I've designed this simple circuit that uses a Silvertel AG9205 PoE PD module in order to extract 5.65V from the RPI PoE header pins.

Here it's the schematic:


In order to be careful, I've placed a SS34 Diode as reverse voltage protection directly at the +VDC output pin of the module.

Now, here comes the strange thing. When powered up, the board itself produces a strange noise (like a whistle) that comes from the capacitors, after three or four seconds, the Poe PD module goes in toshutdown mode.

When I remove the diode and bypass it with a jumper wire, the problem disappears.

Initially, my doubt was regarding a faulty diode, but measuring with a multimeter the diode it shows a voltage drop of 180mV, so it seems okay. In addition, I've tried another board and it shows the same symptoms

Where is the problem? I really can't understand it.


1 Answer 1


That's a bad idea - look at at the schematic on page 3 of your datasheet: Placing your diode where you do disable C35's ability to flatten the ripple of the half-wave rectified current induced in the secondary side of the transformer.

The primary side senses that the energy in the transformer core isn't drained, and consequently shuts down. Good device!

Also, when looking at that schematic, you'll see that your reverse voltage protection does nothing useful, as there's already a diode in place, directly at the transformer.

  • \$\begingroup\$ Thanks Marcus, I've completely missed that! My fault! \$\endgroup\$
    – VirtApp
    Jan 18, 2021 at 20:01

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