I have to flash a TYWE3S module with tasmota. Ive flashed a esp8266 before using 3.3V from an arduino uno. So from what I understand the whole idea behind these 3.3V-5V converters is that these modules are so low power that communication must be done at 3.3V. So I want to check the voltage before doing so, but Im not entirely sure how these converters work.

PL2303

When I measure 3.3V pin against GND pin I get 3.50V.
When I measure 5.0V pin against GND pin I get 5.15V.
When I measure Tx or Rx against GND pin I get 5.15V.

So this means this module will fry my TYWE3S?

My USB-TTL cable:

When I measure red Vcc pin against GND pin I get 5.16V
When I measure green Tx pin against GND pin I get 3.62V
When I measure white Rx pin against GND pin I get 0V

 

Check the datasheet of the maximum input rate for your TYWE3S.
The voltage will probably drop a little bit if you put the Tx of the PL2303 to the Rx of the TYWE3S (since this is a load).
I doubt you will fry your TYWE3S.

 

From what I can see in the data sheets yes, the max. input voltage is 3.3V see here:
https://fccid.io/2AFNL-TYWE3S/User-Manual/Users-Manual-3525098

But this can be done very simply with a 2 resistor potential divider for the TX line at least. Youneed more care with the RX as it is an input so it should really only be pulled to 5V with a high resistance internally, so should be OK to connect directly.. but best to test with say a 1K resistor in series.

I see you can get some of those USB boards with select-able 3.3 or 5V.

 

Looking at the datasheet I see that Vcc is maximal 3.6V (Table 4).
Vih (Voltage input high) maximum rate is Vcc (so 3.6V aswell) (Table 4).
The voltage will probably drop because you attach Tx to the Rx of the module (so you attach a load).
I think you will be fine.