I blew up a... diode?

[skillz21]

Hi....
So I was prototyping with my Arduino nano when I smelled some magic smoke being released, I think the Arduino might have been flashing (the LED's) or something, but I unplugged it. I took it out of the breadboard and took a look underneath....  this component was blown and there was a corresponding burn mark of the breadboard as well (check attachments for photo). that was a while ago, today when I plugged the Arduino in for a tinkering session, I realised the computer won't recognise the device, it gets completely ignored. the Arduino has power but that's how far it goes.  I tried shorting the component for some reason and it shorts the whole power system and the LED's go dark and the computer makes the"device disconnected" noise... then when I remove the short, it makes the "device connected" noise. which is weird, because the Arduino doesn't actually show up... the Arduino does get power though. When I try to upload to it, I get the message:

"avrdude: ser_open(): can't open device "\\.\COM3": The system cannot find the file specified.


Problem uploading to board.  See http://www.arduino.cc/en/Guide/Troubleshooting#upload for suggestions."

What does that component do, is it a diode (I thought it was), what does it do and can I fix it?

Thx
PS. Sorry if the photo quality is low

[sleemanj]

Schottky diode, marking B2, MBR0520, it's diode D1, bottom of this schematic

https://www.dfrobot.com/wiki/images/3/31/Arduino_Nano_Schematic.png

It prevents power back feeding to USB if you have both USB and external power applied (be it directly or indirectly by something else which is powered feeding to the 5v rail).

[Rerouter]

It is there to protect the computer from you abusing the 5V rail, it allows current to only flow from the pc to the device,

Its likley you had a short between the devices power supply and ground, causing too much current to flow through the diode, and popped it. if your confident you have not got anything shorted now, you should be fine to replace it with a suitable shottkey diode.

[skillz21]

If it's shorted, then shouldn't the board still be working? And when I short it with a wire, the board disconnects from the computer, so what's going on? Also, how does this affect the ability to communicate between the Arduino and PC?

[sleemanj]

If you have nothing else connected to said Nano, and you short out the diode, and connect it to USB, then yes, it should work.  If it doesn't, buy another, it's not worth spending time on it realistically.

[skillz21]

But when I short the diode the nano disconnects...

[sleemanj]

Because when you short the diode some other fault, further down the line, is shorting your USB port.  The "device disconnected sound" is almost certainly telling you that your USB port itself, or a hub if you have one, has shutdown because it was shorted out.

Unplug the USB cable, short the diode, and measure between the USB V+ and the Ground.

[skillz21]

Wait, are you saying that the USB port is broken?

[janoc]

Wait, are you saying that the USB port is broken?

NO. Read what he has said again.

He is saying that when you make a short circuit on a device powered from an USB port (like your Nano), the USB hub (either the one on your desk or the one in the PC) will detect that and turn the affected USB port off. It is a protection so that nothing else gets destroyed due to a faulty device, such as your computer's motherboard.

That is why your Windows made that *ding* sound - the hub has disconnected your device because it was overloading the USB port. The disabled port will usually come back once you remove the offending device - test with e.g. a known good USB stick. If not, then after you unplug/replug the hub to the PC (if it is an external hub) or after a reboot (if it is an internal one). If it doesn't come back even after power cycling the PC, then you have likely fried it, I am afraid.

You are actually lucky - not all PCs have this functionality. On some the PC power supply will turn the entire computer off because it detects a short circuit on the 5V rail. And some others will happily let you destroy the motherboard by burning out the USB hub integrated in the CPU chipset (southbridge) when you cause a short circuit. Both are poor design but I have encountered both cases - the first one usually in a normal desktop PC and the second one in laptops.

So, as a corrolary, if you want to be safe and make sure your $10 Arduino doesn't smoke your $1000 computer, make sure that:

a) No "tinkering" while the thing is plugged in, short is easy to make and could end up expensive

b) Better - use a powered external hub between the PC and the Arduino. A $10 hub is much cheaper to replace than $200 motherboard should the inevitable mistake happen.

c) Best - if you are tinkering with higher voltages than 5V - e.g. 12V for motors and such, use an USB isolator. You can get some cheap ones online, they were mentioned here in the forum too. Those are plenty good for Arduinos. PCs definitely do not like having 12V shortcircuited into a USB port and something will blow up. My colleague has destroyed his laptop exactly like this - he was tinkering with some stepper motors driven from an Arduino and 12V from the motor supply somehow got into the USB lines.

And re shorting that diode causing a disconnect - that looks like that not only the diode is dead. There is very likely a short circuit on the Arduino's side of the diode, basically anything connected to the +5V rail in that schematics could be bad and causing a short. That is what @sleemanj is asking you to measure.

It is probably cheaper to buy a new Nano but it would be a very educational experience for you trying to reason your way about the board and finding that damaged component(s).

[skillz21]

Wow... thanks for such a descriptive answer! I tested the USB port, it still works (I'm not using a USB hub BTW, Just the USB port in my laptop). So what exactly do you want me to test with the Arduino? What do you need me to measure?

[Aodhan145]

As they have said, The reasons the schottky diode died must of been a short on the +5V side on the arduion. This means when you short the diode connections you still have a short on your arduino which drags down your power rail with it. Do you have anything connected to the arduino? If you don't it may be a problem with the board its self. Try measuring the resistance of +5V to GND when your board is powered off.

[skillz21]

I measured the resistance between 5v and GND when nothing is plugged in.... it's 1.65k ohms.... I'm assuming that's not normal? Also, I forgot to mention, after the diode blew, the 5v power rail actually dropped to 2.95v.... now sure why I didn't mention it earlier.

[EPTech]

Did you actually measure the resistance between +5V and GND on the USB side. What we really need to know is the resistance on the application side, so between +5V and GND on the board itself, or on the board the Arduino is attached to, because that is probably where the fault is. When you say you measured something, you also need to say on which points in the circuit.

Happy short hunting.

[tablatronix]

You probably have a dead short somewhere, i have done the same, and since a usb port supplies more than the diode can handle it goes up in smoke rather quickly.