Raspberry Pi PoE HAT

Raspberry Pi PoE HAT


Raspberry Pi 3 B+

Back in March 2018, a new version of the Raspberry Pi was released; the Raspberry Pi 3 B+. One of the changes on this new board was the addition of 4 extra pins adjacent to the existing GPIO header. These 4 additional pins enabled support for Power over Ethernet (PoE) by means of a separate PoE HAT board. Unfortunately, however, although announced at the time of release, this PoE HAT has only recently become available for purchase.

4 pin PoE header on Raspberry Pi

Raspberry Pi PoE HAT

The Raspberry Pi has had alternative PoE solutions previously available to it. However, a defining feature of the new PoE HAT is that, unlike these other PoE solutions, this HAT requires no RJ45 connector. This detail means that the board itself is more compact, thus leaving room for other features. One such feature is a cooling fan which, according to the product brief, is controlled via the I2C interface.

For temperature control of the fan on the PoE HAT to be enabled, ensure that you have fully updated the software on your Raspberry Pi. To do this, execute the following terminal commands:-

sudo apt update
sudo apt upgrade
sudo apt dist-upgrade

Note – In order to enable temperature control of the fan it may also be necessary to update to the latest firmware. To do this, execute the following terminal command:-

sudo rpi-update

Raspberry Pi PoE HAT

Underside of PoE HAT

Setting Up The PoE HAT

The PoE HAT comes with 4 9mm standoffs and 8 machine screws, and set up couldn’t be simpler. The 4 standoffs require to be screwed to the underside of the PoE HAT. Once this is done, the HAT can be pushed into place on the Raspberry Pi in the usual way. Correct alignment of the 40 pin GPIO header will automatically line up the 4 PoE pins. Following this, the remaining 4 machine screws can be used to secure the PoE HAT to the Raspberry Pi.

Following set up, the Raspberry Pi needs to be connected in the usual way, apart from the power supply.

Powering the Pi

In order to power the Raspberry Pi by means of the PoE HAT, a PoE enabled ethernet cable must be connected to the RJ45 socket on the Pi. The power to this ethernet cable needs to be supplied by either a PoE enabled router or switch, or a PoE injector.

We did not have access to a PoE enabled switch or router, so instead opted for a Power over Ethernet injector.

PoE Injector

Raspberry Pi powered through PoE HAT

HAT Stacking

From the day that the Raspberry Pi 3 B+ was first released, concerns were voiced regarding compatibility with HATs. Indeed, certain HATs have been found to be incompatible with the Pi 3 B+ because of the positioning of the 4 PoE pins. The same is also true for some enclosures.

To further compound this issue, the PoE HAT itself does not extend the GPIO header. This, in turn, complicates the process of stacking HATs on top of the PoE HAT. Some reviews of the PoE HAT have suggested that merely adding an extended 40 pin GPIO riser will allow stacking of HATs. This, however, is not the case, since doing so will prevent the 4 pin PoE socket on the PoE HAT from contacting with the PoE pins on the Pi (see image below).

PoE HAT on extended 40 pin GPIO riser

To overcome this issue, we found that a standard (not extended) 4 pin header was also required.

Note – 2 x 2 pin headers will work equally well as an alternative to a 4 pin header.

Placing this header onto the PoE header effectively extended these pins to make contact with the PoE HAT. The image below shows examples of the 2 headers required.

GPIO and PoE Headers

Here we can see the 2 headers in place on the Raspberry Pi 3 B+.

Headers on Raspberry Pi

The PoE HAT is placed onto the extended 40 pin GPIO riser and 4 pin PoE header.

PoE HAT on headers

The extended PoE pins are now able to contact the PoE socket on the PoE HAT.

Note – Ideally, to secure the PoE HAT with the extended GPIO header, the supplied 9mm standoffs should be substituted with 15mm standoffs.

Extended PoE pins

Finally, we can place a HAT on top of the PoE HAT.

HAT on PoE HAT

Compatibility

Since PoE power is supplied to the Raspberry Pi by means of the 4 pin PoE header, the PoE HAT is only fully compatible with the Raspberry Pi 3 B+ board. Having said this, however, the fan (including the temperature control feature) on the PoE HAT will still work on older models of Raspberry Pi.

This means that the PoE HAT could, for example, be used simply for cooling on a Raspberry Pi 3 model B. In order to do so, the Raspberry Pi would be powered in the normal way, with the PoE functionality of the PoE HAT effectively becoming redundant.