Oliver Ridler1. Project Overview
The APL80501K is a high-performance, compact 1000W programmable electronic load designed for serious bench work. While conventional high-power loads are typically large, heavy and expensive, the APL80501K achieves an compact, lightweight footprint by leveraging a unique thermal path and architectural approach. It trades pure, ripple-free DC loading for a lightweight design, making it ideal for high-power testing where cost and bench space matter most.
Despite its small size, it offers high-end test accuracy with 20-bit voltage and current measurements. Safety is at the core of the design, featuring multiple hardware and software-level automatic shutdowns. For higher currents and power, the system scales seamlessly; multiple units can be daisy-chained and controlled via a unified Web App (Android/Windows) to act as a single high-power load.
You can find a short video demonstrating it in operation here:
2. The Problem & Motivation: A Better Thermal Path
Typical electronic loads rely on large aluminium heatsinks to move energy from a transistor into the air. This "Transistor-to-Heatsink-to-Air" path is bulky, heavy, and relies on expensive components to prevent silicon failure.
The APL80501K takes a different approach by burning energy directly in Nichrome wire. Shifting the thermal load to the resistors themselves simplifies the thermal path, eliminating heavy transistors and heatsinks, and reducing the cost and weight of the unit without sacrificing raw power handling capability.
3. Key Specifications
- Power: 1000W Continuous (Tested beyond 1100W) at ambient temperatures up to 30°C.
- Voltage/Current: Up to 80V or 50A continuous.
- Precision Metrology:
- Voltage Accuracy: +- 0.1% +- 0.01V
- Current Accuracy: +- 0.5% +- 0.03A (75ppm TC Shunt)
- Resolution: 20-bit.
- Calibration Traceability: Calibration is traceable to the International System of Units (SI) via national metrology institutes that are signatories to the CIPM Mutual Recognition Arrangement.
- Operating Modes: Constant Current (CC), Constant Power (CP), Constant Voltage (CV), and Constant Resistance (CR).
- Advanced Testing: Battery discharge (with impedance measurement), Solar Panel/OCP/OPP testing with V vs. I plotting.
- Range Limits: Controllable current range from 100mA to 50A; controllable power range from 1W to 1000W. Minimum effective load resistance is 0.2Ω.
- Data Capture: 100Hz control loop frequency with full measurement logging to file for post-processing.
4. System Architecture
The digital and power sections are galvanically isolated (200V functional) to protect the 12V supply and allow connection of multiple units.
- Control: An ESP32-C6 handles core logic, protection routines, and Bluetooth LE.
- Measurement: Isolated I2C communication with a Texas Instruments INA228 and a 0.5mΩ sense resistor.
- Power Stage: 4 MOSFETs switching at 20kHz (interleaved), yielding a combined 80kHz ripple frequency. Each FET switches a dedicated 0.8Ω resistor. Isolated gate drivers with optimized gate resistors ensure tight control over edge rates, balancing FET power dissipation and EMC emission reduction.
- Input Filtering: An LC filter (3.5uH inductor rated for >50A continuous / 48 x 2.2uF X7R ceramics) minimizes current ripple and smooths the load seen by the battery or power supply being tested.
- Safety Interlocks:
- Real-time fan tachometer and exhaust temperature monitoring.
- Continuous over voltage, current and power monitoring.
- Continuous 12V supply monitoring with auto-disengage if voltage dips too low.
- Physical "Kill Button" on the front panel for immediate disengagement.
- Connectivity: Front panel features 4mm banana sockets (up to 30A) and 6mm screw terminals for full 50A high-current runs.
5. Thermal Management & Aerodynamics
The load utilizes 4 x 0.8Ω resistors, each constructed from 7 parallel coils of 0.2mm diameter Nichrome wire (1.4m of wire per resistor). Even at 250W per resistor, the resistive coils stay well below "red glow" temperatures thanks to the large surface area and high-velocity airflow.
The resistors sit inside a 60mm x 60mm channel fed by a speed-controllable fan. A custom air-straightening nozzle equalizes pressure across the channel, ensuring no thermal hotspots. This design results in an exhaust air temperature rise of less than 50°C even when dissipating a full kilowatt.
6. Software & Web Interface
The Web App provides a real-time dashboard with data updates every 10ms, featuring a 10-second rolling V&I plot alongside a command and error log window.
- Independence: The device remains safely engaged and maintains its internal safety limits even when disconnected from the Web App.
- Data Logging: Allows capturing full, high-rate data logs directly to a file for external analysis.
- Scalability: Integrated UART TX/RX connectors allow daisy-chaining multiple units together. One automatically becomes the "Leader" and the others "Followers." The Leader continuously monitors the voltage and current of the followers, reports unified telemetry to the web app, and triggers a synchronized shutdown if an anomaly occurs.
- Open Access: We are providing the Bluetooth control interface specification, allowing users to integrate the APL80501K into custom automated test scripts and environments.
7. Current Status & Roadmap
The existing prototype is performing flawlessly through characterization testing. The final design revision is underway to migrate the 12V input to a standardized USB-C PD connector for easier laboratory integration.
Next Steps: Full verification testing and long-term soak tests. Add ability to update eSW from the web app over Bluetooth.
I am actively seeking community feedback—are there specific features, test modes or safety edge-cases you’d like to see implemented in the final design?