With several users reporting problems with the recently released Raspberry Pi Power-over-Ethernet HAT, the Raspberry Pi Foundation is offering to refund customers that have purchased the faulty board.

In the days since its release in late August, users had been reporting limitations in the power supplied by the Raspberry Pi PoE HAT. The HAT is an add-on to the popular Raspberry Pi 3 Model B+ SBC. Over the intervening weeks, engineers at the Raspberry Raspberry Pi Foundation have been wrestling to figure out the nature of the problem. An interesting play-by-play can be followed on the Raspberry Pi forums.

Raspberry Pi PoE HAT alone (left) and fitted on Raspberry Pi 3 Model B+
(click images to enlarge)

In a story appearing on Tuesday in The Register, the problem was explained by Eben Upton, foundation co-founder and CEO of Raspberry Pi Trading. According to Upton, “it’s an interaction between the fairly low-frequency switching regulator on the HAT, and one of the two brands of USB current limiting switch that we use on the main board.” Upton explained that because the regulator operates at a fairly low frequency, each time it switches it moves quite a large chunk of energy into the three USB reservoir caps via the current limiting switch: this large instantaneous current is fooling the switch into thinking that a genuine over-current event is occurring.

“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,” said Upton.

A blog post on the Raspberry Pi Blog is expected to provide all the details, but such a post hasn’t not appeared at the time we posted this story.

The key points of the Raspberry Pi PoE HAT problems, and the steps being taken were spelled out in the letter Upton sent to The Register, and also posted on Raspberry Pi forums.

They are as follows:

• “A significant proportion of HAT/Pi pairs are limited to delivering less than 200mA of downstream current to USB. This is generally enough for mice and keyboards, but not for e.g. hard drives.”
• “We will fix this issue in a subsequent spin of the PoE HAT.”
• “In the meantime we’ll be adding a note where the HAT is sold, documenting this limitation.”
• “We will provide a couple of hand-mod options for adventurous users. These are likely to be:
  • “Removing reservoir caps from the main board (an easy, clean mod if you can use a soldering iron, but limits USB hotpluggability).”
  • “Inserting a small amount of series impedance in the current path from the HAT (this one will be a bit fiddly to implement).”
• “Users who have bought a HAT and are inconvenienced by this issue should return it for a refund.”

Upton summed up the lessons learned saying “The moral of the story: do more testing, particularly where we have multiple vendors for key bits of silicon.”

The Raspberry Pi 3 Model B+ emerged as the winner of the annual LinuxGizmos reader survey of 116 Linux/Android hacker boards.

 
Further information

The original blog post announcing the Raspberry Pi PoE HAT may be found here. The product page with links to resellers is here.