Just blew up an LED on my Arduino by Probing

[modointeriore]

Hi,

I have been getting back into electronics after being interested in the subject as a child. This forum has been invaluable as I have been researching what I need to get, etc.  I have just received my first Arduino and my first scope (Rigol DS1054Z). I ran the basic blink firmware for the Arduino. This blinks the LED at pin 13, which is already on the board (I have an Uno clone). I wanted to use the scope to visualise the pulse and so and put the ground clip on the ground side of the LED (it was not in contact with any other point). Without having a chance to probe the other side there were some sparks and the LED blew. I cannot figure out why.

I am powering the Arduino through USB. It is connected to a powered hub, which is not mains earth referenced. No other device connected to the hub is mains earth referenced (including my laptop).

Included some photos on the off chance they will be of any use in diagnosing the problem. My thanks for any help.

[DimitriP]

Take a look at "EEVblog #279 - How NOT To Blow Up Your Oscilloscope!"
link below:

[modointeriore]

I outlined above that nothing is mains earth referenced. Am I missing another detail from that video?

[WN1X]

I am powering the Arduino through USB. It is connected to a powered hub, which is not mains earth referenced. No other device connected to the hub is mains earth referenced (including my laptop).

How did you confirm the powered hub is isolated from mains ground?

[macboy]

...
I am powering the Arduino through USB. It is connected to a powered hub, which is not mains earth referenced. No other device connected to the hub is mains earth referenced (including my laptop).
...

Most laptop power adapter outputs are in fact mains earth referenced. What reason do you have to believe that yours is not?

[alsetalokin4017]

Use your DMM to check for a voltage between the ground clip of the scope, and the Arduino ground, with everything else connected (except the scope)  as when you blew the LED.

The "ground side of the LED" (cathode) on the Arduino plain Uno is connected to the Arduino common ground and the anode of the LED is connected via a pair of resistors to the Pin 19 output of the ATMega, and to the Pin 13 Arduino output connector, and also to the Pin 3 of the ICSP connector.

The Uno R3 is slightly different, with the anode of the LED connected (through a resistor) also to an op-amp, the non-inverting input of which is connected to the Pin 19 of the ATmega 328. Cathode again connects directly to the common Ground.

I can't figure out how connecting the scope ground to the _ground_ side of the LED could blow the LED, even if there is a voltage present between the scope ground and the Arduino ground, if nothing else is connected.

Are you quite sure you connected to the Cathode (ground) side of the LED? It is the side with the green marking on it.

[ThomasDK]

The fact, that the laptop was floating might be the problem here.
The input filter in the charger will put the ground at a 115V offset from mains earth. It can hardly supply any current, but might be enough to blow up a LED.

[jitter]

We do not have enough info yet. Two things I wonder about:
1) Was the laptop powered by the adapter or running off the battery?
2) If adapter, is it mains earth referenced?

2) can only be answered straight away with "no" if the mains plug is two prong. If it is a three prong plug, continuity must be measured between earth prong and output (most likely: -).
I have come across laptop adapters with two prong cords (double isolated) and three prong cords of which some were mains earth referenced and some were not.

[alsetalokin4017]

Sure, but how could connecting a "hot" ground to the _ground side_ (Cathode) of the LED possibly blow the LED? Where is the current path through the LED in that case?

https://www.arduino.cc/en/uploads/Main/arduino-uno-schematic.pdf
https://www.arduino.cc/en/uploads/Main/Arduino_Uno_Rev3-schematic.pdf

I can understand it if the OP actually connected the scope probe reference (ground) lead to the _anode_ of the LED, and there was a voltage differential between the USB ground and the scope ground. But how could the LED blow if the scope ground was connected to the cathode (common grounded to the Arduino USB connector ground)?

[jitter]

A voltage differential is only a problem if a loop is formed and current can flow.
Assuming that was the case (there were sparks, after all), perhaps too high a reverse voltage fried the LED.

[gaijin]

Switching psu used on the laptop or usb hub?
use a multimeter and test for ac voltage between the scopes ground and the arduino's.
I had a laptop a while ago with a metal shell that was annoying to use while plugged in.
You could measure 85vac between it's shell and ground.

[jitter]

You could measure 85vac between it's shell and ground.

Again: only a problem if current can flow. Besides, you would probably only measure this voltage with a standard 10 MOhm input DMM. Put 10 k across the terminals (or use an Agilent U1232A in low input impedance setting, approx. 3k) and it's probably as good as gone.

To test the theory of floating voltages, I subjected myself to a to a little test a while ago: I used a variac with isolated windings (NOT an autotransformer!) and increased the voltage in steps of 10 V. Each time I would touch the secondary, floating, side. I went all the way up to 230 V without ever feeling a thing! Why? No current could flow (not entirely true, the capacative leakage of the transformer was enough to light up a neon screwdriver but not enough for me to even feel the slightest sting).

[Seekonk]

The laws of physics still apply.  One thing they teach in a Decision Analysis course is at least half of what people tell you is not true.  Even Robert McNamara in FOW said " Seeing and believing are often both wrong."  Connecting the ground lead to the common side of the LED would not blow it, it might do other things. That would be the hardest way to connect onto common. So right there the story falls apart.  Take a break, go back, and the problem will be obvious.  And you better find that problem before you do anything else.

[modointeriore]

Thanks for the input so far, sorry the reply is late, Aussie here.

The way I know that nothing is mains earth referenced is that no plug in my setup has an earth pin. My laptop is a Macbook.

I just confirmed that I still get the spark even with the Arduino connected to my Macbook via USB. Away from board right now but will test with multimeter and share results, and will try probing with the Arduino powered by battery to make sure the problem is with the USB power.

[VK5RC]

As a fellow aussie,  remember the neutral is connected to ground,  I speak with experience!

[Monkeh]

Thanks for the input so far, sorry the reply is late, Aussie here.

The way I know that nothing is mains earth referenced is that no plug in my setup has an earth pin. My laptop is a Macbook.

I just confirmed that I still get the spark even with the Arduino connected to my Macbook via USB. Away from board right now but will test with multimeter and share results, and will try probing with the Arduino powered by battery to make sure the problem is with the USB power.

Not even the scope?

The scope should have an earth pin.

[alsetalokin4017]

The laws of physics still apply.  One thing they teach in a Decision Analysis course is at least half of what people tell you is not true.  Even Robert McNamara in FOW said " Seeing and believing are often both wrong."  Connecting the ground lead to the common side of the LED would not blow it, it might do other things. That would be the hardest way to connect onto common. So right there the story falls apart.  Take a break, go back, and the problem will be obvious.  And you better find that problem before you do anything else.

Exactly. I can't see how connecting to the _ground_ side of the LED could possibly blow it, even if there were some voltage difference between the Arduino ground and the Scope ground. There isn't a current path from the Anode side of the LED that would allow current to flow, that I can tell, without also blowing the microprocessor as well.

In addition, I can't even see how it's possible to connect the Rigol probe ground cliplead to the LED directly, at all. See the image below for the relative sizes of the parts. Perhaps if the probe's spring ground prong were installed and used it might be possible....

So I'd like the OP to confirm which side of the LED,  and how, the connection to the LED was made. I can't even see the green Cathode marking on the LED without using a magnifying glass.

[sleemanj]

But how could the LED blow if the scope ground was connected to the cathode (common grounded to the Arduino USB connector ground)?

If the current limiting resistor is moved between cathode and ground that would do it.

Not impossible the board in question does so instead of the schematics provided, especially if it is a china sourced one.

[alsetalokin4017]

But how could the LED blow if the scope ground was connected to the cathode (common grounded to the Arduino USB connector ground)?

If the current limiting resistor is moved between cathode and ground that would do it.

Not impossible the board in question does so instead of the schematics provided, especially if it is a china sourced one.

In that case the current path must have gone through the microprocessor, or at least the op-amp in the R3 schematic, right? So wouldn't these parts also be fried? I am assuming that the OP's Arduino still works, and just the LED is fried. I guess this might be an unwarranted assumption...   

[Maxlor]

In that case the current path must have gone through the microprocessor, or at least the op-amp in the R3 schematic, right? So wouldn't these parts also be fried? I am assuming that the OP's Arduino still works, and just the LED is fried. I guess this might be an unwarranted assumption...   

The weakest part gives first. Besides, it might well be that the part driving the LED is also dead.

[alsetalokin4017]

In that case the current path must have gone through the microprocessor, or at least the op-amp in the R3 schematic, right? So wouldn't these parts also be fried? I am assuming that the OP's Arduino still works, and just the LED is fried. I guess this might be an unwarranted assumption...   

The weakest part gives first. Besides, it might well be that the part driving the LED is also dead.

Sure. But remember that this scenario depends on the LED's resistor being on the ground side of the LED rather than the Anode side as in the official Arduino circuit diagrams. But the R3 uses a 4-resistor array for the "pin13" LED, the Rx and Tx LEDs and something else. It's the part marked "102" in the OP's photo, and you can see that it's connected to the anode side of the LED.
I've flipped the OP's mirror-image photo to the right orientation below. You can clearly see the trace that connects the resistor array's pin 1 to the Anode of the LED.

ETA: There is the full 5V bus voltage on the Pins 2 and 3 of that resistor array. If these were somehow shorted to Pin 1 which is connected to the Anode of the LED ..... 

[ludzinc]

ETA: There is the full 5V bus voltage on the Pins 2 and 3 of that resistor array. If these were somehow shorted to Pin 1 which is connected to the Anode of the LED .....

Ladies and Gentlemen, I think we have a winner.

[modointeriore]

Okay, I've had another look, and I think alsetalokin4017 might be right. I was using the probe alligator clip connected to a through hole component leg to get a fine enough 'probe' for the ground. When I wasn't getting a trace I went in with the alligator clip to eliminate the leg as a problem. I didn't notice it touching any other part of the board, but it happened fast and I pulled it away once it sparked. Stupid.

I had no idea that neutral was connected to ground in Australia. That's good to know.

I just traced around the blown LED and fortunately the board seems undamaged except for the LED (the signal was as expected) so I will replace it. My first SMD job. Glad I got a $7 board.

Thanks so much for everyone helping out, much appreciated.

I do have a residual question if anyone has the time to answer it for me. Why—if I had placed the probe ground on the anode—would I have potentially blown the LED? Wouldn't it just short the current so that it never reached the LED? Does this scenario rely on main earth bonding or not? I would like to understand the various way I could have screwed up so I get the benefit of multiple mistakes for the price of one!

[retrolefty]

 You simply created a short circuit with your ground clip not being connected to where it should have been. In the future I would suggest you just use a jumper wire from any one of the three arduino ground pins on the shield connectors for your scope ground clip. Then you are free to probe without worry, unless you touch more then one point with the scope probe tip. 

[modointeriore]

Yes, sorry, it wasn't clear that my last question was a hypothetical one. I pick up from the previous comments that if I had placed the ground of the probe on the anode that might have accounted for the problem, and I was wondering why that could have blown the LED. No problem if people don't have time to answer, just thought I'd learn something by asking!