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Sssssmokin'! (256MB Model B issue)

Posted: Mon Sep 16, 2013 10:15 pm
by LemmeFatale
Today, I took delivery of some accessories (a USB WiFi dongle and a ModMyPi case) for a first-batch Raspberry Pi that's been sitting here for well over a year, waiting for a project. This RPi has never been booted up before now (I'd pre-prepared its SD Card in my main RPi, before the accessories arrived), and I temporarily hooked the machine up to my main RPi's Lapdock setup, to make setting it all up a bit more convenient.

Since I understand that early Model B units cannot be "back-powered" like the later ones, due to a polyfuse which limits the current that can go through the USB ports (is my understanding of this correct?), I connected the RPi to a suitable known-good external power source, and assumed that it would only be able to use the Lapdock's USB interface to make use of the keyboard, rather than for power as well.

Anyway, this seemed to be the case, and I then got to work with the WiFi dongle setup, which was ridiculously easy.

All was going well, until about five or ten minutes later, when suddenly smoke (fine, whispy, white smoke that didn't smell of anything - so, I don't think it was Magic Smoke) started pouring out of the area around the micro-USB connector and the SD Card slot! (Mostly the micro-USB connector, though.) The Raspberry Pi didn't seem to have noticed this at all and kept happily chugging along, but I immediately yanked the power cable out, and unplugged the RPi from the Lapdock, too, and quickly pulled the case off to find..... Absolutely nothing.

The board was clean, there was no sign of any reason for it to have been smoking*, and nothing was hot - the board was simply the "lukewarm water" sort of temperature you'd expect to find in running or recently-running electronics.

*All I did spot was a tiny little piece of paper/fluff that for some reason was stuck to the back of the SD Card slot (I took the RPi out of its bag and put it straight into the case, so it must have come out of the factory like that; I didn't notice this at the beginning). I have no idea if this touching the SD Card slot would cause smoke - it didn't seem hot enough to burn paper/fluff, at all.

I cautiously plugged it back in, and to my surprise, it was absolutely fine, as was the SD Card (no corruption or anything). It didn't start smoking again this time, but I also didn't run it for as long - only for long enough to check that it was still working, and that my just-completed WiFi setup hadn't been lost when I cut the power.

I have a suspicion as to what apparently happened, but would like to be sure before I put this thing in the room where I'm intending to set it up; I noticed when I unplugged the power but before I disconnected it from the Lapdock, that the unit kept pinging between being off and the rainbow screen. It looks like it was, regardless of the fuse, taking in a little power from the Lapdock via the USB port, which I didn't think it was supposed to be able to do.

Is this apparent taking in of too much power likely to have been what caused the issue, or would it have done this anyway for a different reason that I'm not aware of?

Does anyone know if this has happened to any other early Model B units, and what the cause was?

Or is this an indication of a fault with this now-out-of-warranty unit, meaning that I really shouldn't use it if I don't want to risk it doing this again?

Any pointers and insight will be greatly appreciated - thanks very much in advance to anyone who might know something. :)

Re: Sssssmokin'! (256MB Model B issue)

Posted: Mon Sep 16, 2013 11:01 pm
by johnbeetem
LemmeFatale wrote:Since I understand that early Model B units cannot be "back-powered" like the later ones, due to a polyfuse which limits the current that can go through the USB ports (is my understanding of this correct?), I connected the RPi to a suitable known-good external power source, and assumed that it would only be able to use the Lapdock's USB interface to make use of the keyboard, rather than for power as well.
...
I have a suspicion as to what apparently happened, but would like to be sure before I put this thing in the room where I'm intending to set it up; I noticed when I unplugged the power but before I disconnected it from the Lapdock, that the unit kept pinging between being off and the rainbow screen. It looks like it was, regardless of the fuse, taking in a little power from the Lapdock via the USB port, which I didn't think it was supposed to be able to do.
If you hook up Lapdock to a RasPi USB A port, Lapdock will try to power RasPi unless you cut the 5V line in the USB cable. USB fuses F1 and F2 limit current, but they won't stop it.

The continual reboot you're seeing when your old Model B is only powered by Lapdock is the usual behavior when trying to power an old Model B through fuse F1 or F2. They are only rated for 140 mA. This enough to get the boot process started, but as boot proceeds the SoC needs more and more current until the voltage drop across the fuse increases to the point where the CPU fails. At that point, the CPU reboots and starts the process all over again.

I have an old Model B as well. I replaced F1 and F2 with 1A single-blow SMT fuses which lets me power it from a Lapdock with a single USB cable. Others simply short out F1 and/or F2, which makes the old Model B look like a new Model B. Board modifications are at your own risk.

I don't know what your smoke was, but one possibility is that there was some solder flux or PC board cleaner left on the board and it was boiling off. Since you didn't feel anything hot and you didn't smell the smoke, it's probably benign. I don't think it has anything to do with Lapdock back-powering through F1 or F2.

These opinions are offered without any warranty -- JMO/YMMV

Re: Sssssmokin'! (256MB Model B issue)

Posted: Mon Sep 16, 2013 11:38 pm
by LemmeFatale
johnbeetem wrote: If you hook up Lapdock to a RasPi USB A port, Lapdock will try to power RasPi unless you cut the 5V line in the USB cable. USB fuses F1 and F2 limit current, but they won't stop it.
Ah! I didn't know that! Thanks.

I'm a software-inclined type, and my knowledge of electronics is not brilliant. :lol:
The continual reboot you're seeing when your old Model B is only powered by Lapdock is the usual behavior when trying to power an old Model B through fuse F1 or F2. They are only rated for 140 mA. This enough to get the boot process started, but as boot proceeds the SoC needs more and more current until the voltage drop across the fuse increases to the point where the CPU fails. At that point, the CPU reboots and starts the process all over again.
That's what I thought and expected. At least I understood that well enough. :)
I have an old Model B as well. I replaced F1 and F2 with 1A single-blow SMT fuses which lets me power it from a Lapdock with a single USB cable. Others simply short out F1 and/or F2, which makes the old Model B look like a new Model B. Board modifications are at your own risk.
This one was only temporarily hooked up to the Lapdock while I got it set up - it's actually going to be sitting under a TV once my wireless joypad arrives for it, so I'm not going to have any reason to modify it. :)
I don't know what your smoke was, but one possibility is that there was some solder flux or PC board cleaner left on the board and it was boiling off. Since you didn't feel anything hot and you didn't smell the smoke, it's probably benign. I don't think it has anything to do with Lapdock back-powering through F1 or F2.

These opinions are offered without any warranty -- JMO/YMMV
Warranty or not, that sets my mind at ease, as it seems logical considering what occurred. I didn't think of this possibility until a while after posting, but I figured it might come up anyway. :lol: (To give a bit more of a description of the smoke, it looked rather steam-like, as well as everything else.)

I don't think I'm so worried about plugging it in in its intended location, now - thankyou. :)