Electronics > Beginners
Why do my ICs keep dying when scope probing?
Johnny10:
Then do what Gyro suggested use two probes and the invert function on your oscope.
ptricks:
The chips I have had fail like this were from the motor being disconnected while powered, the ground path not being a star configuration, or EMI entering from wires that were too long.
Might want to look over this link and see if anything sticks out:
https://www.analog.com/en/analog-dialogue/articles/staying-well-grounded.html
Mike Ro:
--- Quote ---Might want to look over this link and see if anything sticks out:
https://www.analog.com/en/analog-dialogue/articles/staying-well-grounded.html
--- End quote ---
Very good article, thanks!
Mike Ro:
--- Quote ---Or, assuming a two channel scope, use the poor man's version... Add the two channels with one channel set to invert. Make sure the ground clips stay well out of the way.
--- End quote ---
I followed this approach and the result was interesting. I discovered that in spreadCycle mode there were large voltage peaks on the driver outputs.
I removed my fixed voltage supply and supplied the driver at a lower voltage (12V) and set the internal drive current to 100mA. Even at standstill, the output was reaching 24V peak. I then very gingerly ran the motor slowly forward and the peaks increased to around 28V.
Now, I may be misunderstanding the concept of chopper drivers here (and the Trinamic spreadcycle one in particular), so please be patient with me!
I thought that the idea is the chopper will overdrive the voltage to achieve a fast current rise time with the inductance of the motor coils effectively regulating the maximum voltage achieved.
However, from the gentle test that I made, it looks like the voltage can significantly overshoot the supply voltage and I would have thought that this is a problem for the driver MOSFETs.
I haven't found anything in the datasheet that describes the output voltages, only lots about the current / sense resistor voltage.
Interestingly, when I configure the driver for 'traditional', constant off time with fast decay time (i.e. chopper mode, chm = 1) then the differential output looks more normal and is limited to the supply voltage.
Also, in spreadCycle mode (chm = 0) then the motor has an audio whine both at stand still and when moving slowly. The pitch of this whine changes slightly when probing the outputs, so there must be some coupling going on.
I have noticed in the data sheet about choosing appropriate sense resistors. The module has .11R which is optimised for higher currents, so maybe there isn't enough resolution for running at 100mA?
--- Quote ---or EMI entering from wires that were too long
--- End quote ---
This was my first port of call when I saw the voltage overshoots. I wondered if the long (~50 cm) coiled motor leads were having an effect, but cutting them down to 50mm didn't make any noticeable difference.
rstofer:
Put a 1k resistor in series with the scope ground lead and connect the other end of the resistor to your normal test ground point. Measure the voltage drop with a DMM. If there is significant current, there will be a voltage drop.
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