DIY M8 Electronic Load & Resistance Tester

If you're interested in a DIY M8-based microcontroller constant current digital electronic load and AC internal resistance tester, this article is worth checking out.

Core Features

· Constant current electronic load

· Battery capacity testing

· Maximum working current test for switching power supplies

· AC internal resistance measurement

Specifications

· A/D Conversion: 10-bit ±1 (using 64x oversampling)

· Voltage Display: Up to 50V, ±50mV (with 64x oversampling, reduced to ±10mV accuracy)

· Current Display:

  - Up to 5A: ±5mA (using 0.05Ω sampling resistor, current = 0.25V / 0.05Ω)

  - Up to 30A: ±30mA (using 0.0083Ω sampling resistor)

· Current Adjustment:

  - 13-bit PWM D/A

  - Minimum resolution: 1mA (5A range), 4mA (30A range)

· AC Internal Resistance Measurement Range: 0 – 500mΩ (measured at 10mA, 1kHz)

  - Adjustable using R7:

    - R7 = 50Ω → 0 – 500mΩ

- R7 = 100Ω → 0 – 1000mΩ

Circuit Configuration

· PC4 (ADC4) of the M8 is used for alarm output (100Hz signal)

· Remove R60, and use PB2 (OC1B) as the 1kHz AC source output for internal resistance testing

· Remove R25, and use PC3 (ADC3) as the input for AC resistance ADC measurement

· PC2 (ADC2) serves as the voltage input for the low-voltage range (0 – 4.5V)

M8 Fuse Bit Settings

Additional Functional Enhancements

· Current Display Zero Calibration: To reduce offset errors caused by amplifier temperature drift

· Startup Current Display Zeroing: For better accuracy at power-on

· Serial Output Format (once per second, values without decimal points):

  - Current (mA), Voltage (mV), Capacity (mAh), DC Resistance (mΩ), AC Resistance (0.1mΩ)

· Added ADC2 Voltage Input:

  - Can use two 100K resistors for voltage divider

  - Enables accurate measurement of voltages below 5V

  - Includes V1 (ADC2) calibration

1. Modified Serial Output Format

2. Added Serial Port Virtual Button Controls

Finished.