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Power over Ethernet (PoE) basics and beyond

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257 points voxadam | 5 comments | 16 Oct 25 14:09 UTC | HN request time: 1.038s | source
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skopje ◴[22 Oct 25 00:53 UTC] No.45663732[source]▶
PoE is awesome. My custom home security system is all CCTV PoE with a gstreamer backend running on four-core fanless linux box. Way to go. Complete control. No batteries, no wares spying on me, no personal data getting scraped by big guys. (Cloud connectivity sucks because I have segmented mp4s and jogging through them hurts but I only care for events after they happen, not while they happen.)
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shadowpho ◴[22 Oct 25 03:44 UTC] No.45664711[source]▶
Poe is great for many things, but it’s not as efficient as direct connection (for both low and high power.)
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1. rjdj377dhabsn ◴[22 Oct 25 07:31 UTC] No.45665889[source]▶
Why's that?
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2. jansper39 ◴[22 Oct 25 08:53 UTC] No.45666423[source]▶
You have to convert the power to 48v and back down to whatever you need on the other end which has losses, plus you have losses over the cable too.
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3. rubatuga ◴[22 Oct 25 14:13 UTC] No.45669483[source]▶
Well quite frankly 48V has lower losses than 12V
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4. shadowpho ◴[22 Oct 25 16:11 UTC] No.45671318[source]▶
At higher power the Ethernet drops quite a bit of voltage/power compared to the wiring in your walls. Furthermore 48v vs 110v is double the amperage/more loss on top of that.

Then there’s double/sometimes triple conversion (120:48 and then 48:dc; 120:48 and then 48:12, and then 12:dc).

Furthermore magnetics are a must on both side of the PoE which also isn’t great.

At lower power there’s more circuitry to run and multiple conversions aren’t great compared to a simple cheap flyback.

For more technical feel free to check here, although it isn’t quite end to end: https://e2e.ti.com/cfs-file/__key/communityserver-discussion...

5. shadowpho ◴[22 Oct 25 16:13 UTC] No.45671353{3}[source]▶
110v -> 48v (long thin cable) -> 12v

Versus

110v (long thick cable) -> 12v

Top has more conversions and more current running on smaller gauge