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--- poe [2024/07/17 17:06] – created river
+++ poe [2024/07/27 20:50] (current) – river
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 Cheap passive PoE injectors probably use pairs (4,5) and (7,8) exclusively for power, and disconnect them from data, so probably will only work at 100 Mbps.
+==== Testing one ====
+I bought some cheap PoE injector/splitters from ebay. They claim gigabit speed, and a quick test shows they negotiate at that.
+Pins 1,2,3,6 are directly connected through. Pins 4,5,7,8 seems to be connected with a serial DC blocking cap of what my multimeter says is ~93nF.
+Cheap component testers says:
+  * 0.41mH power to combined side pin 4.
+  * 0.73mH between combine side pins 4 and 5.
+It seems to be a bias T! I was expecting transformers with central tap, as is used on the ends normally.
+====== Voltage / Cable resistance ======
+I thought 12V would be enough for low power stuff / short wires, but with the Pi in a box even 10 meters is pushing it at 12V.
+The Pi crashed unpleasantly, causing file system corruption, when running with 12V into a high resistance cable of 2.5R per wire.
+I will install box 0 with a 20V supply I found.
+<file>
+pi draws up to 3a in normal non PD mode at 5V - 15W.
+Heltec board up to about 2.5W.
+Other bits maybe max 0.5W.
+W needed.
+W @ 12V about 1.5A + conversion loss, say 1.7A.
+V supply is 3A rated.
+Converter is fine at 3A (tested).
+CAT5e I have is 24 AWG.
+AWG copper: 88 Ohm / km
+For 10m: 0.88 Ohm
+For a pair: 0.44 Ohm / 10m.
+For a pair including return: 0.88 Ohm.
+It's more though, because of twisting, copper is longer than cable length.
+Say 1 Ohm per 10 meters. It might be more actually.
+Max R (including both sides) 1.75 Ohm.
+m is therefore too long for this cable / voltage.
+With 20V, in max acceptable R is about 4.48R.
+So then 40m is ok.
+Apparently Cat 6 is usually AWG 23.
+</file>
 ===== Raspberry Pi PoE header =====