Tcepsa wrote:I think I might have figured it out. I took the reading off of pins 1 and 3 on the FTDI connector as you suggested and it was the expected 3.3 V, so my 3.3 V supply regulator is still good. However, in order to get the temperature (and other value) readings, I have been leaving my FTDI cable connected to that header so that I can get the serial values in the terminal. Then I remembered that some FTDI cables are 5V rather than 3.3V, so I also measured pins 1 and 3 of my FTDI cable and sure enough it's a 5V cable. So whenever I have the FTDI cable plugged in, it is getting 5V. I'm counting myself lucky that I have not burned out any of the components!
Should the board maybe be redesigned so that the power pins from the FTDI cable are also on the 5V side of the 3.3V regulator, just in case?
I don't think Paul is planning to make any more of the non LCD chargers, and the LCD chargers won't be using an FTDI board. Also, connecting the FTDI board to the 5V side of the regulator might cause 3.3V boards to be fried anyway.
If it's just a matter of getting incorrect temperature readings if you have a 5V cable connected, then it's maybe not worth it. But if there's other damage that I (and other unsuspecting users) might be doing, then it's probably worth at least considering, or putting a warning or something (and if there was an "only use 3.3V FTDI cables" warning somewhere and I just missed it then that's on me--ideally such warnings wouldn't be necessary, but it sounds like it is late enough in the design process, at least for the 2-cell model, that making that change would probably be overly onerous).
I don't think it would actually damage the board as such - the 328P MCU can be run off of 5V and I'm pretty sure that the 18 bit ADC is also OK with 5V. I can't think of anything else that would be damaged by 5V.
In the meantime, I'll try adding two 1N2004 diodes between pin 3 of my FTDI cable and pin 3 of the FTDI connector on the board and seeing whether my temperature readings get a little more realistic.
I wouldn't do that - you don't want to run the controller off of a lower voltage than you're feeding into any of the input pins (e.g. transmit, DTR on the FTDI board) otherwise you may end up frying those pins. You're better off using dupont male to female cables to only connect ground and receive on the FTDI cable - i.e. pins 1 and 5 on the charger. That way, you're not sending 5V into the controller when it's running at 3.3V. In the longer term, I think that you should just buy a 3.3V FTDI cable since that would be a lot safer. They're less than $8 off Ebay as long as you don't mind waiting for them to come from Hong Kong!