Electronic load

This repository contains hardware designs and firmware for my electronic load.

Hardware

This project is based on an analog electronic load PCB from Aliexpress: XHDZ-FZ-2g. Originally, I just put it in a box and used it as-is with a 10-turn potentiometer. However, then the pot failed, and I decided to upgrade it with digital control. The control loop is still analog utilising the original circuitry, but the reference signal is generated by a microcontroller instead of the potentiometer.

My electronic-load-controller PCB contains an ATmega8 microcontroller, a PWM DAC to generate the reference voltage, UART optoisolators for remote control, and circuitry for measuring the load voltage and current. It connects to the analog PCB via the original connectors with just one wire needing to be soldered to a non-standard pad on the board.

The user interface consists of an SSD1306 OLED display and a rotary encoder.

Space for improvement: I didn't spend much time designing the user interface and as a result, I had to use a small font that is hard to read from a distance.

Software

The firmware on the ATmega microcontroller performs several functions. It measures the voltage across the load, the load current, calculates the power loss and implements over-power protection, monitors the heatsink temperature and controls the cooling fan. It uses the DAC to generate the current setpoint signal for the analog board based on the value set by the user via the rotary encoder, and it implements a SCPI interface on the serial port for remote control capability.

SCPI commands

A text-based remote control interface is available on the serial port (9600 baud, 8N1).

The following table explains the individual command (v is a placeholder for an integer argument).

command	description
ISET v	Set load current. E.g. ISET 1000 sets the current to 1A.
ISET?	Get load current setpoint value in mA.
IGAIN v	Settings: set conversion coefficient for current measurement.
IGAIN?	Settings: get IGAIN value.
VGAIN v	Settings: set conversion coefficient for voltage measurement.
VGAIN?	Settings: get VGAIN value.
SPGAIN v	Settings: set conversion coefficient for current setpoint.
SPGAIN?	Settings: get SPGAIN value.
SPOFF v	Settings: set conversion offset for current setpoint.
SPOFF?	Settings: get SPOFF value.
DUTY?	Get PWM DAC duty cycle value. Useful for debugging.
I?	Get measured current in A.
V?	Get measured voltage in V.
P?	Get measured power in W.
TEMP?	Get heatsink temperature in degrees C.
*BOOTLOADER	Jump to bootloader to allow updating firmware.
*SAV	Save settings to EEPROM.
*RST	Reset the microcontroller.

Calibration

Calibration is performed using SCPI commands on the serial interface. The goal is to make the load current and voltage readings accurate and to make the current setpoint match the actual load current.

TODO document calibration procedure.