When 3.3v isn't actually 3.3v: USB to Serial adapters

I picked up a couple of USB to serial adapters recently, both of which claimed to switch 5v to 3.3v. It turns out that everything is not a cut and dry as it seems.

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[robert]

I'm glad I watched this, I wasn't aware of the problem. I guess a logic converter / shifter might work also. I checked mine and it did switch to 3.3V. I have a few, recently purchased, and they are similar to your first one, with 6 pins only without the switch. From one end I have 5V - Vcc - 3.3V. A jumper from Vcc to 5V gives 5V outputs on serial lines and moving the jumper to Vcc - 3.3V gives 3.3V outputs to serial. It is good to check.

[HariWiguna]

Wow! I didn't know that. I'll check the next time I use one of mine. Thanks David!

[PLOBEXRIME]

You are only missing one small thing with that CH340G adapter - a jumper :-) In order to switch to 3.3v TTL you have to set jumper on 3V3 and VCC. 5V pin will become 3.3v VCC, and Rx Tx will also work with 3.3v logic.

[DontShootBob]

Glad I found your video. Never had to use a USB to TTL before and want to use one to open up a wireless access point. The requirement is for 3.3v. I had been looking at the switchable one. But will now get the second model. Thanks :-)

[javierpallalorden]

Nice video and you make a good point there, always check your voltages!

[jensschroder]

I have already noticed that there is 470 ohms resistance in TX and RX. Together with the clamping diodes at the input, this results in 3V3.
To be on the safe side, you can also use a 3 volt zener diode, or blue or white LED to limit.
The ESP8266 can handle 5 volts for a very short time. But not in the datasheet.
I wouldn't expect that from the PI.

[masshysteria]

There's actually a makeshift voltage dropper there using two signal diodes that make 5v into 3v and there's a jumper you connect vcc to and that will switch the operation from 5v to 3v

[De-Notes]

It's very clear information about this specific features for this converter ! Thank you :)

[stevejensen]

Thank you Dave.! I was very frustrated why I still had 5V on RX and TX on my USB to TTL...i almost burned off my ESP8266. Talking about bad Engineering.

[cbmamiga]

Very good video! I have both of USB-serial converters and was thinking that this more expensive with the switch is better and more reliable. So I was wrong :)

[snimavat]

I have USB to TTL CH340G and it outputs 3.8V in 3.3V mode - how safe would it be for pi pico UART pins

[NOTuNOTme]

Great vid. To be honest I know that I should be testing the levels out of anything that isn't OEM.

[huskylfp]

Despejaste todas mis dudas!!! Muchas Gracias

[dpmkumbhar]

Hi I have one question.... What will be voltage between Rx and Gnd pin and Tx and Gnd pin....when USB TO SERIAL is connected as shown in this video...?

[kausarhasnain]

Hi Mr David I have a combi oven pcb / mcu which have 5 pin flashing port it doesn't mention rx tx vcc and other how can I know the position with Multimeter, pls suggest

[cosmosaodyssey]

I just so happened to be checking the same thing on my ftdi and found that while it was outputting 3 volts when I switched it over from 5v, it was actually 3.6v... which is right at the edge of tolerance for my device.

[lungfish]

I'm a little confused... With the first adapter, you only tested the RX line, showing that Vcc to RX was 5 V when Vcc was 5 V... Doesn't that just mean that the RX line sank all five volts? Should it have sunk only 3.3V instead, giving a resulting voltage of 1.7 V? Checking the TX line on both makes more sense to me.

[GrahamToal]

If you ever make videos again... I'ld love to see a clear explanation of the difference between USB to TTL and USB to serial - when the signal is inverted (RS232 lines are supposed to be active-low), what voltages are used to drive the serial lines, what voltages are accepted without blowing up your 3.3v devices. RS232 and RS423 can go quite high. Sometimes converters are advertised as "USB to TTL Serial" and I have no idea what they are! Some converters have a pin labelled "RST" that should mean "Reset" but Reset makes no sense in this context - are these just mislabelled "RTS" pins? So many ways to go wrong, and I've never read a post or seen a video that shows the differences and how to recognise them so that you can pick the right device to add serial USB to whatever micro you are building. Thanks for your consideration!

[TheKetsa]

interesting, I never had a problem but will check my cheap chinese ch340 adapters.

[Autotrope]

Do those devices have the same converter chip? What kind of chip is it, a Prolific?