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I'm working on providing a stable supply of 5V power to a Raspberry Pi over the micro USB input port. One possible issue I've noticed is occasional instantaneous dropouts when switching over from battery power back to the feed from the wall after a brownout. These dropouts are very short, less than a tenth of a second at most, but I'm worried that it might cause the Pi to reboot, which has a chance of damaging the SD card.

What I'm unsure of is how short of a delay in power the Pi can handle before shutting off (if any). I know from experience that laptops and desktop pcs can handle a second or two before shutting down if you accidentally pull the battery or the plug on them, but I dont know if that's something that the Raspberry Pi can do as well. Looking inside I see most PCs have decent sized capacitors inside that can probably hold a small amount of charge when the power supply gets cut, but the Pi only has surface mount capacitors, which I would assume are much smaller capacity.

Does the Pi have any ability to ride out extremely short power interuptions at all, and if so, how long?

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    I expect this is something you will have to test yourself. I doubt an official reply will be forthcoming. At the very least an answer will vary with the Pi model.
    – joan
    Jan 11 '18 at 20:47
  • Use a super capacitor.
    – Kong Chun Ho
    Jan 12 '18 at 6:47
  • By far the easiest way to measure this is using an oscilloscope and seeing how it take 5V or 3V3 on GPIO header to drop out. There is not much dedicated charge storage and only a 200 mikes at most on the supply line so I suspect the hold time to be a few ms at best.
    – crasic
    Mar 27 '18 at 22:59
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The RPi has a 47uF capacitor on the 5V power line, and the low-voltage warning triggers at 4.6V. Assuming 750 mA power consumption, it's OK for the RPi to be unpowered for

(5.1-4.6 V) * (47 uF) / (750 mA) = 31 us

This is the time your RPi is guaranteed to survive without any ill effects. It may survive much longer in practice, but you'll be gambling.

The straightforward way to increase the time would be to add another capacitor on the 5V line. To safely cover 0.1s, you will need a capacitor of 0.1F.

Practically speaking, it should be much easier to reduce the unpowered time. Even mechanical relays can have switch-on time of ~10ms, and MOSFETs and SSRs are even faster, so reaching 31us target is not an unreasonable goal.

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It depends on devices connected and the RPi model (different CPU speeds, number of cores). My RPi boots from HDD, so the voltage will drop very quickly for sure. I also guess that if the voltage will decrease for not very long time, it will trigger the APX803 chip to generate reset signal for CPU (with a certain delay). A longer voltage drop will switch off the entire circuit. Determining exact time to shut down needs a measurement of course.

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