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8 years, 12 months ago.
Plugging in USB while externally powered killed my mbed
Hello,
Yesterday I managed to fry a trace on my PCB along with my mbed 1768, and I can't figure out why. My circuit uses an external 5V AC/DC power supply which powers the mbed via the Vin port. The PCB also has 3.3V components which are powered by the 3.3V output on the mbed. Both of these are referenced to a common ground.
I had the mbed plugged in to the circuit and it all seemed to work as normal (blue led on mbed was lit and both power rails were OK). Then I plugged my macbook to the mbed via the USB port and BANG. The trace that exploded was the AC Neutral line running from my IEC receptacle to the AC/DC power supply. This is mains voltage so I assume the current must have been huge for it to explode.
The mbed is no longer working as far as I can tell. Since the circuit was running fine before I plugged my macbook in, I assume the laptop's USB interface is at fault. I understand the mbed has diodes to prevent reverse current conditions, but maybe the macbook had a problem with this and tried to supply current regardless?
I'm going to replace the mbed but want to nail down the problem before I try it again. Am I missing something fundamental?
Cheers
2 Answers
8 years, 12 months ago.
Ground is not always ground. You would be surprised to learn about how many AC outlets are miss-wired. Also, some laptops can have voltages above ground on the shell. the AC/DC converter may not even have a ground since it is probably a two wire, so who knows?
8 years, 12 months ago.
The fundamental error is : you have two paths to the main (AC power) and to the ground and no knowledge of the ground connections .
It is of course needed to connect your Mbed to the Mac (or PC) for your debugging, but you have a choice :
1) use the Mac with his own battery (no main connection)
2)use the Mbed with Vin to a battery (no main power supply) and use your Mac with main connection.
3) use a "safe" AC/DC power supply....but how do you know yours is safe. Look at the documentation of this logic analyzer http://support.saleae.com/hc/en-us/articles/200671790-Protecting-Against-Ground-Loops , go to the download section.