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I have still been able to get it stuck in the reset loop when the DC-DC input is at the low end of its voltage range, with the little bit of inrush current there is causing it to dip below the threshold. The DC-DC is regulated, so it will not be in this range, and the AUX will be monitored before we hit it with high currents. But overall, we just had too much current and too much resistance. So was all this testing worth it for a problem that we might not have actually encountered in the car? From a time perspective, maybe not, but from a learning perspective, definitely!
Main lessons learned:-
Start testing out possible solutions immediately, and document what changes.
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Always check your assumptions (I forgot the multimeter leads had resistance at one point)
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Make sure that you know your board like the back of your hand, then you know where all the extra pieces for testing need to be put (I had to get creative with some of the components placement)
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Make sure you’re using the right tools for the job - a clamp meter trying to measure inrush current on a 1ms timescale will not do anything when your inrush current only lasts for 8ms. Know your measurements timescales and the limitations of the equipment!
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Sometimes seemingly complicated problems can be caused by simple effects → inrush current and ESR causing voltage droop = reset loop
Multimeter loading is a thing - multimeters will typically have 10M resistance between their input terminals. This can (and will) affect the signals that you measure!