Electronics > Beginners
I keep ruining circuits with oscilloscope
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cabalist:
Hey there,

I've fried a couple of Servo control boards and I'm hoping for some guidance to what I'm doing wrong.   

The equipment in question:

Mg996r servos
Hiwonder Digital Servo Tester Servo Controller with Voltage Display (https://www.amazon.com/gp/product/B07485M6PH)
Hiwonder 6 Channel Digital Servo Tester with Over-Current Protection (https://www.amazon.com/LewanSoul-Channel-Digital-Over-Current-Protection/dp/B073XZH264)
Rigol 1054Z
Tenma 72-2660 Portable Power supply (https://www.newark.com/tenma/72-2660/power-supply-handheld-3-75a-12v/dp/56Y1289)

The power supply is connected to the Servo Controller @ 6V limited to 1A.  The servo is plugged in with Signal, Power, Ground in what I believe to be the correct pins. When power is turned on the servo twitches but turning the knob and/or pushing the button seems to do nothing.  I wanted to check the signal out and so I connected the oscilloscope to the Signal pin and the ground pin.  I smelled what I now know was the MCU getting very very hot.   I removed power and the probes and now the controller doesn't work.   I foolishly did this AGAIN and fried another board.   :/

Any hints on what I could be doing wrong?   The second board I fried was using a different servo...  The Power supply doesn't have a ground pin and so I believe I am not creating a ground loop.  Should I suspect the oscilloscope?  The servos? 

The oscilloscope reads its compensation terminal just fine.   I'm new to this and I'd really like to get past the point of mysteriously burning out boards.  Any help would be really appreciated.  Thanks!






StillTrying:
"The Power supply doesn't have a ground pin and so I believe I am not creating a ground loop."

Check it's really floating, just in case. I don't think you're doing anything wrong, but on boards like that never trust the "+" and "-" markings, they're sometimes the wrong way around, check all the "-" are connected to GND. And of course keep the board on something no-conducting.
bdunham7:
The scope probe input has 1M or 10M (you should start in the 10X setting) input resistance, so 6V will get you 6uA at the worst.  That won't heat anything up.  So you only have two connections to the scope--the probe tip and ground, right?  So the only way this happens is if you have current going thru the ground connection. 

First, watch Dave's video:



In fact watch it twice, since you've blown up two boards!  |O

Then use my own oscilloscope safety technique--remove your ground clips!  Just use the probe tip and no ground to measure and observe all of the outputs and the ground connection.  You'll see that your "ground" on the board in fact is neither at ground potential nor isolated, and has measurable potential to ground.  That or the scope has nothing to do with the problem.
Psi:
It does sound like you are shorting something with the scope ground clip.
But from what you describe i cant see why its happening.

Things i would check
- Silkscreen markings on china PCBs are not always correct. Maybe the polarity is backwards somewhere.

- Before connecting the scope ground clip to anything put a small 5-12V lightbulb in series between the ground clip and what you're about to connect it to.
The light bulb should NOT turn on. If it does there is voltage between scope ground and that point. You will short it out if you connect the probe there.

- Do any of those boards have 79xx written on the voltage reg ICs?  It's pretty unlikely, but if those are negative regulators it might make sense why things blow up when you connect to ground.
David Hess:
Something might be using ground side switching or current sensing in which case connecting the oscilloscope ground will short that function out.

Combine two oscilloscope channels using add and inverter mode (or subtraction) and use them as a differential probe with the ground leads connected together and not to anything else.

Also to verify what is going on, try connecting an oscilloscope probe tip to the ground point with the oscilloscope probe ground floating and see what you find.  If the voltage is not zero, then something is going on.
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