plugwash wrote:Basically there are four main possibilities
1: True PoE, this is the least risky soloution because it's got a load of protection systems built in but it requires relatively complex (and therefore relatively expensive) hardware at both ends to inject the power and to extract and convert it.
Not tried that.
plugwash wrote:2: 5V ghetto PoE, this sounds like a good idea at first but due to cable resistance it simply will not work properly over nontrivial cable lengths. The voltage drop will just be too much.
Yep, works fine for modest cable lengths (blog post
plugwash wrote:3: 12V ghetto PoE with a switched mode converter (you DO NOT want to try and use a linear regular, it will waste a lot of power and get bloody hot) at the Pi end. With the right DC-DC converter and all wires used in the cable it should be possible to get up to 50-60M with such a setup. I wonder why the astronomy guy failed, my guess is his ebay adaptors were using using two wires rather than four.
I assume by astronomy guy you mean me, and yes, even with 12V and a DC-DC convert I only got this to work at 30m not 40m (blog post
). Your hunch about the passive PoE adaptors not using as many wires as possibly is intriguing.
plugwash wrote:4: 24V ghetto PoE with a step down converter at the Pi end. This should work fine out to the 100M maximum distance of ethernet (and probablly beyond), main downside is that you will likely have to buy the 24V PSU specifically (whereas a 12V PSU you are likely to have lying around).
This has worked for me at 40m (I don't currently have a longer ethernet cable or need to try any longer distances) (24V PoE blog post
plugwash wrote:I also notice the astronomy guy tried to use higher voltages to compensate for cable loss without using a DC-DC converter. I strongly reccomend against this practice because it will make the voltage delivered to the Pi swing wildly depending on what is currently drawing current and carries a high risk of damaging the Pi.
Again I presume that was me you were talking about - that was a one off experiment (blog post
), and I was monitoring the voltage at the Raspberry Pi via the test points while doing this (rather than ramping up the voltage blindly). One of the nice things about the low cost of the Raspberry Pi is I feel a bit less worried about damaging it
I had the 40m 24V PoE setup running indoors for a few weeks (filming mice invading the house
), and noticed every so often the Pi would lock up. The webserver stopped responding, I couldn't SSH in. After rebooting I could see it had also stopped recording images. I didn't establish if this was an intermittent power issue, some other hardware issue (overheating in the closed case I was trying perhaps) or a software problem.
I'm intending to use the 40m 24V PoE setup in my garden this spring (for monitoring bird nesting boxes rather than telescope usage).