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LED burns when oscilloscope is connected to it

Jean-yves

Oct 20, 2016
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Hello,
Working on a project of mine, I have recently bought a USB oscilloscope. It took me only 30 minutes to damage my project using the oscilloscope. I'd like to understand why it happened please .


Setup:

Arduino Mega fitted with a 4xLED driver shield (http://ledsee.com/index.php/en/ardu...-pwm-high-power-led-shield-0-35-0-7-1a-detail) with one LED connected to shield output 1.

Hantel 6022 BE oscilloscope (http://www.hantek.com/en/ProductDetail_2_31.html), powered solely from USB.



Problem : if both Arduino and Oscilloscope are power from the same PC, when I try to measure voltage on the driver output 1 (tip of probe on +, ground of probe on -), I immediately burn the connected LED.

If Arduino is powered not from USB but from an external power supply (and I remove the USB cable between Arduino and PC), the LED won’t burn and I get a realistic read on the oscilloscope.

I am not skilled enough to understand what happens. It is like my probe makes the constant current driver increase its current output, leading to LED burn.

What shocks me is that I thought oscilloscope probes where isolated from the oscilloscope power supply, allowing hot measure on 230 V circuits without risking to inject high voltage back to the PC.
The said probe is 10 MOhms resistive, so it must be something related to the ground of the probe being somewhat connected to the USB ground, but I don't see the big picture here.
 

BobK

Jan 5, 2010
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Unless the O-scope has an isolated power supply, you cannot connect the ground lead to anything but ground!

Since you are powering the board and the scope from the same PC,they share a common ground. Which means that wherever you connect the ground lead, you have shorted that point to ground.

To get the voltage across the LED, measure each side of it separately, using only the probe lead, and then subtract the two readings.

Bob
 

dorke

Jun 20, 2015
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"What shocks me is that I thought oscilloscope probes where isolated from the oscilloscope power supply, allowing hot measure on 230 V circuits without risking to inject high voltage back to the PC.
The said probe is 10 MOhms resistive, so it must be something related to the ground of the probe being somewhat connected to the USB ground, but I don't see the big picture here."

Most scopes(the vast majority) are not isolated.
The GND of the input channel and probe are connected to chassis GND.

It doesn't matter at all what the resistance of the prob is.
As bob said the only thing to connect the prob to is GND in the tested circuit.

One sure way to damage your scop is to try to measure high voltages (like 230 mains circuits,SMPS circuits etc.) with GND of prob connected at anything but GND point on the tested circuit.
 
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Colin Mitchell

Aug 31, 2014
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The first thing to do is remove the earth pin of the CRO when measuring "hot chassis" TV's etc.
This can be done with an adapter on the "power plug."
Then the whole CRO is floating and you need to touching nothing else or you will get fried.
I am referring to a "mains powered" CRO.





.
 
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(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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Colin's suggestion is not relevant to the OP's problem.
 

Jean-yves

Oct 20, 2016
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Unless the O-scope has an isolated power supply, you cannot connect the ground lead to anything but ground!

Since you are powering the board and the scope from the same PC,they share a common ground. Which means that wherever you connect the ground lead, you have shorted that point to ground.

To get the voltage across the LED, measure each side of it separately, using only the probe lead, and then subtract the two readings.

Bob

Your answer is short and very explicit, thank you a lot.
If I have understood well : since the scope came with two probes, I could connect one to + side, one to - side, probe GND to board GND (but useless in the case where the board is powered from the same PC as the scope) and display subtract of both probes.
 

Jean-yves

Oct 20, 2016
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Most scopes(the vast majority) are not isolated.
The GND of the input channel and probe are connected to chassis GND.

It doesn't matter at all what the resistance of the prob is.
As bob said the only thing to connect the prob to is GND in the tested circuit.

One sure way to damage your scop is to try to measure high voltages (like 230 mains circuits,SMPS circuits etc.) with GND of prob connected at anything but GND point on the tested circuit.

Thank you. Your answer made me think.... What happen if one day I want to probe a 230 device that is isolated (thus no connection to main GND). When I open it, will I find a GND somewhere on the electric board ? I mean, will it be labeled as "GND" even in the case of isolated device ?

Next beotian question : if I want to check a female socket, shall I use one probe like this : GND to socket GND and tip to either left or right socket, OR : probe GND to what I think is neutral and probe tip to what I think is phase ? (that second idea seems very bad because my scope GND may now be connected to a phase!)

If I understood well : the best would be to use 2 probes, both grounded and each tip to each socket , then display 1-2.
 

BobK

Jan 5, 2010
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Your answer is short and very explicit, thank you a lot.
If I have understood well : since the scope came with two probes, I could connect one to + side, one to - side, probe GND to board GND (but useless in the case where the board is powered from the same PC as the scope) and display subtract of both probes.
If it has two probes, it can probably show you the difference between the two, which is exactly what you want.

Bob
 

dorke

Jun 20, 2015
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Well,
using a "standard mains operated non-isolated " scop to measure Mains isn't the way to go.

There are several ways to by-pass this problem:
1. You can use a dedicated "differential isolated probe",expansive.
2. You can use a hand held scope (battery powered hence is isolated),very expansive.
3.If you only need to measure Mains Voltage, a DMM in ACV will do .

Here is a very good video on the issue.
Enjoy
 
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