Charging a phone from 12 V with an HW-688
The HW-688 buck module turns 9–36 V into USB 5 V. Plug a cable into its USB-A port and the phone at the far end sees 5.2 V minus whatever the cable drops. This is how car chargers and solar USB outlets work.
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How it works
The HW-688 regulates its USB port to about 5.2 V. Through 1 m of 24 AWG (standard) (loop 288 mΩ), at 0 A:
V_phone = 0 V − 0 A × 288 mΩ = 0 V
Input side: 0 W out ÷ 90 % efficiency ≈ 0 A from the 12 V adapter.
Try it on the bench
- Try to plug a USB-C → C cable into the HW-688 (set the cable type in Connect with): it is rejected, because the module has a USB-A port.
- Raise the phone load to 2.5 A and select the cable: watch the drop grow. Then switch to 20 AWG.
- Turn on Tools → Power & heat overlay to see where the watts go.
Parts on the bench
In the course
Zen’s Electronics Design Mastery, module “Capstone — Connected Tracker”.