this post was submitted on 17 Mar 2025
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high voltages on usb to TTY UART? (lemmy.one)
submitted 3 weeks ago* (last edited 3 weeks ago) by [email protected] to c/[email protected]
6 comments fedilink hide all child comments
 

Hi, Any1 have experience with this usb to TTL 6 pin converters? I just want to know if I have a dodgy one that I bought from Ali or normal behaviour.

Doesnt seem right. I have it plugged to a powerbank, following voltages read from pinout, without any jumper link.

5v = 5.3 Vcc = 3.9v 3.3 = 3.2v Tx RX = 3.9v

If I use jumper link from 5v to VCC, then tx RX = 5.3v, if I jumper link VCC and 3.3, RX and tx = 3.9v.

Thanks for reading

Photo of product http://0x0.st/81fc.jpg

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[–] [email protected] 2 points 3 weeks ago

I use the ones from ftdi all the time, it sounds similar in concept. (Though I only use Rx/into the PC and ground).

[–] [email protected] 1 points 3 weeks ago (1 children)

https://learn.sparkfun.com/tutorials/logic-levels/all

If i read your question correctly then you are wondering if the voltages you see are going to work for TTL devices. As you can see in the linked tutorial it is not just about nominal Vcc levels. Every chip is different. :)

[–] [email protected] 1 points 3 weeks ago

"On certain 3.3 V devices, any voltages above 3.6 V will cause permanent damage to the chip. " Well thats not good. I guess I have my answer 3.9v is high 🤷

[–] [email protected] 1 points 3 weeks ago (1 children)

The 5.3V is from your computer, that's not the fault of the USB UART.

3.2V is perfectly acceptable for a 3.3V rail.

The 3.9V is a bit weird. Can you post a photo of your USB UART board? Maybe the main chip has an inbuilt 3.3V regulator separate to the external one.

[–] [email protected] 1 points 3 weeks ago (1 children)
[–] [email protected] 1 points 3 weeks ago* (last edited 3 weeks ago)

That's a CH340G, it has an in-built 3.3V regulator. But there is no external regulator on the board.

Maybe the chip is running off its internal 3.3V, but the board designers put a tie-up resistor on one of its pins to 5V, which results in the weird 3.9V. Dunno. Try attaching a 1K resistor between that pin a GND, see if that makes the problem disappear.