Power over Ethernet HAT
les pounder has the power! And now it comes over Ethernet. Can he ditch the micro USB power supply and finally live with just an Ethernet cable?
Les Pounder has the power! And now it comes over Ethernet. Can he ditch the micro USB power supply and finally live with just an Ethernet cable?
Power over Ethernet (PoE) is nothing new. In fact, it’s been with us for many years. But it has eluded the Raspberry Pi… until now. The PoE HAT is an official Raspberry Pi product and is designed to work only with the new Pi 3 B+, via the PoE pins present on that board. To use the PoE HAT, we need to attach the HAT to all 40 pins of the GPIO, and the four pins of the PoE header. We then supply power using the Ethernet connection, rather than using a micro USB power source. This necessitates using either a PoEcompliant switch/router or a PoE injector that will send 48V of power and data over a single Ethernet cable. Be careful using this with your network, because non-PoE compliant devices may be damaged.
Supplying power via this board requires us to have the latest version of Raspbian installed. But once all that’s done, the board just works – and all we need is one cable!
The PoE HAT is dominated by a 25mm 5V fan that’s designed to cool the CPU on the Raspberry Pi. When the fan is at full speed it does produce an audible whine. This is slightly annoying, but can be tolerated. However, there’s a problem with the PoE HAT, and it’s quite a big one.
It’s a cover-up
Because the board covers all of the GPIO, we can’t attach any add-on boards or make direct connections to the GPIO. There’s a workaround that involves a 40-pin header extension and a four-pin extension for the PoE header, but the necessary components aren’t included. This is a shame, because makers will be using the PoE HAT for powering outdoor projects, typically involving sensors. Adding HATs on top of the PoE HAT is possible, but you’ll also need brass spacers to stop the PoE HAT from making contact with the add-on board. Worse still, the clearance between the fan and the add-on board is mere millimetres and generally just heats up the board.
Fan control is automatic, triggered by a temperature threshold set using a config file that communicate with the onboard Atmel chip using I2C. At 45 degrees C the fan will kick in and cool the system. The higher the temperature, the faster the fan will spin.
There’s a quirk with the PoE HAT, specifically the USB ports. Connecting keyboards and mice is fine, but any device that draws more than 40mA may trigger the system to shut down power to the USB ports until the current draw is removed. This issue has been addressed by Eben Upton, who said that the cause was due to “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.” Furthermore, testing of heavy current loads was undertaken with one brand of current-limiting switch, and that “dumb luck” saw the lower loads being tested with this troublesome component. So there’s a chance that we can’t plug in external USB devices requiring more than 40mA. That’s unless we wish to start soldering components to the PoE HAT.
Is the PoE HAT any good? It is, if you need a single cable solution for your project. The problem is that the PoE HAT restricts access to the GPIO, unless we add the extra headers. The GPIO is what makes the Raspberry Pi such a versatile device. Without this functionality, it’s really just a low-power Linux computer.