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Discovery BNC burnt


weaverdouglas22

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My question is what did I do wrong that caused my Discovery BNC to go up in smoke?

I was trying to measure a millivolt signal generated by an abs sensor on a car.   I was not receiving any signal after testing several sensors.   After double checking settings, I wanted to verify that the oscilloscope function was working at all so I connected the test leads to a 12v car battery.    I did see the 12v signal on the scope, however the leads became untouchably hot almost instantly.     

I then looked up the internal resistance of the analog discovery 2 (1M Ohm) and confirmed this resistance using my multimeter.     Touched the oscilloscope leads to the battery again and they heated up instantly.

I then ran the scope in series with the amp meter in the multimeter.  It read 0.001V and there was no excess heat generated on the scope leads.  I found this very strange and decided to try the scope directly to the battery one more time and this time a circuit on the Discovery BNC overheated and burned the board.

I don't understand why this happened. I have always tested the scope with a direct connection to a 12v DC source for long periods of time and have never had an issue before.   A 1M ohm resistance on a 12v circuit should be a very safe current.    I'm open to any thoughts!   Thank you.    Doug

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Hi @weaverdouglas22,

I don't know exactly how you have the Analog Discovery 2 and BNC Adapter connected (AC or DC coupled, connected to the correct oscilloscope connectors and not accidentally the Wavegen outputs as the Wavegen outputs are sensitive to back-powering), but I have a few questions to hopefully help narrow down the cause.

- The BNC Adapter has single ended inputs and the Analog Discovery 2 has differential inputs but they are not floating (as it is referenced to the GND to the connected host computer). When measuring the voltage of the battery did you have the ground clip of the probes connected to the chassis (or at least the negative input on the battery if it is disconnected).
Though if you are measuring voltage across a battery, there isn't going to be a reference ground point to begin with, so I would have also expected that measuring the voltage across a battery, car battery or otherwise, with a 1M Ohm resistance on analog inputs would not be an issue in any scenario.

-When you say the Discovery BNC burnt, are you referring to the BNC Adapter specifically, or the Analog Discovery 2?

Thanks,
JColvin

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Thanks for the response JColvin!   I'm very sorry but I did not receive a notification that anyone replied and just happened to jump on the forum today and figured I'd check.

I see where I went wrong.   I did not understand that two of the probe connections on the Discover BNC adapter are for Waveform outputs.   I thought that one probe connected to CH1 and the other probe connected to W1 for ground. (I honestly assumed W1 was just a non-intuitive abbreviation for CH1-)  I had been using it this way for YEARS and still received seeming accurate measurements from hall effect sensors!   Apparently I was just lucky until this last experience where the W1 circuit received too much current.

To answer your question, yes, it was the BNC adapter that was burned on the W1 circuit.

Thank for helping me figure this out.

Doug

 

 

 

 

 

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Hi @weaverdouglas22,

Understood. The W1 and W2 labels on the BNC Adapter were chosen because that matched the labels of the two waveform generator channels shown on the plastic of the Analog Discovery 2 and 3.

If you have a different labeling scheme in mind that would help make the product more intuitive to users, I would love to hear it so that I can pass it on to R&D.

Thanks,
JColvin

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The labeling makes perfect sense now that I understand the product better.   I didn't realize previously that the BNC Adapter worked with the waveform generator (obviously I didn't read the manual!) and being more familiar with multimeters, I assumed that I needed two leads attached to the adapter for signal and ground. I didn't realize that a single BNC probe has both signal and ground in the same lead until your response above made me question if I'm using it properly.

Thank you so much!

Doug

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