MMI Modular wrote:
The Meanwell brick has low voltage lockout if the input drops below 4.2V.
I was doing some testing last night and this appears to be the root of the issue.
My setup consisted of an Anker USB battery bank, an Amazon basics USB cable (aka the "good" ones), and a 200mA 12V fan.
It's pretty well known that fans like that have rather large inrush current since it's basically just running current through a wire so it made for a pretty decent test.
At work I powered the fan up and noticed inrush of about 1A at 12V. Assuming near perfect efficiency that would be about 2.4A on the 5V rail, reaching the limits of most USB chargers out there.
So what happens when I try to boot the fan from the USBPower?
This graph shows the 5V rail direct from the battery bank. The resolution isn't as fine as it could be, but you'll notice that the peaks reach very near to the 4.2V threshold and very likely reach/pass it as both 12V rails flash on and off until I either unplug everything or give the fan a jump start with some compressed air. Once the fan gets going, it is rather happy to keep spinning, though you can see how much the 5V rail likes to droop under a 200mA load on the 12V rail.
This is all the info I have for now. It's a reasonable assumption to say that USB cables with poor conductors could drop even more voltage and lower the inrush current threshold for boot looping. Another possibility is that the USB power source could have a hard limit on how much current can be pulled in a short amount of time, which would cause the voltage to drop suddenly as well. If I find anything else I'll make it known, though I consider this a pretty good explanation for now. Maybe a V2 with higher current rating and inrush current limiting? You never know