The official Power over Ethernet (PoE) HAT is an add-on board that sits on top of the Raspberry Pi 3 Model B+ and allows the computer to be powered using an Ethernet connection. The HAT uses the 802.3af PoE standard, which allows 15W to be delivered to be the board.
Yesterday Eben Upton, foundation co-founder and CEO of Raspberry Pi Trading, responded to claims that some PoE HATs were failing to deliver sufficient power to the Pi 3 Model B+, in an open letter on the Raspberry Pi forums.
SEE: Hardware spotlight: The Raspberry Pi (Tech Pro Research)
He said the problem stemmed from an interaction between a regulator on the HAT and a USB current-limiting switch on the main Raspberry Pi 3 Model B+ board.
The upshot of this problem, he said, was that a significant proportion of PoE HATs were delivering less than 200mA of downstream current to the USB, enough for keyboard and mouse, but not enough for attached hard drives.
“We missed it in product testing because, dumb luck, our heavy-load testing was done on boards with the other brand of switch, and most of our field testers were only using the board to power mice and keyboards, which works fine on all the HAT/Pi pairs we’ve tested,” he said in the letter.
The foundation is taking a number of steps in the wake of the discovery, including adding a note to store pages for the HAT describing this limitation and offering a refund to those who return a HAT affected by this issue.
Upton said the issue would be resolved in “a subsequent spin of the PoE HAT”. He added that the Foundation will publish details on how to mod the hardware of affected PoE HATs to workaround the issue.
He said this mod would likely involve: “Removing reservoir caps from the main board (an easy, clean mod if you can use a soldering iron, but limits USB hotpluggability)” or “inserting a small amount of series impedance in the current path from the HAT (this one will be a bit fiddly to implement)”.
The PoE HAT was launched last month and sells for £18, and the problems with the PoE HAT were first reported by The Register.