Raspberry Pi PoE Redux

[Martin Rowan] was lucky enough to get his hands on the revised Power Over Ethernet (PoE) hat for the Raspberry Pi. Lucky for us, he wrote it up for our benefit, including inspection of the new hat, it’s circuit, and electrical testing to compare to the original hardware.

You may remember the original release of the PoE hat for the Raspberry Pi, as well as the subsequent recall due to over-current issues. In testing the revised board, [Martin] powered a test load off the USB ports, and pulled over an amp — The first iteration of the PoE hat would often trip the over-current protection at 300 milliamps.

This afternoon, the redesigned PoE board was officially released, and the post mortem of the problem documented in a blog post. It’s a lesson in the hidden complexity of hardware design, as well as a cautionary tale about the importance of thorough testing, even when the product is late and the pressure is on.

The PoE hat converts 48 volt power down to a 5 volt supply for the Pi using a flyback transformer. The problem was that this transformer setup doesn’t deliver clean steady 5 volt power, but instead provides power as a series of spikes. While these spikes were theoretically in spec for powering the Pi and usb devices, some Raspberry Pis were detecting those spikes as too much current pushed through the USB ports. The official solution essentially consists of better power filtering between the hat and the Pi, flattening that power draw.

We’re looking forward to getting our hands on this new and improved PoE Hat, and using it in many project to come.

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