Poe UPS Pi

A super-cap based Power-over-Etherent Uninterruptable Power Supply for a Raspberry Pi 3B+ and a hungry HAT too.

A Class-3 PoE PD front-end takes PoE power from an RPi 3B+ 4-pin header, a commercial Cosel DC-DC converter isolates and converts from 48V to 5V at 2A, a charger circuit keeps two series 25F super-caps topped up, a balancer circuit makes sure the super-caps stay in their voltage range, a boost converter circuit supplys the RPi with 4.9V, a microcontroller supervises operations and also provides a watch-dog function. All on a HAT sized RPi PCB, there's even room for a 20mm fan, super-capacitors are off-board but there's a full 40pin through-header to accept another HAT on top (eg a RAK2249 lora concentrator & GPS board).

The supercaps provide about 30s of backup time at 800mA load, that is enough to cover 10 seconds of power outage plus another 15 seconds during which the Pi is cleanly shut down (with the help of a service script) if necessary prior to power removal.

The impetus for this project was the construction of a LoRa gateway that must be highly reliable - trusted not to trash it's SD card and can be remotely located and managed.

I currently have a prototype up and running but have several bug fixes to implement on a revision-B PCB and am still working on the firmware for the microcontroller. Watch this space and let me know if you're interested.

Project Elements

RevB board in use (JPG, 410.4 KB)

PoE powered RPi3B+

PoEUPSPi_Back (JPG, 439.81 KB)

Rear view of assembled board

RPi3B+ modification (JPG, 85.96 KB)

Adding the 2 pin RUN/PEN header

PoeUPSPi_Schematics.PDF (PDF)

Full schematics

Gerber file pack (ZIP)

PoEUPSPi RevB Gerbers (with JLCPCB.com naming convention)

PoEUPSPi.xls (XLS)

Bill of Materials

PoEUPSPi.zip software archive (ZIP)

PoEUPSPi.hex V030120 + SerialIn.py V3 + PoEUPSPi.sh helper script

PoEUPSPi.PDF (PDF)

Updated documentation PDF 3/1/20

Updates from the author

fishbeetle 5 years ago

I've made some more minor improvements to the microcontroller code and there's a new HEX file now in the ZIP archive PoEUPSPi.zip. There's also a revised python script to receive V3 of the serial debug stream.

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fishbeetle 5 years ago

I have just updated the microcontroller HEX file and the PDF documentation to reflect some changes I made to the watchdog system. The watchdog time-out procedure now includes a pre-emtive shut-down request to ensure that an otherwise running OS is not asynchronously reset. It became apparent that when used with a RAK concentrator and TTN gateway packet forwarder that if network connectivity was lost then the packet forwarder would stop accessing the concentrator module thus activating the watchdog, however the OS was still running fine and certainly not requiring a reset. This issue is now worked-round by including a shut-down request as part of the watchdog timeout process.
The new code also includes an improved algorithm for predicting reminaing hold-up time in the event of power loss.

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fishbeetle 5 years ago

FYI I have documented my RPi-based LoRa gateway integration here:
http://www.marvellconsultants.com/LoRa

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fishbeetle 5 years ago

All information, design files and documentation are now uploaded. If you're thinking of building this board please note that it IS possible to hand solder it, U7 has a thermal pad for which I used just a dab of thermally conducting grease. It's advisable to test the balancer circuit prior to installing the supercaps as a voltage in excess of 3V across a capacitor will drastically shorten its life. I test my balancers by temporarily mounting two large regular capacitors of different values eg 15000uF and 30000uF, applying power and checking, preferably with an oscilloscope, that they both charge equally.
I can highly recommend JLCPCB.com for supplying the PCBs they're unbelivably cheap and if you spend what you save on delivery the board will arrive surprisingly quickly from HK. If you want to operate it without PoE you'll need to somehow provide a PGOOD signal to the PIC & balancer circuit to ensure proper operation. Be carefull not to short the output voltage, when I did that U7 blew up - but I did manage to successfully swap it out. Good luck.

Joshua 5 years ago

Very useful project.
Can it be used as simple UPS if PoE is not used?
How are the signals managed by the Raspberry Pi?

fishbeetle 5 years ago

It can be used without PoE but there are plenty of other products readily available that would perform the UPS function. I believe the pdf documentation file above should answer your questions.

Patt1607 4 years ago

Hey,
Is it possible to geht a füll shematic with all parts?

I want to make my own Board for rpi4

Rgds

fishbeetle 4 years ago

Full schematic re-uploaded and available now.

3 Comment(s)

fishbeetle 5 years ago

The revision B PCB has now been successfully tested so I've posted updated schematics, gerber files, bill of materials and an example service script for the Pi to support programmed shutdown & watchdog functions. I still have some wrinkles to iron out on the PIC microcontroller code, once done I'll post the HEX file and a bit later some more detailed documentation.

0 Comment(s)

fishbeetle 5 years ago

The revision B PCB is currently in manufacture. Once it's built and tested without any issues I'll upload gerbers, BOM and firmware HEX file. The image shows the prototype board in service supporting an RPi3B+ and RAK2247 LoRa gateway working very reliably.

Lora Gateway with PoE & super-cap UPS (789kb)

Sir Cer 5 years ago

Hi!
Nice project.
Are the project files available?
It should be very useful to unpopulate the PoE part, and use it only as managed UPS for mains failures.
Jesus.

fishbeetle 5 years ago

Project files coming as soon as I've debugged RevB PCBs.

1 Attachment(s) 2 Comment(s)

fishbeetle 5 years ago

PIC microcontroller code (in XC8) is now complete and fully functional for the prototype hardware. Shut-down timing is computed in real-time based on the state of charge of the supercaps and their rate of voltage drop. The mechanism provides 15 second warning of critical capacitor voltage loss independent of o/p power consumption and size of the super-caps. A bash shell script running on the Pi (as root) initiates a clean shutdown upon request from PoEUPSPi via a GPIO pin.

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fishbeetle 5 years ago

Image shows scope trace of discharge process with 800mA fixed load, note that the 4.9V output is maintained for 32 seconds. Re-charge time (from zero Vcap volts with no load) was measured at 40 seconds.

Yellow trace: Vcap-top, Blue: V-cap-mid, Magenta: output voltage. (1663kb)

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fishbeetle 5 years ago

The photos show the revision-A board under test, the board works after some minor modification. Revised schematics have been produced and new PCB artwork is currently being finalised. Code for the PIC microcontroller is under development. The revision B board features improved functionality and some smaller, cheaper components.

Revision A prototype under test (587kb)
Revision A prototype under test (532kb)

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Discussion (2 comments)

BerndGaykema 5 years ago

Very interested project.
My LoRa gateway crashes once every 2 to 3 months, no clue why. Most of the time the SD-card can be trashed.

My gateway: Raspberry Pi 2B (Stretch-Light) with IMST iC880A, build according the site https://github.com/ttn-zh/ic880a-gateway/wiki

fishbeetle 5 years ago

I hope Elektor will pick it up, but I've heard nothing from them so far. I might do a small production run myself if they don't - but it won't be a cheap board.

BerndGaykema 5 years ago

I now use a powerbank (for charging phones on the way) which stops the Pi from crashing due to a power failure, but it doesn't nicely close the software before the voltage becomes too low. And your PoE-solution solves the necessity for a seperate power unit and cabling.

I have 3B's lying around, so that is not a problem. Will your product become available as an Elektor product?

fishbeetle 5 years ago

OK, as this is PoE powered it really only works with Pi3B+, other UPS solutions are available for regular 5V power set-ups, eg PiJuice. PoE is a great solution for powering a gateway though.
One issue I've noticed at least with RAK's gateway packet forwarder is the HUGE number log lines it generates, if there are any kind of errors it can spew >6MB/Hr of log content. If you're not taking action to keep this under control it can soon fill the SD Card.