Author Topic: Can only power from FTDI after reversing battery polarity(doh!)  (Read 2205 times)

CallOne_Phil

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Had myself a proper bonehead moment and flipped the wires on the lipo battery port, this caused *some* magic smoke to release.  It's an R1 from way back when and that has been working perfectly until I decided to beat it up.

I have the FTDI programmer connected and it powers up fine, the screen displays correctly, radio works and I even have incoming messages printing to the debug terminal.  However, I am not able to power at all from the USB port and was wondering if you had some recommendations on what I should look at first to try and bypass the damage I have caused.  It's obvious that the lipo charger is toast, but maybe I can replace a regulator to get the USB power working again... unless the regulator is part of the charger?

Felix

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Re: Can only power from FTDI after reversing battery polarity(doh!)
« Reply #1 on: June 07, 2018, 10:03:34 AM »
R1 huh!  8)
Just for fun can we see a photo of it?
It may reveal other problems too. But there is no charger on the Moteino ;)

From your description I think most likely the MCP regulator has been sacrificed for the sake of the rest of the board.
Replacing that will likely fix the problem.

CallOne_Phil

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Re: Can only power from FTDI after reversing battery polarity(doh!)
« Reply #2 on: June 07, 2018, 11:02:37 AM »
Here is the pic you requested, I have a macro lens if you want a closeup of anything. It was installed with an old phone lipo for years and I was changing to an 18650  when I tired to murder the poor thing. It has been running like a clock since 2015, it even has the collector edition red jumper bypass for the pcb error!  ;D


Felix

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Re: Can only power from FTDI after reversing battery polarity(doh!)
« Reply #3 on: June 07, 2018, 01:30:30 PM »
Nice to hear this was working reliably. Mine has been solid too :)

Ok I think your charger (MCP73831) and probably LDO are fried.
One way to determine this is try to apply 5v of power to various points on the board and see if you get 3.3v or not.
If that doesnt work I would desolder the charger and try again.
If still no dough then the LDO is probably toast as well.
Are you able to do this level of debugging?

CallOne_Phil

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Re: Can only power from FTDI after reversing battery polarity(doh!)
« Reply #4 on: June 07, 2018, 02:26:39 PM »
Aside from what that jacked up solder job from 3 years ago says, I shouldn't have a problem with the repair.  I have some MCP1703's on hand, but not a single MCP73831.  Adding insult to injury, it's one of those parts that cost under a buck with murderous shipping charges.  Can you recommend any sort of semi-simple bypass that would get me back up and running with just replacing the likely toasted LDO?  I can live without any lipo backup features until I place a proper parts order next month for the MCP73831.

Is there a schematic anywhere for the R1? I found the R3 in the guide, but nothing before that.  I always figure it's best to ask the creator before Frankensteining something in an attempt to forge my own path.

Felix

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Re: Can only power from FTDI after reversing battery polarity(doh!)
« Reply #5 on: June 08, 2018, 12:58:44 PM »
Here's the R1 power path schematic:


CallOne_Phil

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Re: Can only power from FTDI after reversing battery polarity(doh!)
« Reply #6 on: June 09, 2018, 04:36:18 PM »
I was able to remove the MCP73831 charger with my hot air rework station and it seems that the LDO is working fine. I got the power on beeps and everything turned right on, however I seem to have a problem with the boost circuit heating up.  I assume it is due to my lack of knowledge with boost circuits in general, but hopeful you can shed some light on it.

Using a less that reliable laser thermometer I am reading the boost IC, inductor and possibly the flash breaking 43.3C/110F and climbing to the point where you start to smell something.  The targeting of the thermometer extends past the laser dot, but something in the boost area is definitely warming.  I'll admit I have never recorded how hot the boost circuit runs under normal conduction, but I not want to run it longer than 45-60 seconds to see if the heat leveled off or it smoked.

Voltages are:
USB port source: ~4.9v
Pi-PWR(after switch is turned on): v4.32
LDO: 3.34v

I have not reconnected it to the Pi, because that ~4.3v off the booster seems a little troubling.  I have some knowledge, but I'm definitely not a trained electrical engineer. Any suggestions?


 

perky

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Re: Can only power from FTDI after reversing battery polarity(doh!)
« Reply #7 on: June 09, 2018, 05:35:21 PM »
There is a DC path in the booster circuit from power in through the inductor and the diode to the output, what you're seeing is 4.3V which is a diode drop below the input. I.e. the booster IC is not functioning properly.

Mark.
 

CallOne_Phil

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Re: Can only power from FTDI after reversing battery polarity(doh!)
« Reply #8 on: June 09, 2018, 11:39:23 PM »
It was most definitely the diodes/boost circuit, smooth sailing after bypassing.  My setup is hacky at best, but at least it successfully boots again until I can get the proper replacement parts.  Thanks for the info guys  8)