Forum Rule: Always post complete source code & details to reproduce any issue!
Results 1 to 5 of 5

Thread: Don't blow your Teensy 4.1 Ethernet PHY when using passive PoE

  1. #1
    Junior Member
    Join Date
    Sep 2020
    Posts
    4

    Don't blow your Teensy 4.1 Ethernet PHY when using passive PoE

    Hot-plugging a passive PoE connection will blow your teensy PHY. Even though the PHY is DC-isolated from the jack, the unbalanced condition that occurs while hot plugging a powered cable will cause a transient spike of the order of your PoE voltage on your PHY. If you actually make this mistake, only the PHY is damaged and you can fix your teensy by replacing it (PN:dp83825irmqr). Robin very generously sent me some spares they had around and I was able to bring my dead teensies back to life. Thanks Robin!!!

    The solution to this is easy: TSV diodes on the ethernet data lines on the PHY side + small series resistors. Implementing this on the next teensy revision or the ethernet adapter board or on your custom PCB will protect your PHY from hotplugging passive PoE.

    This application note explains all this in detail. http://ww1.microchip.com/downloads/e.../00002157B.pdf

  2. #2
    Senior Member
    Join Date
    Dec 2015
    Location
    LA
    Posts
    197
    Thanks for finding this, very interesting. I use a Netgear GS305P POE that I'm always connecting and disconnecting test equipment, rPi and computers to, as if it was a regular ethernet connection. While the simplistic visual aids in the instructions show plugging in the power last no specific mention is made of the necessity. So far no damage to my stuff, but I will now take precautions, thanks.

    Looking at schematics of the Raspberry Pi 3+ and 4 that were designed for POE, I see no TSV. In fact looking at 4 devices so far I see no protection, unless it is built into the magJack or hidden somewhere.

  3. #3
    Junior Member
    Join Date
    Sep 2020
    Posts
    4
    You probably wont have this problem with active PoE because the power isn't enabled until negotiation, so hotplug events wont happen the same way they would with passive PoE.

  4. #4
    Senior Member
    Join Date
    Dec 2015
    Location
    LA
    Posts
    197
    @mamdos,
    Thanks, wondered what passive meant.

  5. #5
    Senior Member PaulStoffregen's Avatar
    Join Date
    Nov 2012
    Posts
    23,951
    @mamdos - Can you tell me which hardware you're using for passive PoE? If I wanted to experiment with various diodes and resistors, which gear should I buy to give me the PHY-destroying scenario you had?

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •