USB’s widespread adoption makes it possible to control solenoids operating at 24 VAC in older automatic irrigation systems through USB-PD. The opensprinkler.org project examined four methods for this purpose: unipolar PWM, dual-voltage, DC-to-AC waveform generation, and a bipolar square wave.
The bipolar square wave method produced results close to a true sine wave while keeping power electronics requirements low. The study shows that USB can be adapted to older systems using 24 VAC in cost-effective and various ways.
Why it matters
This study shows that USB-PD is not merely a standard for powering new devices, and that existing 24 VAC infrastructure can also be adapted for use. Thus, instead of replacing the entire control architecture in older automatic irrigation setups, the fundamental issue becomes how to adapt power and signal generation; this has direct implications for those maintaining and continuing to use older equipment. The options examined show that adaptation is not tied to a single circuit approach, but that a trade-off is required between circuit complexity and the resulting waveform. The open question is how the compatibility of these methods will vary across different solenoids and field conditions.