Solar Panel I-V Curve Tracer

Note

The project was built on top of the template project provided by ESP-IDF. The example is used by command idf.py create-project that copies the project to user specified path and set it's name. For more information follow the docs page

How

The project was developed with the VSCode IDE using the ESP-IDF extension. The project can be built and flashed to the ESP32-C3 using the ESP-IDF tools. The web server can be accessed by connecting to the ESP32-C3's Wi-Fi network and navigating to the IP address of the ESP32-C3 in a web browser.

A good starting point is:

Copy the settings file for VSCode to enable ESP-IDF extension features:

cp .vscode/settings.json.example .vscode/settings.json

Then choose the esp32c3 target by clicking on the target in the bottom bar of VSCode, or by pressing Ctrl+Shift+P and typing ESP-IDF: Set Espressif device target. The project was developed with ESP-IDF version 5.5.1, so make sure to use the same version or a compatible one. You can set the ESP-IDF version by clicking on the ESP-IDF version in the bottom bar of VSCode, or by pressing Ctrl+Shift+P and typing ESP-IDF: Select Current ESP-IDF Version.

By doing this, the ESP-IDF extension will use the correct toolchain and settings for the ESP32-C3.

To connect to the ESP32-C3's Wi-Fi network, the default SSID is "ESP32_PLOT" with no password. Then navigate to http://192.168.4.1 in a web browser and the web interface should load. The /data endpoint is used to fetch the data for the chart, it returns a JSON object with the voltage and current data as a list of x,y points.

Flashing

Because Secure Boot is enabled, the standard ESP-IDF flash button will fail when writing the SPIFFS partition. Use the provided script instead:

./secure_boot_flash.sh

This builds the project and flashes all binaries in the correct order, working around the Secure Download Mode limitation on large partition erases.

OTA Updates

If the device is already running and connected to Wi-Fi, firmware and storage can be updated over the air without a USB connection:

1. Download the latest app.bin and storage.bin from the GitHub releases (or build them locally).
2. Navigate to http://<device-ip>/ota in a browser.
3. Under App firmware, select app.bin and click Upload and Flash.
4. Once the device reboots, go back to /ota.
5. Under Storage / SPIFFS, select storage.bin and click Upload Storage.

The device reboots automatically after each upload.

[!NOTE] I'm experimenting with SECURE_BOOT Right now some boards may have *.pem file applied.

SECURE_BOOT is enabled on the test device. The signing key secure_boot_signing_key.pem is committed to the repo for convenience since this is a experimental project with no secrets to protect.

Warning

Committing the signing key is only acceptable here because Secure Boot is used for experimentation, not security. In any real deployment, never commit the key — if an attacker obtains it they can sign and flash arbitrary firmware. For production, store it offline in a password manager or encrypted storage.

Additionally, once Secure Boot is burned into a device's eFuses it cannot be disabled. If the key is ever lost, the device can never be updated again and is permanently bricked. See LESSONS.md for the full story.

What

The Circuit is based on an ESP32-C3 microcontroller, INA219 current sensor, and an op-amp based voltage controlled current source (VCCS) to load the solar panel. The ESP32-C3 reads the voltage and current from the solar panel using the INA219 sensor, and adjusts the load using PWM to control the VCCS. The data is then sent to a web server hosted on the ESP32-C3, where it can be visualized in real-time.

A prototype PCB was mounted on a perfboard, and the firmware was developed using ESP-IDF. The web interface is built using HTML, CSS, and JavaScript, and uses the Chart.js library for data visualization. For more information about the circuit design and implementation, please refer to the notebook.

Prototype PCB

Measurement setup

Web interface

The solar panel used is a Luxen 10W 12V panel. LN-10P

References

• Rashid, M.H. (2013) Power Electronics: Devices, Circuits, and Applications. 4th Edition, Pearson Education, Harlow. Chapter 16 - Introduction to Renewable Energy.
• Practical Guide to Implementing Solar Panel MPPT Algorithms - 00001521a.pdf
• Modeling Photovoltaic Cells - Theory 1/2 - YouTube
• Modeling Photovoltaic Cells - LTspice model part 2/2 - YouTube
• INA219 Zerø-Drift, Bidirectional Current/Power Monitor With I2C Interface datasheet (Rev. G) - ina219.pdf
• Using An Op Amp for High-Side Current Sensing (Rev. A) - sboa347a.pdf
• operational amplifier - High voltage adjustable constant current source controlled by MCU - Electrical Engineering Stack Exchange
• An_Easy_Solution_to_Current_Limiting_an_Op_Amp - sbva011.pdf
• Implementation_and_Applications_of_Current_Sources_and_Current_Receivers\ - sboa046.pdf
• operational amplifier - MOSFET - OPAMP circuit - Electrical Engineering Stack Exchange
• voltage - Driving an IRLZ44N Logic MOSFET with a 2N2222 NPN Transistor from an ESP32 - Electrical Engineering Stack Exchange
• microcontroller - Micro-controller controlled current source - Electrical Engineering Stack Exchange
• Unstable Feedback in Opamp+MOSFET circuit for Voltage Controlled Current Source - Electrical Engineering Stack Exchange
• Power MOSFET gate driver fundamentals - AN90059.pdf
• PWM DAC (Rev. A) - slaaec5a.pdf
• Using PWM Output as a Digital-to-Analog Converter on a TMS320F280x (Rev. A) - spraa88a.pdf
• Using PWM Timer_B as a DAC (Rev. A) - slaa116a.pdf
• Dual-Output 8-Bit PWM DAC Using Low-Memory MSP430™ MCUs - slaa804.pdf