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Will this Ups Design work?

Posted: Thu Aug 03, 2017 11:29 am
by Chaoticstickman
First off I want to say I'm very new to this. I'm currently am in the process of making a car pi however with the hard shutdown I keep having problems. I have looked for many ups designs however they are far too complex for me. I have found this easy to understand instructions and just wanted to confirm it would work before I wasted time and money on it. (I'm a student so I'm on a tight budget)

http://www.instructables.com/id/Simple-USB-UPS/

Re: Will this Ups Design work?

Posted: Fri Aug 04, 2017 12:53 am
by BudBennett
I don't think that it will work for you. The supercapacitors will just hold up the 5V supply for an additional 80 seconds before the power fails, so it will still force a hard shutdown on the Pi. UPS is hard to do. A real UPS has to inform the Pi that the power has failed and allow it to perform a shutdown sequence to prevent SD card corruption.

Re: Will this Ups Design work?

Posted: Fri Aug 04, 2017 1:00 am
by Chaoticstickman
BudBennett wrote:
Fri Aug 04, 2017 12:53 am
I don't think that it will work for you. The supercapacitors will just hold up the 5V supply for an additional 80 seconds before the power fails, so it will still force a hard shutdown on the Pi. UPS is hard to do. A real UPS has to inform the Pi that the power has failed and allow it to perform a shutdown sequence to prevent SD card corruption.

So what your saying is that this isn't enough time to start and finish a shutdown sequence (this is what I wanted to do.. Should have clarified)
And if this isn't enough time is there any way I could make the hold time longer?

Or it it that it can't tell the pi when to shutdown? I thought that you could get the pi to detect when power drops to automatically start the shutdown

Re: Will this Ups Design work?

Posted: Fri Aug 04, 2017 4:06 am
by BudBennett
From what I could tell by reading the article you referenced, the circuit doesn't do anything but hold up the 5V supply for a few extra seconds. The Pi is not aware that the power has failed until it's too late and will just hard reset when the voltage drops below its minimum requirement. You would have to add a separate circuit that detected that the power had failed and signal that fact to the Pi -- usually via a GPIO input.

But it gets tricky. If the power comes back on while the Pi is powering off then it will hang in the power off state. Then you must intervene manually to reboot the Pi by cycling the power to the Pi.

This is why UPS systems are so complicated.

Re: Will this Ups Design work?

Posted: Fri Aug 04, 2017 5:04 am
by Chaoticstickman
BudBennett wrote:
Fri Aug 04, 2017 4:06 am
From what I could tell by reading the article you referenced, the circuit doesn't do anything but hold up the 5V supply for a few extra seconds. The Pi is not aware that the power has failed until it's too late and will just hard reset when the voltage drops below its minimum requirement. You would have to add a separate circuit that detected that the power had failed and signal that fact to the Pi -- usually via a GPIO input.

But it gets tricky. If the power comes back on while the Pi is powering off then it will hang in the power off state. Then you must intervene manually to reboot the Pi by cycling the power to the Pi.

This is why UPS systems are so complicated.
So it's possible but just more difficult then one would think.

Thanks so much for the help ☺