Just blew up an LED on my Arduino by Probing - Page 1
Forum section: electronics / beginners

author: modointeriore
author_rank: Contributor
timestamp: September 22, 2015, 12:30:11 pm
content: 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.

author: DimitriP
author_rank: Super Contributor
timestamp: September 22, 2015, 12:58:33 pm
content: Take a look at "EEVblog #279 - How NOT To Blow Up Your Oscilloscope!"link below:

author: modointeriore
author_rank: Contributor
timestamp: September 22, 2015, 01:15:19 pm
content: I outlined above that nothing is mains earth referenced. Am I missing another detail from that video?

author: WN1X
author_rank: Unknown
timestamp: September 22, 2015, 01:32:05 pm
content: Quote from: modointeriore on September 22, 2015, 12:30:11 pmHow did you confirm the powered hub is isolated from mains ground?

author: macboy
author_rank: Super Contributor
timestamp: September 22, 2015, 02:11:23 pm
content: Quote from: modointeriore on September 22, 2015, 12:30:11 pmMost laptop power adapter outputs are in fact mains earth referenced. What reason do you have to believe that yours is not?

author: alsetalokin4017
author_rank: Super Contributor
timestamp: September 22, 2015, 04:50:21 pm
content: 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.

author: ThomasDK
author_rank: Regular Contributor
timestamp: September 22, 2015, 05:06:53 pm
content: 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.

author: jitter
author_rank: Frequent Contributor
timestamp: September 22, 2015, 05:34:35 pm
content: 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.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: September 22, 2015, 06:22:51 pm
content: 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.pdfhttps://www.arduino.cc/en/uploads/Main/Arduino_Uno_Rev3-schematic.pdfI 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)?

author: jitter
author_rank: Frequent Contributor
timestamp: September 22, 2015, 06:44:17 pm
content: 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.

author: gaijin
author_rank: Contributor
timestamp: September 22, 2015, 07:06:01 pm
content: 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.

author: jitter
author_rank: Frequent Contributor
timestamp: September 22, 2015, 07:38:49 pm
content: Quote from: gaijin on September 22, 2015, 07:06:01 pmAgain: 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).

author: Seekonk
author_rank: Super Contributor
timestamp: September 22, 2015, 07:40:54 pm
content: 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.

author: modointeriore
author_rank: Contributor
timestamp: September 22, 2015, 08:27:05 pm
content: 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.

author: VK5RC
author_rank: Unknown
timestamp: September 22, 2015, 08:30:16 pm
content: As a fellow aussie, remember the neutral is connected to ground, I speak with experience!

author: Monkeh
author_rank: Super Contributor
timestamp: September 22, 2015, 08:32:49 pm
content: Quote from: modointeriore on September 22, 2015, 08:27:05 pmNot even the scope?The scope should have an earth pin.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: September 22, 2015, 09:16:41 pm
content: Quote from: Seekonk on September 22, 2015, 07:40:54 pmExactly. 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.

author: sleemanj
author_rank: Super Contributor
timestamp: September 22, 2015, 09:39:02 pm
content: Quote from: alsetalokin4017 on September 22, 2015, 06:22:51 pmIf 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.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: September 22, 2015, 09:45:44 pm
content: Quote from: sleemanj on September 22, 2015, 09:39:02 pmIn 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...

author: Maxlor
author_rank: Frequent Contributor
timestamp: September 22, 2015, 09:50:42 pm
content: Quote from: alsetalokin4017 on September 22, 2015, 09:45:44 pmThe weakest part gives first. Besides, it might well be that the part driving the LED is also dead.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: September 22, 2015, 10:22:17 pm
content: Quote from: Maxlor on September 22, 2015, 09:50:42 pmSure. 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 .....

author: ludzinc
author_rank: Unknown
timestamp: September 22, 2015, 11:24:10 pm
content: Quote from: alsetalokin4017 on September 22, 2015, 10:22:17 pmLadies and Gentlemen, I think we have a winner.

author: modointeriore
author_rank: Contributor
timestamp: September 23, 2015, 12:11:35 am
content: 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!

author: retrolefty
author_rank: Super Contributor
timestamp: September 23, 2015, 12:19:57 am
content: 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.

author: modointeriore
author_rank: Contributor
timestamp: September 23, 2015, 02:59:26 am
content: 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!

author: alsetalokin4017
author_rank: Super Contributor
timestamp: September 23, 2015, 03:00:44 am
content: Quote from: modointeriore on September 23, 2015, 12:11:35 amWell, let's see. If your laptop is isolated from the mains ground, then the Arduino's ground, while it is connected to the USB ground and hence to the laptop's case/motherboard/psu ground, doesn't make a complete current path back through the mains to the scope's ground clips. So simply touching the scope probe ground to the Anode would not have done anything. But while you were probing around, if you inadvertently shorted the Pin 2 of the "102" resistor array over to the Pin 1/LED Anode, the LED would have immediately turned redorange and then fried, since the Arduino's 5V bus is connected directly to the Pins 2 and 3 of the resistor array. This shorting is very easy to do since those pins are so close together; even a pointed pin probe might do it. It is essentially connecting the LED directly to a +5V voltage regulated source without a current-limiting resistor, and the other side of the LED is connected to the ground return of the 5V source.I'm still not 100 percent convinced this is what happened but it seems very plausible, according to your description. I'm still not understanding where the "sparks" are coming from though. Once the LED is blown and presumably open rather than shorted, repeating the Pin2 to Pin1 short should not have done anything, no sparks.ETA: Did you perhaps have another scope probe ground connected somewhere else? All the probe ground clips are connected together at the scope chassis BNC connectors, and also to the Mains ground pin.

author: modointeriore
author_rank: Contributor
timestamp: September 24, 2015, 12:31:25 am
content: No the scope ground was only connected. Thanks, your explanation helps!

author: nbritton
author_rank: Frequent Contributor
timestamp: September 24, 2015, 12:59:43 am
content: Quote from: alsetalokin4017 on September 22, 2015, 06:22:51 pmDiodes have a breakdown voltage at which point they start conducting electricity in the reverse direction. If the voltage potential between the cathode and anode side was 120V you could have definitely hit the breakdown voltage threshold.

author: nbritton
author_rank: Frequent Contributor
timestamp: September 24, 2015, 01:17:42 am
content: Quote from: modointeriore on September 23, 2015, 12:11:35 amNeutral is bonded to ground, in the main service panel, throughout most of the world.http://www.esgroundingsolutions.com/why-do-you-have-to-bond-the-neutral-and-the-ground-wire-in-the-main-panel/