Known issues with Chinese 3.3V UART to USB adapters

UART to USB adapters are very useful for programming microcontrollers or connecting to sensors. This post is to document issues I’ve found with 3.3V UART to USB adapters off Aliexpress.

Note: I’m talking about these specific boards and not all boards using the specified chipset.

CH340G “D-SUN-V3.0”

This board uses the CH340G chipset and looks very promising because you can easily switch between 3.3V and 5V logic levels. However, the “3.3V” output is actually nearer to 3.8V because this board doesn’t use a voltage regulator but instead uses two diodes to reduce the voltage instead.

Thus, I do not recommend this board if you want to use 3.3V logic.

CP2102 “CJMCU” “Micro”

This is a very nice and small board using the CP2102 chipset. The micro-USB port (rather than a built-in type A connector) also allows you to put this board with your project and provide USB connectivity.

I consider the CP2102 chipset more robust than the CH340G chipset because it has an internal voltage regulator to produce 3.3V so you’re not going to have any issue with the board designer cheaping out and not using a voltage regulator… or will you?

One big issue is on some boards (including mine), the voltage on the TX/RX pin is actually 4.2V instead of 3.3V! This issue has been documented online several times and the most clear documentation is this blog post. Basically, there’s a mistake on the PCB design connecting the RST pin to 5V causing the voltage to leak to TX/RX. This can be fixed by cutting that link with a sharp object such as a small drill bit or a mechanical pencil (without leads).

Larger CP2102 boards do not appear to exhibit this issue.