal Management: While the MOSFET is rated for very high peak currents, the small PCB has minimal heatsinking capability . For continuous operation above 5-10A, provide active cooling (a fan) or attach a small heatsink to the MOSFET. Keep the MOSFET temperature below 80°C for long-term reliability .

• Power Supply Voltage: Note the minimum supply voltage requirement. The MOSFET’s gate is driven to 50% of the load supply voltage. At lower voltages, the gate may not turn on hard enough, causing increased resistance and heat. For currents above a few amps, a minimum 9V supply is recommended .

Wiring Guide (Controlling a 12V DC Motor with Arduino)

Basic Arduino Example Code

This code demonstrates simple on/off control and PWM speed control.

// LR7843 MOSFET Module Example

int mosfetPin = 9; // MOSFET SIG pin connected to PWM-capable pin 9
int speed = 0;     // Variable for PWM speed

void setup() {
  pinMode(mosfetPin, OUTPUT); // Set the pin as an output
}

void loop() {
  // 1. Simple On/Off Control (Relay Replacement)
  digitalWrite(mosfetPin, HIGH); // Turn load ON (e.g., motor full speed)
  delay(2000);
  digitalWrite(mosfetPin, LOW);  // Turn load OFF
  delay(2000);

  // 2. PWM Speed Control / Dimming
  // Gradually increase speed/brightness
  for (speed = 0; speed <= 255; speed++) {
    analogWrite(mosfetPin, speed);
    delay(10);
  }
  // Gradually decrease speed/brightness
  for (speed = 255; speed >= 0; speed--) {
    analogWrite(mosfetPin, speed);
    delay(10);
  }
  delay(1000);
}