I have like 15 of these boards so it would be nice if I could fix them, but even more importantly we don’t want whatever is going on to be inherited by subsequent boards that are derived from this one. That would ruin yet another batch.
Basically, this batch of Lepton 2.0 boards (tried 3 different ones ) suffer from some kind of noise issue. Serial communication is extremely unreliable, so is I2C, when powered from 24 volts. So are interrupts, interrupts spontaneously fire, depending on how the wires are arranged etc. (probably due to capacitive coupling in some way). It works fine if I power the MCU over the SWD connector (which is apparently backwards, you aren’t supposed to feed 5 volts into that connector but it seem to work fine :P)
I read recently using ceramic caps can lead to noise issues, paradoxically, but it looks like that’s not what’s happening.
I made three videos, one showing the voltage spikes on the ground plane when powered with 24 volts, outputting 2 volts programmed at 2 radians per second, no motor connected. Large spikes of more than 10 volts are seen on the ground plane, but of very short duration, the oscope is set to 5 volts , 0.1 microseconds per division. It may be the spikes are actually larger but extremely short and therefore the peak is not visible.
similar spikes seem to occur on the positive side, but it’s not clear. I have to change the oscope to 10 volts per division and that renders the spikes on the ground side hardly visible and so I wouldn’t expect the positive side spikes to show up when I do that, just some kind of defect in how the oscope works I guess. It would be nice to know if the spikes were positive in nature on the positive side, indicating something to do with inductance, or if they were only dips, which would say to me that the mosfets were giving a very low resistance path to ground during some periods.
The large 1000uf capacitor gets warm even when the motor is not connected.
I connected the motor, and the spikes get much bigger, at least 50% bigger, you can see in the video.
I tried removing the capacitors by destroying them/prying them off with pliers (tried to solder the off but didn’t seem to be working). Didn’t work, the voltage spikes are basically the same as before, apparently.
I would have thought the capacitors would blunt the spikes considerably, maybe the capacitors are too small, a manufacturing error. Even so it would be good to address the source of this noise a little closer to the root instead of just trying to absorb it.
Any ideas on the cause? I previously experimented with dead time and PWM frequency to see if I could solve it and could not.
Now that I have things all set up, now’s the time to investigate if anyone has any ideas.
no motor connected
with motor (iirc it was about 24 ohms for both phases of the motor in series, i.e. terminal to terminal. Power supply said 0.08 A when no motor connected and only 0.1 A when connected but I think that’s a bit off.)