Comment on Current limiting

• litchralee@sh.itjust.works ⁨1⁩ ⁨week⁩ ago

  Ah, I entirely missed the sense pin when skimming the datasheet.

  That said, using a shunt for an inductive load like a motor may have to contend with the corresponding spikes caused when switching the motor. This just means the thing doing the sensing needs to tolerate the spikes. Or mitigate them, with either a snubber or a flyback diode (is this actually doable with an H bridge?).

  As for the op-amp and ADC, if we already accept the additional of the op-amp part, it is also feasible to use a comparator with a reference voltage set for the max safe current. The digital output of the comparator can then be fed directly to the microcontroller, providing fast reaction without the sampling time of an ADC. But this would be so quick that the spikes from earlier could get picked up, unless mitigated.

  Still, these solutions are adding to the part count. If that’s a concern, then I’d look for a motor driver with this functionality built in.

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  • boojumliussnark@lemmy.world ⁨1⁩ ⁨week⁩ ago

    OK… I understand very little of this :( I am a software person. But could this comparison not be done with some hysteresis? The part count is not important as long as the parts aren’t terribly expensive, since this is exclusively for my personal use.

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    • litchralee@sh.itjust.works ⁨1⁩ ⁨week⁩ ago

      But could this comparison not be done with some hysteresis?

      It can, but analog design is also not my forte.

      The part count is not important as long as the parts aren’t terribly expensive, since this is exclusively for my personal use

      In that case, the original suggestion of using an ADC and an op-amp would be the most flexible for software. You would, however, need to do some research on wiring an op-amp to amplify the sense voltage to something your microcontroller’s ADC is capable of resolving.

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• boojumliussnark@lemmy.world ⁨1⁩ ⁨week⁩ ago

  That sounds very cool - and way beyond my own understanding :-)

  Is this something you could actually help with sketching out for me?

  I found the datasheet for the L298 and I can see that for my use (I am using just out 1 and out 2) it should be pin 1 (Sense A) that should be relevant, right?

  This looks like it is connected to gnd on the driver board (I can easily cut this connection).

  Unfortunately I don’t understand what a current shunt is? :-)

  It has me slightly worried that it says that pin 1 can go to -1v which I think would break the ADC input?

  As far as I can tell the ADC input on the PICO accepts from 0 - 3.3v right?

  Thanks a bunch for looking at this!

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  • cmnybo@discuss.tchncs.de ⁨1⁩ ⁨week⁩ ago

    A current shunt is a resistor that can handle the full current of the motor. You would probably want a 0.1Ω resistor rated for at least 1 watt.

    You measure the voltage drop across it to determine the current. The voltage will be quite low. With a 0.1Ω resistor and a 2A load, it will be 0.2V. You want to amplify that up closer to the full scale input of the ADC to get a decent resolution. An op amp with a gain of 10 would work well for that. Use a rail to rail op amp that can run from the 3.3V power supply.

    Here is a schematic:
    Schematic

    R4, D1 & D2 protect the op amp input from any spikes the motor produces.

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    • boojumliussnark@lemmy.world ⁨1⁩ ⁨week⁩ ago

      Sorry I forgot to ask: I should cut the SENSE-A connection to ground right?

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    • boojumliussnark@lemmy.world ⁨1⁩ ⁨week⁩ ago

      Oh, wow!

      Very nice! Thank you so much for doing this!

      Is it correctly understood that the “Sense” line can be hooked up directly to the ADC input on the Pico?

      Now, just because I am quite ignorant when it comes to all this, I will venture to ask: This is better because it is closer/faster/more exact than using the Hall current sensor, or? With my complete lack of understanding, I had the impression, that I could more or less “just” connect the sensor to the “right spots” and directly to the ADC input on the Pico, and my work in the Pico would be similar to this setup?

      Don’t get me wrong - I can see this is much more elegant - and won’t be bothered by whatever else might use power on the same rail. But since it might be simpler for me to use a more “out of the box” component like the sensor, instead of my bad soldering of several discrete components… Also however ridiculous it might seem it is much easier for me to source 5 modules - than 5 of each of these components (I would have to buy many of each, and waste most).

      So I have to ask. :-)

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      • cmnybo@discuss.tchncs.de ⁨1⁩ ⁨week⁩ ago

        The sense output from the op amp connects directly to an ADC input. The output is 1 volt per amp. If sense_a is already connected to ground, you will have to disconnect it.

        The hall effect current sensor will work fine too. They do tend to have an offset, but you can calibrate that out in software. Just take a reading when the motor is off and subtract it from the reading when the motor is on. If you do use the hall effect sensor, you could connect it between sense_a and ground so it will only measure the motor current.

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