Why a Benchtop Electronic Load is One of the Most Awesome Arduino Projects You Can Build

If you want to test power supplies, measure battery capacity, or characterize solar panels, a multimeter and a power resistor won't cut it. You need a constant-current electronic load. While commercial units from Rigol or BK Precision cost $300 to $800, building your own is easily one of the most awesome Arduino projects you can tackle on the workbench. It forces you to deal with analog control loops, I2C bus management, and power electronics thermal limits all in one build.

This guide walks through building a 3A, 26V precision electronic load using an Arduino Nano V3, an INA219 current sensor, and an MCP4725 DAC. We will cover the exact component specs, the PI control loop firmware, and how to debug the inevitable I2C faults.

System Architecture and Component Selection

An electronic load works by forcing a MOSFET into its linear (ohmic) region, acting as a variable resistor. The Arduino reads the actual current via the INA219 shunt monitor, compares it to your dial setpoint, and adjusts the MOSFET gate voltage via the MCP4725 DAC to maintain a constant current regardless of the input voltage.

Below is the exact bill of materials with 2026 pricing and critical limits. This data-dense spec sheet ensures you don't buy under-rated clones.

Wiring and Pin Mapping

The system relies heavily on the I2C bus. Because the Arduino Nano V3 uses the ATmega328P, it does not have internal I2C pull-up resistors enabled by default in the Wire library. You must ensure your MCP4725 and INA219 breakout boards have 4.7kΩ pull-ups to 5V, or add them externally to the SDA and SCL lines.

This build targets the Arduino Nano V3 (ATmega328P, 5V logic). If you are using a Nano 33 IoT or ESP32, you must use logic level shifters for the 5V I2C LCD and adjust the analog reference voltages in the code.

For the high-current path (the device under test), use minimum 14 AWG wire from the binding posts, through the INA219 Vin+/Vin- terminals, through the IRLZ44N Drain/Source, and through the 0.1Ω resistor back to ground. Do not route high-current return paths through the Arduino breadboard ground rails.

The Firmware: Constant Current Control Loop

The firmware uses a Proportional-Integral (PI) control loop. A pure proportional loop leaves a steady-state error (the actual current will always be slightly below the setpoint). The integral term accumulates the error over time to eliminate this offset. The code includes explicit error handling to halt the load if the I2C sensors drop off the bus, preventing a runaway overcurrent event.

#include <Wire.h>
#include <Adafruit_INA219.h>
#include <Adafruit_MCP4725.h>
#include <LiquidCrystal_I2C.h>

// --- PIN DEFINITIONS ---
#define POT_PIN A0
#define ENABLE_BUTTON_PIN 2

// --- I2C ADDRESSES ---
#define LCD_ADDR 0x27 
#define DAC_ADDR 0x60

// --- CONTROL LOOP TUNING ---
const float Kp = 1200.0;  // Proportional gain
const float Ki = 400.0;   // Integral gain
const float MAX_CURRENT_MA = 3000.0; // 3A limit
const unsigned long LOOP_DT = 50;    // 50ms loop time

// --- OBJECTS ---
Adafruit_INA219 ina219;
Adafruit_MCP4725 dac;
LiquidCrystal_I2C lcd(LCD_ADDR, 16, 2);

// --- STATE VARIABLES ---
float integral = 0.0;
float last_error = 0.0;
bool load_enabled = false;
bool last_button_state = HIGH;
unsigned long last_loop_time = 0;

void setup() {
  Serial.begin(115200);
  pinMode(ENABLE_BUTTON_PIN, INPUT_PULLUP);
  
  Wire.begin();
  
  // Initialize LCD
  lcd.init();
  lcd.backlight();
  lcd.setCursor(0, 0);
  lcd.print("Elec Load Init..");

  // Initialize INA219 with error handling
  if (!ina219.begin(&Wire)) {
    Serial.println("Failed to find INA219 chip");
    lcd.clear(); lcd.print("INA219 FAIL");
    while (1) { delay(10); } // Halt safely
  }
  
  // Initialize MCP4725 with error handling
  if (!dac.begin(DAC_ADDR, &Wire)) {
    Serial.println("MCP4725 not found at I2C address");
    lcd.clear(); lcd.print("DAC FAIL");
    while (1) { delay(10); } // Halt safely
  }

  // Ensure load is OFF at startup
  dac.setVoltage(0, false);
  
  lcd.clear();
  lcd.setCursor(0, 0);
  lcd.print("Set: 0.00A");
  lcd.setCursor(0, 1);
  lcd.print("Act: 0.00A OFF");
}

void loop() {
  // 1. Handle Enable/Disable Button (Debounce)
  bool current_button_state = digitalRead(ENABLE_BUTTON_PIN);
  if (current_button_state == LOW && last_button_state == HIGH) {
    load_enabled = !load_enabled;
    if (!load_enabled) {
      dac.setVoltage(0, false); // Kill gate drive immediately
      integral = 0; // Reset windup
    }
    delay(50); // Simple debounce
  }
  last_button_state = current_button_state;

  // 2. Read Setpoint from Potentiometer (0.0 to 3.0A)
  int pot_raw = analogRead(POT_PIN);
  float setpoint_mA = (pot_raw / 1023.0) * MAX_CURRENT_MA;

  // 3. Execute PI Control Loop at fixed interval
  unsigned long now = millis();
  if (now - last_loop_time >= LOOP_DT) {
    last_loop_time = now;
    
    float actual_mA = ina219.getCurrent_mA();
    
    if (load_enabled) {
      float error = setpoint_mA - actual_mA;
      
      // Anti-windup clamping
      if (integral > 2000.0) integral = 2000.0;
      if (integral < -2000.0) integral = -2000.0;
      
      integral += error * (LOOP_DT / 1000.0);
      
      float output_dac = (Kp * error) + (Ki * integral);
      
      // Clamp DAC output to 12-bit limits (0-4095)
      if (output_dac > 4095) output_dac = 4095;
      if (output_dac < 0) output_dac = 0;
      
      dac.setVoltage((uint16_t)output_dac, false);
    }
    
    // 4. Update UI
    lcd.setCursor(5, 0);
    lcd.print(setpoint_mA / 1000.0, 2);
    lcd.print("A  ");
    
    lcd.setCursor(5, 1);
    lcd.print(actual_mA / 1000.0, 2);
    lcd.print("A ");
    lcd.print(load_enabled ? "ON " : "OFF");
    
    // Serial logging for debugging
    Serial.print("Set:"); Serial.print(setpoint_mA);
    Serial.print(" Act:"); Serial.print(actual_mA);
    Serial.print(" DAC:"); Serial.println(dac.readData());
  }
}

Debugging: When the I2C Bus or MOSFET Fails

Embedded hardware rarely works on the first power-up. When dealing with mixed-signal I2C buses and high-current linear MOSFETs, failures usually fall into specific patterns. Here is how to troubleshoot the most common faults.

Exact Error Strings and Ranked Causes

Error 1: "Failed to find INA219 chip"
This prints to the Serial Monitor and halts the MCU. The Arduino Wire library failed to receive an ACK on address 0x40.

• Cause A (Most Likely): Missing I2C pull-up resistors. The Nano's internal pull-ups are ~30kΩ, which is far too weak for 100kHz I2C. Solder 4.7kΩ resistors from SDA to 5V and SCL to 5V.
• Cause B: The A0 address jumper on the INA219 breakout is bridged, shifting the address to 0x41 or 0x44. Check the breakout board silkscreen and adjust the ina219.begin() parameter if necessary.
• Cause C: Voltage drop on the 5V rail. If the LCD backlight is drawing too much current, the Nano's onboard 5V regulator may be browning out, causing the INA219 to reset.

Error 2: "MCP4725 not found at I2C address"
The DAC is not acknowledging on 0x60 or 0x61.

• Cause A: The ADDR pin on the MCP4725 breakout is pulled high, changing the address to 0x61. Ground the ADDR pin or update DAC_ADDR in the code.
• Cause B: Capacitive loading on the I2C lines. Long wires between the Nano and the DAC act as capacitors, rounding off the I2C square waves. Keep I2C wires under 6 inches.

The First Three Things to Check When It Fails to Regulate

If the code compiles, the LCD turns on, but the current reads 0.00A or spikes to the maximum when you turn the load on, check these three physical layer issues:

1. MOSFET Gate Threshold (Vgs): The IRLZ44N is a "logic level" MOSFET, but its transfer curve is non-linear below 3V. If your MCP4725 is powered by 3.3V instead of 5V, it cannot output enough gate voltage to fully open the MOSFET at low current setpoints. Ensure the DAC VCC is tied to 5V.
2. Shunt Resistor Kelvin Connection: The INA219 measures voltage across its internal 0.1Ω shunt. If you soldered the high-current wires to the same pads as the I2C/Sense traces, the voltage drop from the high current will corrupt the sensing. Wire the high-current path to the large Vin+/Vin- pads, and let the INA219 breakout route the sense lines internally.
3. Oscillation (Squealing Inductors): If you hear a high-pitched whine from the device under test or the breadboard, your PI loop is oscillating. Lower the Kp value in the code from 1200 to 500, or increase the LOOP_DT to 100ms to slow down the control loop.

Extending and Simplifying the Build

Depending on your bench needs, you might want to strip this project down to its bare essentials or scale it up for heavy-duty battery testing.

How to Simplify (The PWM Dummy Load)

If you don't need 1mA precision and just want to dump 2A to test a PC power supply, drop the MCP4725 DAC entirely. You can drive the IRLZ44N gate directly from an Arduino PWM pin (Pin D3) passed through a simple RC low-pass filter (10kΩ resistor and 1µF capacitor) to create a pseudo-analog DC voltage. This reduces the BOM cost by $2 and eliminates all DAC-related I2C debugging, though you lose the fast transient response of a true 12-bit DAC.

How to Extend (10A and Constant Voltage Mode)

To push this into the 10A+ territory required for testing LiFePO4 battery banks or ATX power supplies, a single IRLZ44N will exceed its SOA and melt. You must parallel three IRLZ44N MOSFETs. Critical rule: Do not just tie their gates together. Each MOSFET must have its own dedicated 0.1Ω source resistor to force current sharing, and each gate needs its own 10Ω gate-stopper resistor to prevent high-frequency parasitic oscillation.

To add Constant Voltage (CV) mode for testing solar charge controllers, wire a second INA219 (with the A0 jumper bridged to address 0x41) across the input terminals to monitor Vbus. Add a toggle switch to the code to switch the PI loop's feedback source from getCurrent_mA() to getBusVoltage_V(), allowing the load to hold a specific voltage while drawing whatever current is necessary.

Building an electronic load bridges the gap between writing code and manipulating real-world power. Once you have this on your bench, you will never go back to guessing battery capacity with a multimeter and a stopwatch.