PoEPT (Pass through) heat vs voltage

spiccinini edited this page Dec 12, 2018 · 5 revisions

LibreRouter phase 2 (MegaBoard 1.1, Core board 0.2) with the following modifications:

  • solded R1 of 82ohm (R2 no load)
  • solded R4 of 82ohm with 10nF in series to gnd
  • solded R5 of 82ohm with 10nF in series to gnd
  • changed R32 with 71kohm
  • removed D1 (SMBJ24CA)

With dragino 2 wires PoE

Using Programmable DC Electronic Load and using both pairs to PoEPT (4, 5 and 7,8) while using Dragino PoE injector with only one pair (wires 5 and 8)

PoEPT power[W] DC IN [V] Total consumption [W] Temperature after 5' [°C] Conditions
5 19 9 40 radios off
8 19 13 60 radios off
12 24 19 68 radios off
12 32 19 55 radios off

With 4 wires "normal" passive PoE

Using Programmable DC Electronic Load and using both pairs to PoE Passthrough (4, 5 and 7,8) and injecting power to Port 1 using 2 pairs (4,5 and 7,8)

PoEPT power[W] DC IN [V] Total power [W] Temperature after 5' [°C] Conditions
12 24 16 42 radios off
12 32 16 37 radios off
12 32 22 44 1 radio full tx
12 24 22 58 1 radio full tx
12 18 18 59 radios off
10 18 21 70 1 radio full tx after 1'
10 24 20 57 1 radio full tx
10 32 21 46 1 radio full tx
6 12 12 58 radios off
6 12 - - 1 radio full tx -> board reset
0 12 4 36 radios IDLE in AP mode
0 12 8 49 1 radio full tx

Some notes:

  • Other temperatures: Full Tx radio (at txpower=23dBm) 57°C at mPCI heatsink, radio 2 IDLE at 40°C, CPU 49°, ambient temp 26°C
  • verified that there is no voltage without PoEPT activation

PoE pins: VCC: 4, 5 (blue, blue-white) GND: 7, 8 (brown-wthite, brown)

Conclusions

  • PoE is safe in the range of 12-32V when using four wires (4,5 - 7,8) to inject power
  • PoE may be safe at 32V when operating from only two wires
  • PoE Passthrough is safe only when operating from four wires and >= 24V (note that 24V must be present at the input of the LibreRouter, voltage drop in the cable should be measured)
  • Do not use PoE passthrough with less than 24V
  • If you really need power from 12V (batteries, solar power, etc) use the input power and not PoE
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