"Unexplainable" magic smoke episode - Page 1
Forum section: electronics / repair

author: MtheMan
author_rank: Newbie
timestamp: February 24, 2016, 10:12:16 pm
content: HelloI've recently experienced a magic smoke episode that I'm not able to explain myself. Hopefully somebody else can help!I was using DJI guidance system. Which is a board that does on-board image depth calculations and spits out the images via USB.I was also using Beagle Bone Black(single board computer).The DJI guidance system requires external power in the 12-24V range, which I provided with a 4cell LiPo battery.The Beagle Bone can be powered with a mini-USB (5V 2A recommended).When the Beagle Bone was powered from my laptop USB, while I've had the DJI guidance system connected to the 4cell LiPo, as well as with a USB to the Beagle Bone with no problems whatsoever (communication between those two working and everything).Then I've powered the Beagle Bone with a variable power supply, which also has a 5V 1A USB output "for phone/camera charging". The Beagle Bone still worked just fine - however, when I've connected the DJI guidance system (which was already connected to the 4cell LiPo) to Beagle Bone (trough the USB, for communication purposes) a lot of smoke came out of thee DJI system. In the process the 5V usb cable from Beagle Bone to the DJI system also got very hot.After the episode both boards were appeared to be not working. My initial investigation shows that the micro usb power input on the Beagle Bone was burned (the plastic inside the socket). Opening the DJI systems shows an obviously blown diode, but nothing else.My own theories:Some kind of ground/earth loop shortNone of the equipment was every earthed (not the computer with everything working, none the 5V power supply)The 5V power supply failingThis seems to be the most plausible option for me. Maybe I drew too close to the 1A max of the output, which cause it to over-voltage the 5V output? Or maybe short it? Is that even something that happens?My next steps are to hook up the power supplies 5V output to different resistive loads (20mA -> 1A), and look at the output with a scope.I hope somebody can help me with this detective work! Maybe I'm missing something?

author: nogood
author_rank: Contributor
timestamp: February 25, 2016, 11:52:27 pm
content: Since I don't exactly know how everything is set up with both products I may be wrong but:It seems that there was a short on the usb line coming from the DJI.The USB cable got hot because you were powering it from the Li-Po battery which can source enough current for that.And I guess the diode that blew on the DJI was some sort of protection diode against wrong polarity, and was blown due to overcurrent.This is what I came up with, but after re-reading your post, it seems that you would need to have some sort of regulator in between because of the 12-24V input,Maybe this is what blew and shorted a component on the Beagle Board ?Do both devices still work ?

author: TerraHertz
author_rank: Super Contributor
timestamp: February 27, 2016, 02:36:39 am
content: You leaned the drill against it, didn't you?

author: android
author_rank: Regular Contributor
timestamp: February 27, 2016, 05:34:08 am
content: Quote from: TerraHertz on February 27, 2016, 02:36:39 am

author: VK5RC
author_rank: Unknown
timestamp: February 27, 2016, 07:41:22 am
content: I have been 'done' before by a bit of solder splatter on the desk! Mutter-mutter-mutter.

author: mich41
author_rank: Regular Contributor
timestamp: February 28, 2016, 09:31:43 am
content: I've heard about type C cables with swapped 5V and GND, maybe somebody made a type A-B cable of this sort as well?