Electronics DIY
Building a compact USB power delivery negotiator for testing PD devices and custom charging profiles.
A compact USB PD negotiator guide that blends practical testing, safe handling, and modular design ideas to explore power profiles, negotiate protocols, and validate PD devices in diverse environments.
Published by
Justin Walker
July 30, 2025 - 3 min Read
Designing a portable USB Power Delivery negotiator begins with a focused objective: to verify PD controller behavior, assess cable performance, and compare voltage profiles under controlled conditions. The project prioritizes a compact form factor without sacrificing measurement accuracy or signal integrity. Selecting a capable microcontroller with native USB-C support and a variegated set of VBUS and CC negotiation options allows the device to emulate power sink and source roles. Precision instrumentation, including a small, accurate voltage reference and a high-bandwidth current sensor, ensures that the negotiator can log real-time data for later analysis. A modular layout also supports swapping connectors, adapters, and calibration references without redesign.
At the heart of the build is a PD controller implementation that can perform Standard USB PD, USB PD 3.0, and optional USB-C Alternate Modes. The firmware must model Rd/Rd and Rp resistances, accommodate fast role swap, and manage PD messages efficiently. A robust state machine helps track the contract lifecycle, from discovery to capability advertisement and contract negotiation. Logging capabilities capture negotiated voltages, current limits, and sink/source states in a timestamped ledger. Safety considerations include current limiting, short-circuit protection, and isolation between the measurement circuitry and the host interface. Documentation within the code helps future contributors extend tests and profiles.
Real-world testing workflows for PD devices and charging profiles.
A compact instrument like this benefits from a clean hardware skeleton: a small development board, a PD front-end, a measurement module, and a user interface layer. The front-end must present CC pins, detect cable orientation, and switch Rp/Rd configurations deterministically. The measurement module should monitor VBUS, IMAX, and temperature, providing watchdog protection to avoid runaway conditions. A simple display or companion app can present current negotiation state, detected PD profiles, and error flags. The software design emphasizes non-blocking I/O, efficient interrupt handling, and deterministic timing to ensure repeatability across tests. Coupled with a USB interface, data logs can be exported for deeper analysis on a PC.
In practice, calibrating the device is essential to produce reliable results. Calibrations align voltage readings to a known standard, characterize channel losses, and verify the accuracy of current measurements under varying loads. A careful calibration routine accounts for board trace resistances and connector parasitics which influence perceived voltage at the test point. Establishing a baseline with a known benign load helps validate the measurement chain before engaging in more aggressive tests. Documented calibration procedures enable users to reproduce results and compare performance across different builds. This documentation also clarifies how environmental factors, such as temperature, influence measurement drift.
Benefits of modular design, calibration, and reproducible tests.
The first workflow focuses on device compatibility testing. A candidate device presents a PD capability profile, negotiates into higher voltages, and then gracefully returns to a known state. The negotiator should populate a log with timing data for each stage: discovery, capability exchange, and contract confirmation. This data helps compare devices and identify any deviations from expected behavior. A repeatable test harness ensures consistency across trials, while a controlled environment isolates variables such as cable length and connector quality. Users can then map out how particular devices respond to different power profiles and what safeguards are triggered during negotiation.
A second workflow emphasizes custom charging profiles. The tester supplies derived voltage/current curves, simulates ripple conditions, and records how the PD controller handles deviations from nominal profiles. The ability to enforce specific PD contracts enables exploration of non-standard charging strategies—useful for battery research and device tuning. The negotiator should present a simple method to load and apply profiles, possibly via a small configuration file, while preserving a detailed history of negotiation steps. Documentation should include caveats about profile realism, stability under load, and any regulatory considerations when applying unconventional charging schemes.
Strategies for safe operation and responsible experimentation.
A modular approach helps separate concerns and extends the lifespan of the device. The PD front-end is one module; the measurement subsystem is another; and the user interface sits on top, enabling independent upgrades. Each module communicates through clean interfaces, which makes it practical to swap components as technology evolves. Modularity also aids debugging, as issues can be traced to a specific subsystem rather than the entire system. By isolating power paths from microcontroller logic, the risk of noisy measurements corrupting control decisions is reduced. The result is a reliable platform suitable for hobbyists and professionals alike.
Reliable calibration becomes a recurring discipline, not a one-off task. Periodic checks with standardized references help ensure ongoing accuracy, particularly when the device operates in diverse environments. A well-documented calibration schedule guides users through routine checks and notes any drift observed over time. It is wise to archive calibration certificates alongside test results for traceability. When calibrations are integrated with firmware updates, the user maintains confidence that tests remain meaningful after changes. The combination of calibration rigor and thoughtful logging makes the power negotiator a trustworthy tool in any lab bench.
Final thoughts on building, testing, and refining PD negotiators.
Safety-first design choices protect both the user and the device under test. Key elements include current limiting, overvoltage protection, and thermal monitoring with automatic shutdown thresholds. The device should clearly communicate fault conditions and provide an easy path to recover from transient faults. A robust user manual helps prevent hazardous setups, explaining appropriate cable selections, safe handling practices, and how to interpret test results. In addition, designing for easy disassembly enables component replacement or upgrade without compromising safety. Producers should also consider EMI/EMC implications and implement shielding or filtering where needed to minimize interference.
Documentation and community engagement enhance the project’s value. A clear bill of materials, wiring diagrams, and firmware changelogs empower others to replicate or improve the device. A test protocol appendix with example sessions helps new users get started quickly, while a troubleshooting guide supports common issues. Publishing example datasets and performance graphs fosters openness and invites peer review. Encouraging community contributions—whether code, hardware ideas, or test scenarios—accelerates iteration and expands the device’s applicability across different PD ecosystems and chargers.
Embarking on this project blends practical electronics with careful experimentation. A compact PD negotiator becomes a versatile companion for engineers evaluating chargers, batteries, and PD-enabled accessories. The design philosophy emphasizes simplicity, traceability, and modularity so the device remains useful as standards evolve. By coupling accurate measurements with a transparent test workflow, users can build confidence in the results and iterate rapidly. The project also invites learners to explore how PD contracts affect real-world charging behavior, fostering curiosity about power delivery protocols and their practical implications.
With thoughtful planning, this compact negotiator can scale from hobbyist experiments to a helpful lab tool. Start with a minimal viable version that covers core negotiation, logging, and safety features, then incrementally add profiles, alternate modes, and advanced analytics. The journey teaches: how PD messages are structured, how to interpret voltage/current changes under load, and how to document results for future reference. As you refine calibration, expand test coverage, and improve user interfaces, you unlock a practical, enduring device that supports discovery, education, and robust device testing across a range of PD scenarios.
