Tuesday 2 October 2012

24V passive POE for Raspberry Pi

Today my 24V power supply arrived - the final part I needed to try a 24V passive power-over-ethernet (POE) solution down a 40m cable to drive my Raspberry Pi and two webcams. Good news - it works!
Raspberry Pi and two Xbox Live Vision webcams
using 24V passive power over ethernet (POE)

I'd previously succeeded in running two webcams directly from the Raspberry Pi (when mains powered), and managed to run one webcam from RPi using 12V passive POE over 30m - but that cable only just reached the bird box with camera at the end of the garden so I wanted a longer network cable. I searched the house for stronger power supplies,  giving two different flat screen monitor 12V DC supplies a go - but had no joy at 40m (even without the cameras).

The only thing for it was more volts! And 24V seemed the way to go. While waiting for the parts to arrive, I realised I'd hit on the same idea as plugwash's ghetto POE solution on the Raspberry Pi Forums - also highlighted by Frambozenbier.

I ordered an adjustable 24V to 5V DC-DC step-down unit from eBay for a few pounds, searching specifically for a KIS-3R33s based unit based on advice I'd read online. The claimed specifications for anyone wanting to buy something similar were:
  • IS-3R33S integrated with adjustable MOSFET MP2307DN
  • Size: 45mm by 32mm
  • Input voltage: 4.75-24V
  • Output voltage: 0.93V-18V
  • Efficiency: up to 98%
  • Output current: rated 3A, max up to 4A peak 
  • Programmable Soft-Start
  • Low ESR Ceramic capacitors is recommended at output
  • Internal Frequency: Fixed 340kHz
  • Output ripple: 20M bandwidth (for reference only)
  • Protection: Cycle-by-cycle over current protection.
The rubbery coating on mine didn't look too good, but it works OK. Notice the small screw on the blue block between the large orange capacitors - that controls the output voltage:

KIS-3R33S based DC step-down

I attached this to a couple more  2.1mm  barrel connectors from Maplins for ease of use - given that's what the rest of my setup uses:
More DIY soldering to add connectors

A quick reminder image from my previous blog post - the simple £3 passive PoE kit from eBay:
PoE female power injectorPoE male power out

Surprisingly Maplins didn't have any 24V DC supplies in store, so again I turned to eBay and ordered a 24V 1200mA power supply (aiming for a reasonable power rating), which was the last part to arrive.

Initially I set the DV output to 5.25V to match what the original power supply I'd used gave. That worked but under load this fell to just 4.8V, so by fine tuning the DC step-down converter I could ensure the the Raspberry Pi was getting the full 5V it wants even with the two web cameras attached.

The setup works as follows: Mains --> 24V power power adapter --> 2.1mm plug --> Ethernet injector --> 40m Ethernet cable --> Ethernet splitter --> 2.1mm plug --> 24V:5V step down --> USB micro --> Raspberry Pi --> USB --> Webcameras.

The observant may wonder why the green LEDs on the Xbox Live Vision web cameras are not on - that's because I've disabled them for astro-photography and use in a bird nesting box (the camera on the right has four infra-red LEDs added to it).

My next project will be to install the Raspberry Pi and voltage step-down module in a suitably water tight container to mount in the garden, and run the 40m ethernet cable to it. Then I should finally be able to monitor the bird box remotely.

Update: Successful indoors testing.

Update: Successful outdoor testing.


  1. How does the pi manage to power the both webcams over just its U.S.B? i thought you would need a hub???

    1. Using powered USB hub is probably needed if you want to connect lots of power hung devices at once. However, using a good power supply directly connected to the Raspberry Pi as usual, there is no problem running two of the web cameras and using the wired ethernet connection (which is also USB internally) - but I do have nothing else connected, no sound, no video, no data pins, etc. In my experiments at least, this is also possible sending the power via this kind of basic POE cable.

  2. Thank you for the article I'm interested in doing this can you take a photo of the PSU and show how it's connected to the Ethernet cable or maybe just explain in the comments? Also if you can provide a link to the PSU that would be great

    1. I bought the 24V power supply on eBay, it just had a standard barrel connector which could plug directly into the PoE adapter to "inject" power. Very simple.

    2. Thanks for the quick response, I have a ac/dc laptop charger with variable voltage up to 24v @ 3a/3000ma do you reckon this power supply would be suitable?

    3. That should work, you have over twice the claimed power of my eBay adapter. Good luck.