Energy requirement for shutdown

Allan Herriman · 2016-03-09T21:43:20+00:00

Hi, I've been asked to create a "graceful powerdown" circuit for a Raspberry Pi 3. The idea is to detect input power failures, then request that the Pi shutdown by (e.g.) changing the state of a GPIO, then supplying power from (e.g.) a capacitor and a DC/DC converter until the Pi has completed its shutdown. That doesn't sound too hard, but I don't know how long it takes a Pi to shutdown, and what sort of power it's using during that time. I'm guessing a few seconds at a watt or two, for a total energy of a few J. Any improvement on that? BTW, I'd measure it, but I don't actually have a Pi. I have one coming on overnight delivery but it's been a week so far. :( Thanks, Allan

Mar 09, 2016 84 Replies

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Ahem A Rivet's Shot 10 years ago

More to the point the original proposed 47000uF has more than ten times the required capacity - which sounds like a safe margin for a robust design.

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The Natural Philosopher 10 years ago

Not quite that bad

Hers a 10F 2.7v cap

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Realistically you would need three or so to get to 7-8V

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Andy Burns 10 years ago

But in series they'll only be 3.3F ...

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Folderol 10 years ago

Another point is that a moderately high voltage standard electrolytic can actually be your PSU *reservoir* cap, so no need for any fancy switching.

Just had a look for DC-DC converters:

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Not exactly expensive.

W J G

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The Natural Philosopher 10 years ago

Actually the easy way to do all this is a classic mains transformer, bridge rectifier, reservoir cap, 5V regulator, and then simply have a mains loss detector feeding some Pi pin.

If the 5V regulator is switched mode, well OK.

Or you can even go for mains->bridge rectifier->400V reservoir cap-> switched mode PSU->5V.

If you want best efficiency.

I am sure there are in fact wall warts or similar doing exactly that already. Just add mains fail detector

Or for a bit more DIY

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5V at up to 8A, with loads of capacitor in it, and just add mains fail detector.

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