LED burns when oscilloscope is connected to it

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.

S----y oscilloscope doesn't have isolated ground.
I did completly destroyed one laptop the same way with similar oscilloscope few years ago.

Not sure if I can picture what you described, but I also accidentally burned equipment connected to the oscilloscope, on two occasions. In the first case, the oscilloscope was connected to the power socket without isolating transformer, and the ground of the probe shorted to earth of theelectric mains. In the second case, it was measuring 2 signals with different references, and the common ground of the probes made a short circuit.

Honestly, I'm not sure if a high value of series resistance is able to safetly protect the equipment connected to the oscilloscope; everything depends on the characteristics of the circuit connected.

Jean-yves06 said:

Hello,



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.
.

Click to expand...

Absolutely incorrect assumption.

You may consider yourself fortunate, that you only measured low voltage DC and not powerline level AC.
Not only would your component have been damaged but your computer as well.

Most likely when you connect the probe ground on the (-) side, you are bypassing a low side current sensing resistor, which provides the required feedback for the current regulating loop.

Thanks. I think I am going to connect this Oscillo to a cheap laptop until I have a better grasp.

Can you please give me a more detailed description of the first case ?