[edit: updated the links]
This is the 3rd revision of the load first discussed here:
https://www.eevblog.com/forum/projects/programmable-electronic-load-0-5a/As pointed out by several forum members, the previous revision had some issues:
- The PWM ref voltage was unstable as it relied only on the stability of the AMS1117 LDO - which is not great
- The MOSFET was not good for a load. A linear one gives way better performance, both in therms of max continuous power but also because it has a DC region specified in the datasheet SOA
- The reverse polarity was not ideal as the MOSFET went into the linear region at low voltages and heated up considerably
- The opamps (TL071) were too noisy for this application
- Several other smaller issues - like the screen being too small and hard to read, lack of SD card for saving the data, incomplete schematic (opamp voltages were generated outside, no fan mosfet on the PCB) etc
I've been working on an improved version during the past month and I finally managed to get something working.
Specs:
- 0-4A (it was 5A in rev2, dropped now due to shunt de-rating)
- 1mA, 1mV resolution. Accuracy should be similar if calibrated
- 0-30V
- 100W continuous with the recommended heatsink & FAN
- 3D Printed case
- Reverse polarity protection (opamp driven, improved from rev2)
- 2 & 4 wire measurements (rev2 had only 4 wire)
- Autoranging for both V&A using the ADS1115 PGA to get the max resolution/accuracy I can get from that part
- Temperature sensing
- Fan control (mosfet on pcb this time)
- Fully programmable: it can execute a programmed sequence of operations and output the data through serial
- SDCard for data saving (a csv per session)
- 320x240 Touchscreen
- Proper linear power MOSFET (IXTQ 42N25P)
- Voltage reference REF3120AIDBZR (rev2 used PWM for this)
- Precision DAC DAC8571IDGKR (rev2 used PWM)
- Digital section on the PCB (rev2 PCB had no room for the esp32)
- USB Powered (rev2 needed 3v3 input and +/- 12V for the opamps)
The software has the following features (some are WIP)
- Voltage/Current/Power/Resistance display
- Energy - both Wh and Ah
- Graphs for the whole session
- Constant Current/Power/Resistance Modes
- SD card data saving
- Time/Energy/Voltage limits
I had to switch to a 4-layer PCB for rev3 in order to fix the esp32.
Possible improvements for a rev4:
- Better shunt, double the current capability
- Use a raw esp32 module instead of a devkit, to cut BOM costs
- 2-3 power mosfets, each with load regulation for way more power
I made a video with it in action:
https://youtu.be/jkzM7wHbVtESchematic:
https://easyeda.com/jeanlemotan/jlm-electronic-loadGithub project:
https://github.com/jeanlemotan/jlm-electronic-loadLet me know what you think.
I plan to do some more videos analyzing the schematic - including TINA sims for the load regulation and so on. I'm a SE, not an EE and I don't want to spread false info so your scrutiny is appreciated.
[Edit]
Second video here with the PCB & Schematic and details about the load regulation.
https://youtu.be/gcrGE7rbWiYHopefully I haven't made any embarrassing mistake