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Thread: My Teensy caught fire

  1. #1
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    My Teensy caught fire

    My teensy just caught fire... literally!

    I'm trying to understand what happened and I hope someone here can help me.

    I had the teensy on my breadboard with a small circuit while I was writing code on my MacBook Pro. Everything was working fine for a couple of hours until my laptop was running out of power and I plugged in the charger and *POOF* ... flame and smoke from the MCU on the teensy. I wasn't really doing much with the circuit... just messing around with the code. The teensy was powered from USB and I had no other power source on the breadboard.

    I checked my breadboard for shorts afterwards but found none (resistance between VCC and GND is 2k ohm). I guess there could have been a short between 2 pins on the MCU itself (I was fixing a bad solder joint on the pin header I put on the Teensy earlier today) but then why didn't my circuit misbehave when I was running off the laptop battery?

    I wonder if something could be wrong with the laptop or charger? Normally, my macbook will disable USB ports that draw too much current but that didn't happen here, even though it took me several seconds to disconnect the burning teensy. Also, I charged my laptop (with the teensy removed) after the incident and got a small electric shock when I touched the charger plug to remove it from the socket (I was wearing sneakers, so I guess it could have been worse). This is a german style plug that exposes GND on the sides of the plug "casing", so I'm guessing that's where the shock came from.

  2. #2
    Senior Member PaulStoffregen's Avatar
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    Quote Originally Posted by rasmus_b View Post
    The teensy was powered from USB and I had no other power source on the breadboard.
    Did the breadboard have *any* other non-power wires to the world? Like maybe connection to something else which is connected to earth ground? Or something that should have been grounded, but was actually "hot"? Because that sort of path would be the most likely explanation why it worked when running from the laptop battery, but then fried when your laptop got connected to the rest of the world.

    Could also be some sort of problem with your laptop, but that seems unlikely. If your laptop's USB port is malfunctioning so badly that it sends high voltage without current limiting capable of burning Teensy, it would probably be destroying any other USB devices you plug into that port.

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    No other connections, just the teensy hooked up to the laptop. I happened instantly when I plugged in the charger.

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    Is the charger an actual Apple charger and not a counterfeit?

    I seem to recall from somewhere in the past where I read about fake chargers being dangerous. Here's an article that I found just now:

    Safety tests of 400 fake Apple chargers bought online reveal that 99% are dangerous

  5. #5
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    Even with genuine chargers it is quite possible for laptops and phones to have half the mains voltage between the chasis and the building gnd. So on possibility here is that the USB gnd was not at gnd for your bench and current flowed that way. If you have a meter checking the voltage of the laptop case or the USB gnd to various parts of your bench setup may be informative. I have a powerboard with integrated USB that sits around 100V with respect to the USB of the grounded desktop computer next to it.

    It would be very odd that the Teensy caught fire on purly USB power since by design USB ports should constrain the amount of current provided and normally shut down when things go wrong. There is also a polyfuse on the Teensy itself in case that does not work and the onboard regulator should prevent the 3.3V that powers the CPU doing much either. There is a couple of watts potentially available but normally you would need some careful engineering to tune the extraction of it to get a burn. Not impossible that it 'just happened' but more likely contender is something beyond the pure USB 5V was in play.

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    The charger is a genuine apple charger.

    @GremlinWrangler I'll try to poke around with my multimeter but I'm not sure I understand exactly what you mean? How could the USB ground not be ground for the circuit I had on the board (there were no other connections)?

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    Was not clear from your initial description if you had any wiring running from the Teensy to any sensors/buttons or similar that might expose a different ground potential. If all you really have is a teensy on a cable plugged into your laptop all sitting isolated on your desk then the only other connections should be your desk/bench and yourself, and hopefully neither of those things are conductive enough/at high enough potential to get a watt or so flowing through the teensy but is also easy to check.

  8. #8
    Senior Member PaulStoffregen's Avatar
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    I know we've heard "no other connections", but if the USB ports work with other devices like keyboards, there really isn't any other plausible explanation for how this could happen with everything working and then burning when the power adaptor gets connected. An unintentional connection to ground and a ground difference is the only reasonable way to explain this sort of thing.

  9. #9
    my guess is that you were correct in suggesting there was a short between pins you had been working on. the laptop most likely being low on power because of the battery condition was not feeding anywhere near normal juice to the teensy via the usb and when you plugged in the charger there could have been a surge beyond whatever the max out of the laptop usb port is, or even with a stable max output, caused the failure. mac book can do 1.1A out normal when plugged in.

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    Sorry for not posting earlier, I was travelling.

    I got out my macbook and the breadboard this evening to do some measurements. Unfortunately, the teensy is now permanently shorted so I couldn't really do any testing on the teensy/circuit.

    I did take some pictures of my breadboard but was unable to upload them. The circuit had 4 10k pots, 4 push buttons and 4 74HC595 shift registers, all connected to VCC (3.3V) and GND on the teensy (pots to AGND). One pot was wired to a pin on the teensy, while the other components were only connected to power. There were no other components or wires on the board and all components were hooked up correctly (I double checked).

    The power socket I used when has only phase and neutral (no ground).

    I took the following measurements:

    - Chassis to socket neutral: 0.3V (AC)
    - Chassis to ground on charger plug (not connected): 0,45V (DC)
    - Macbook chassis is connected to USB ground (continuity)

    I used one of the screws on the bottom of the laptop to get chassis.

    Hope that helps, I would really like to understand what happened.

  11. #11
    Senior Member vjmuzik's Avatar
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    I always use my MacBook charger with the extension cable so it has a ground, that being said Iíve never experienced this issue with mine. Although there may have been a short at the MagSafe side of your charger, Iíve definitely been shocked by it before because it was shorted and luckily nothing got damaged.

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    Well, none of the outlets in my apartment have ground (it's an old one) so unfortunately that's not an option. Fedex just dropped off 2 new Teensys but I'm afraid I will fry those too... and considering the amount of power that must have gone through my breadboard I'm also a bit concerned about my own safety.

  13. #13
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    try a UPS which filters the AC, and check if you still get shocks on the magsafe connector?

  14. #14
    Senior Member vjmuzik's Avatar
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    Quote Originally Posted by rasmus_b View Post
    Well, none of the outlets in my apartment have ground (it's an old one) so unfortunately that's not an option. Fedex just dropped off 2 new Teensys but I'm afraid I will fry those too... and considering the amount of power that must have gone through my breadboard I'm also a bit concerned about my own safety.
    The fact that you don’t have ground is likely the reason this happened, the point of the ground is to protect equipment and yourself if a live wire touches a chassis. The current from the short has no where else to go without a proper ground connection, therefore you got shocked when touching the metal housing of the MagSafe connector since that is grounded to the MacBook. Likely the MacBook has short circuit protection on it’s input to the battery charger circuit, but not on the USB side, so the MacBook survived but the USB device did not.

  15. #15
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    @vjmuzik - if you have a loose wire inside the case it will not be a slight shock when you touch it, there will be a continuous burn. Short shocks happen when there your device becomes one plate of a charged capacitor with respect to the floor/bench and you short that capacitance out with yourself (or possibly in this case your Teensy) and produce a short pulse of current as that capacitor bleeds down. Once everything is connected the actual current flow is very low since it is leakage through the insulation layers of the power supply.

    @Ramsmus - given the checks you have done you are probably safe to work on the Teensy but would suggest
    Test the Teensy first with laptop on battery to make sure it really is alive
    Maybe have it unpluged the first time you plug in the laptop charger (confirm can still program, then plug and unplug the charger in case there is a pulse on USB when plugged in)
    With it is connected check for AC on the 5V pin
    Make sure you avoid connecting/placing it on anything that you have not already checked as at the some potential as the teensy Gnd, this includes your soldering iron and any test equipment you use.

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