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.
Here are the ones I would recommend - 2PCS CH340G RS232 Upgrade to USB TTL Auto Converter Adapter STC Brush Module - ebay.eu/2fMPrO2
Here is the other one - ebay.eu/2g87fUJ
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[@robert574]

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!

[@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 :-)

[@jensschroder8214]

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.

[@javierpallalorden]

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

[@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.

[@DavidWatts]

I think the 340g needs an external 3.3v regulator if you want it to output 3.3v logic, might be wrong though.

[@t1d100]

I have this cheapo/Chinese USB to TTL converter, with which to set the configurations of my new GPS unit. The GPS is 3v3 to 5v tolerant. I want the final GPS device to run on 3v3, to aid battery life. So, I want to practice/make certain that the GPS will operate at 3v3, by programming it with 3v3. I believe that the converter is not intended to power the DUT. DMM testing of the open positive pin shows that it is the inverse voltage of the programming logic level. (If jumped to 5v, 3v3 is on the logic pins and the open power pin is 5v. If jumped to 3v3, 5v is on the logic pins and the open power pin is 3v3.) Therefore, the DUT/GPS needs its own power supply. And, that, regardless of the DUT's/GPS's power source, the grounds need to be tied, to enable programming. The converter should be tied to earth, through the laptop. I want to use my bench power supply, to power the GPS/DUT. The PSU can be configured to leave the ground floating, or tie it to earth. I think floating the PSU ground is likely to be better... Thoughts? Tying various power supplies/devices together with my laptop makes me a bit nervous. So, I would appreciate you confirming that I am not about to mess up. Of course, you are not responsible, for a negative outcome, no matter what your thinking/reply. EDIT: I used the powering configuration, as I had planned, above. The unit operated 100%, at 3v3. I was able to set the configurations, in the GPS. It obtained a good satellite lock. The GPS and the USB to TTL Converter were completely unharmed.

[@PhilXavierSierraJones]

Found out that if "V3" pin is connected to GND via 100nF capacitor (default -- no jumper or jumper is on 5V setting) the VCC is 5V and 3.3V regulator is disabled. You need to connect it to VCC in order to enable it.

[@oskarenoksson9626]

I have exactly the adapter shown in the video, and there is no pin marked 3V3 on this adapter. Pin 2 is marked CTS. Just as an experiment I put a jumper between pin 2(CTS) and 3(VC) but it makes no difference - the RX/TX outputs are still 5.0V. Other devices may work with a jumper between pin 2 and 3, but not this one.

[@snimavat]

I have Ch340G - when its in 3.3v mode, it outputs 3.8V between Rx & GND - i believe, unsafe for connecting to pi pico UART

[@masshysteria1589]

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

[@lung0fish1]

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.

[@De-Notes]

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

[@huskylfp1505]

Despejaste todas mis dudas!!! Muchas Gracias

[@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

[@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...?

[@MkmeOrg]

Good stuff David!

[@cbm80amiga]

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 :)

[@DavidWatts]

Thanks. I think some of them with the switches do actually work as they should, it is more of a warning to check.

[@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.

[@0xB8xor0xFF]

ESP8266's GPIO pins are 5V tolerant. 5V on TX isn't a problem.

[@cosmosaodyssey2188]

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.

[@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!

[@NOTuNOTme]

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

[@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

[@Sama_09]

How to go even lower like 1.8 volts ??

[@Autotrope]

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

[@Roby0593]

And when 3.3v is 3.7v is not good either right ?

[@Serg-978]

Why did You measure the RX line, while I think you should have measured the TX line?

[@TheKetsa]

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

[@TheKetsa]

They do switch tx/rx lines but i'm reading 3.8v ... better than 5V I guess.

[@vladimir0rus]

ESP8266 I/O pins are 5V tolerant.

[@pbart9821]

The rx tx 5v is such a low amperage that even 3.3v logic boards are infact not effected

[@zetaconvex1987]

Grrr, so confusing. It makes it difficult to buy these modules, as the seller plasters their page with all the words they can think of, and it's hard to discern whether it's actually 3V on the TX line.

[@terrylay]

Thank you

[@thisoldjeepcj5]

Yep, learned the hard way. Trust no one.

[@jaimeferreira9427]

Hello David, your video is very good and very clear in your explanation, however I have an adapter of these gold and I can not work with it. I've tried a win7 and win10 computers and it does not work, do you know where I can copy the correct drivers? Or is it another problem?

[@DavidWatts]

Have a look at the chip on the board, what does it say?

[@jaimeferreira9427]

CH340G

[@DavidWatts]

Jaime Ferreira I believe that MKme Lab has you covered there - kzbin.info/www/bejne/f6mXpWCVh79qhKs

[@Ernie-Tech]

I find these adapters very deceiving; their supposed to "work in 3.3v compatible" but are only so if the CH340G itself is fed by 3.3v for which you'll need a regulator on the 5v usb power. The incompatibility comes from the Rx line that is kept high internally to Vcc. Meaning; if your Vcc is 5v on the CH340G, the Rx-signal floats between 5 and 2v in case of a 3.3v device Tx line attaced to it, which is not compatible with standard TTL levels. In practice this means that the Tx-signal responce from any 3.3v device behind the adapter fed by the 3.3v line (e.g. Sonoff which you'd like to flash) will not be "seen" by the software on the USB side. You can read this yourself in the following datasheet : images.100y.com.tw/pdf_file/34-WCH-CH340G.pdf (bottom of page 4 "When using 3.3V operation, tie V3 pin to VCC pin and supply 3.3V power." ) which explains my failing flash operations with Tasmotiser on Sonoff switches. I'll have to supply my adapter with 3.3v and remove the 5v line from usb to ch340g.

[@KN100]

This comment was unbelievably helpful. Thank you. Interestingly, I didn't realise the rx/tx pins need to be switched as well as the power source to the Sonoff plug - and I was able to flash one esp8285, but not the other. The only thing I changed was the PSU to my USB hub, it makes me wonder if maybe the power supply I switched to is crappier than the one I had connected before and that meant that the esp8285 stopped being 'tolerant' of 5v. I instead bought the bloody adaptor in this video that has the physical switch after realising that switching the rx/tx voltage was important as well as the voltage, and now I see this video! Oh well, guess I'll voltage divide to 3.3v on the input of the serial to USB.

[@Ernie-Tech]

@@KN100 Hey Kevin, great to read somebody appreciated my input :-) The whole mislead situation also comes from the classic DTE/DCE serial configuration from the old school days: all communication lines had one name on both sides and the definition of who is DTE and who is DCE determined the direction of the signal, input or output. Nowadays most serial interfaces are all DTE's and wires are crossed over cause people in general find that more understandable, I think. These weird adapters with CH340G on them suddenly apply the old original RFC's. Not wrong, but not what we now expect ...

[@ipodmaurits]

Hey, could you make a video about your recommeded module? I've a problem with uploading it to my mini pro

[@tttomisss]

i also have recommended one, i have red light LED and in this video is blue

[@Fumacenta2tempos]

Dude, is there any problem in use the 5v Tx and Rx in a 3v3 board?

[@hanelyp1]

yes, you might burn out the input. CMOS inputs routinely have diodes to limit voltage beyond device supply rails, but those can only take a limited current.

[@subhashtd1436]

Its the Problem with the switch 😅😂 change the switch, don't blame pcb 😅😂 past. 5 years I'm using the first one you blamed

[@hycron1234]

How typical lol 🙄