RoadTest: Raspberry Pi4B (4GB) plus POE Hat
Author: stevesmythe
Creation date:
Evaluation Type: Development Boards & Tools
Did you receive all parts the manufacturer stated would be included in the package?: True
What other parts do you consider comparable to this product?: tp-link TL-POE10R splitter (2A @ 5v) Adafruit PoE Splitter with MicroUSB Plug (2.4A @5V) Adafruit 5V 1.8A Isolated Output PoE Module ID: 3848 (1.8A @ 5V)
What were the biggest problems encountered?: "Out of the box", the fan comes on too often and is noisy. I had to delve around for the fix for this. As supplied, you can't stack another HAT on top of the PoE HAT. There is not really enough clearance to to use a heatsink on the Raspberry Pi with the PoE hAT on top
Detailed Review:
The PoE HAT is an official Raspberry Pi “HAT” that enables a Raspberry Pi 3B+ or Pi 4 (and associated peripherals) to be powered by a power-over-Ethernet (“PoE”) connection. It costs around £18 in the UK.
My aims in this review are to explain what PoE is, how you can use it, and compare the Raspberry Pi PoE HAT with other PoE options for your Raspberry Pi.
Thanks to element14 for providing the opportunity to test it, together with a 4Gb Raspberry Pi 4.
PoE is a networking feature defined by the IEEE 802.3af and 802.3at standards. PoE lets Ethernet cables supply power to network devices over the existing data connection.
The main benefits of PoE are:
Some common applications for PoE are:
The major difference between 802.3af (PoE) and 802.3at (PoE+) is that PoE+ power-sourcing equipment can provide almost twice as much power over a single Ethernet cable.
In my house have a couple of D-Link DES-1008PA 8-Port Fast Ethernet PoE desktop switches (one upstairs and one downstairs), each of which features four 10/100BASE-TX ports supporting the IEEE 802.3af PoE protocol (plus four non-PoE ports). Note that my PoE switches do not offer Gigabit speed, so I was unable to test this throughput with the PoE HAT, although I understand that it is within the specification.
The devices that I have that are powered from my PoE switch are:
In the case of the CCTV camera and the Harting MICA, they were supplied with IEEE 802.3af compliant PoE connections, so it was just a matter of plugging them into the relevant network port. However, the Raspberry Pi Zeros are not equipped with an Ethernet connection of any sort, but I sourced some IEEE 802.3af compliant port-splitters so that I could power the Raspberry Pis. By combining these with a USB-to-Ethernet hub, I can also stream the music over a wired connection - which is more stable than WiFi. I also have a TP-Link 802.3af compliant PoE splitter running a microphone/amplifier/speaker by my front door PoE CCTV camera so I can monitor who is there.
The HAT comes in an antistatic bag in a simple cardboard box with a small bag containing the four standoffs and eight screws needed to secure the HAT to the Raspberry Pi. The Raspberry Pi 4 itself just comes in a cardboard box with a “Safety and User Guide”. The model supplied for the RoadTest was the top-of-the-range one with 4GB RAM.
The Raspberry Pi PoE HAT is an IEEE 802.3af-compliant device and as such it can provide up to a maximum of 15.4 watts per port (compared with 30W for an 802.3at-compliant device). However, some power is always lost over the length of the cable, and more power is lost over longer cable runs. The minimum guaranteed power available at the Raspberry Pi is 12.95 watts.
The PoE HAT works by converting the incoming 48V from the Ethernet lines to 5V using a transformer with an MPS MP8007 in a flyback configuration. The transformer is the large rectangular component set into the middle of the HAT (next to the fan). This means you get a power supply with a galvanic isolation from the power source. On the board is an Atmel ATtiny814 for the fan PWM control and it also emulates a I²C EEPROM to supply the HAT information to the Pi.
The primary side of the transformer is switched across the 48V, and energy is stored in the transformer in the form of a magnetic field. The primary is then disconnected and the magnetic field collapses. This changing magnetic field induces a voltage (scaled based on the transformer turns ratio) in the secondary, which is rectified by a Schottky diode and output capacitance. This output capacitance is formed from the output capacitors on the PoE HAT itself, the capacitors on the Raspberry Pi 5V rail, and, when the switch is on, the 5V USB reservoir capacitors.
As you can see from the photos above, you lose the ability to stack another HAT on top of the PoE HAT, although this can be remedied with the right choice of stacking pin headers. As well as stacking headers for the 40-pin GPIO header, you will also need to add a standard (i.e. not extended) 4-pin header for the PoE connector.
Although I didn’t experience this, some users have reported that the four-pin PoE header on the Raspberry Pi can become detached from the board when you remove the HAT from the board. The online documentation warns against that.
The kit itself comes with no printed documentation but refers the purchaser to the online documentation at http://raspberrypi.org/products/raspberry-pi-4-model-b and http://raspberrypi.org/products/poe-hat . There is a wealth of information about the Raspberry Pi 4 available but not so much about the PoE HAT; in particular how to control the operation of the onboard fan.
The main pre-requisite for using this HAT is the availability of a PoE network connection that adheres to IEEE 802.3af compliant PoE connections. I should also point out that the PoE HAT only works with the later Raspberry Pi 3B+ or the Raspberry Pi 4. This is because other models do not have the four-pin PoE header needed to access the Ethernet connections.
For this RoadTest, I just needed to add an SD card to the Raspberry Pi 4 with my chosen operating system. For simplicity I chose the latest version of Raspbian.
As supplied for the RoadTest, there is no case for the Pi/PoE HAT combination, but I tried a couple of options. The first, and obvious, choice was the official Raspberry Pi 4 case. However I found that this case relies on using the four mounting holes on the bottom of the Pi to clip onto four mounting lugs moulded into the bottom part of the case.
When the PoE HAT is attached as intended, those four mounting holes have screws in, which stops the Pi from fitting onto the lugs. I took out those four screws, leaving the PoE HAT just sitting on the GPIO and PoE connectors.
It works OK, but obviously is not as secure. The other problem is that the official Raspberry Pi 4 case has no ventilation and we all know that the Pi 4 became somewhat notorious when it was first released for getting too hot and “throttling” the CPU. Even with the fan on the PoE HAT running, there is no way for the hot air to escape or cooler air to enter.
As an alternative, I bought a case made specifically for the Pi 4 / PoE HAT combination. This has air flow holes above and below the fan so should be a better choice for the PoE HAT.
The PoE HAT thoughtfully provides a slot above the Raspberry Pi’s camera port, so you can pass the cable through. This slot also helps to isolate the high-voltage side of the board from the low-voltage side. Unfortunately, neither of the cases I tried has a slot for the camera cable but the PoE case “sort of” works if you pass the cable out through the SD card slot at a slight angle.
Testing the Raspberry Pi PoE HAT is relatively straightforward - it only has two functions really:
When the PoE HAT was first released, some users reported that high-powered devices attached to the Raspberry Pi’s USB ports would “cut out”. This was caused by lack of smoothing on the flyback circuit causing the “over current” protection in the USB ports to trip. The board was swiftly withdrawn and a new board with increased smoothing to the 5V output was released.
It was easy enough to test that the PoE HAT provides a good 5V to the Raspberry Pi by sticking a voltmeter across the relevant pins on the Pi’s GPIO header. I also tested whether, powered by the latest version of the PoE HAT, the Raspberry Pi 4 could provide enough power to the Raspberry Pi to deliver the rated current of 1.2A from its USB output ports. Unfortunately, I don’t have a fancy load tester, just a fixed resistance load and an in-line USB power tester but I was able to confirm that it could deliver a nice, stable 1.0A from either of the two USB2 ports.
Due to the proximity of the Ethernet port to the two USB3 ports on the Pi 4, I couldn’t get my tester in without adding an extension cable. This addition caused the output current to drop by around 0.07A which is presumably caused by the extra resistance of the extension cable.
Just to check, I measured the output current through the two USB2 ports when using the extension cable and it dropped by the same amount. This tells me that there is no problem getting at least 1A delivered through any of the four USB ports. In version 1 of the PoE HAT, the overcurrent condition on the USB ports was triggered at around 200mA so it appears that the latest version of the PoE board does not suffer from that problem.
As I don’t have a thermal camera, I used the vcgencmd utility to measure the temperature of the Raspberry Pi system on chip (“SoC”) and compared this with the results obtained using a thermocouple probe attached to my multi-meter. I quickly abandoned the thermocouple method when I saw that it gave readings around 10 deg C lower than the vcgencmd method – which I presume is more accurate as the thermocouple method relies on perfect thermal coupling with the SoC.
Using the PoE HAT in the official Raspberry Pi case, the Pi just ran hotter and hotter until it reached around 60 deg C as the fan had little or no effect on the cooling. If your application is not very CPU-intensive, this might not be a problem. The temperature as measured by the vcgencmd utility hovered around 62 deg C with minimal CPU activity. I believe that the Raspberry Pi starts to “throttle” at or above the 80 deg C point, so this could be an issue if your application is CPU-intensive. Testing with the vcgencmd utility showed that running without a case (or with the PoE case) keeps things running around 10 deg C cooler.
There are several alternatives to using the official Raspberry Pi PoE HAT, three of which I already have. I did test each of them to confirm that they can power a raspberry Pi 4 with an HDMI screen, USB keyboard/mouse and a load of at least 1A on any of the four USB ports. I do not intend to make a detailed comparison of them, but it is worth pointing out the main differences:
1. Adafruit PoE Splitter with Micro USB Plug. Actually, I have a Chinese clone of this. It claims to provide an isolated 12W at 5V (2.4A). It is claimed to be IEEE 802.3af compliant and supports 10/100M data transmission (i.e. not for Gigabit ethernet) This works well with any Raspberry Pi that has an Ethernet port. To use it with a Raspberry Pi 4, you will need a micro USB B to USB C adapter. It seemed to deliver just under 1A (0.95A) from the USB 2 ports. Not sure why. The Adafruit splitter costs around £11 and the Chinese clones around £6 in the UK.
To use it with a Raspberry Pi Zero you will need to add a micro USB hub with an Ethernet connector, although I have experienced some issues with it “shutting down” (particularly when used with an HDMI monitor and USB mouse and keyboard).
2. tp-link PoE Splitter (TL-POE10R). This also claims to be IEEE 802.3af compliant but supports Gigabit Ethernet as well as 10/100 (auto-negotiated). It provides a choice of output voltages (12VDC 1A, 9VDC 1A, 5VDC 2A), so is a good choice if you want to power a range of equipment and/or need Gigabit speeds.
It is supplied with a 2.1mm/5.5mm barrel jack, so a barrel jack to USB adapter is needed to use it with a Raspberry Pi (and also the above USB B to C adapter). This splitter costs about £12 in the UK.
This PoE splitter has worked flawlessly for me in a range of harsh conditions (inside an IP55 box).
3. tp-link Power over Ethernet Adapter Kit (TL-POE200). This is non- IEEE 802.3af compliant but is useful if you do not have an available PoE router. It consists of a PoE injector (with mains transformer to provide the 48V) and a PoE splitter, which also provides a choice of output voltages (12VDC 1A, 9VDC 1A, 5VDC 2A). This kit costs about £18 in the UK.
4. It is also worth mentioning the Seeed Studio Raspberry Pi Case with Raspberry PoE Expansion Board for Raspberry Pi Compute Module. This is a fan-cooled case with PoE board that provides dual CSI camera ports, and is quite a neat solution if you want to use the Raspberry Pi Compute module instead of the Pi 3B+ or Pi 4. This device (remembering that it includes a case for the Raspberry Pi) costs around £50 in the UK.
Adafruit offer at least two other PoE options for the Raspberry Pi
5. https://www.adafruit.com/product/435 (essentially a case-less alternative to the non-compliant tp-link injector/splitter pair shown above). It costs around £6 in the UK.
6. https://www.adafruit.com/product/3848 (Probably the smallest form-factor of any PoE splitter, but only rated to 1.8A at 5V and only works with Pis that have the PoE pins). It costs around £15 in the UK.
I have not had the opportunity to try these last three but they would be worth considering, depending on your application.
I was interested to test the Raspberry Pi PoE HAT device in a couple of use cases that I thought might be appropriate. These are:
I have a Naturebytes Wildlife Cam Case to monitor wildlife in the garden. This currently is WiFi and battery-powered but, even though it only takes a photo when triggered, the battery only lasts a few days as it gets lots of triggers. It was easy for me to run an IP55 Ethernet cable out into the garden as I already have a PoE CCTV camera on my garage roof. What wasn’t so easy was fitting the Raspberry Pi into the case with the PoE HAT on top, but I made it work in the end by using some longer standoffs.
I took advantage of not being constrained by battery power to record videos and timelapse photos using RPi Cam Web Interface.
| {gallery} Fatball birds |
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I was fortunate that some blue tits decided to use my nest box while I was testing the camera, and I got lots of lovely pictures and videos, a small selection of which is posted below.
| {gallery} Nest box |
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The Raspberry Pi 4 and PoE HAT combination works well for this use case, and the extra power of the Pi 4 compared with an older, slower Pi, is a real benefit when processing large video files. As with the Official Raspberry Pi Case, I don’t think that the fan on the PoE HAT helped at all, when it was inside the IP55 case.
Pi-Hole is an application that you can run on a Raspberry Pi that effectively stops popups and other unwanted advertising from slowing down your browser and generally being annoying. It provides a nice dashboard, where you can control "whitelists" and "blacklists" and monitor what is being intercepted.
I have one of these running already, but I thought it would be simpler just to run it off PoE and stick it next to the network switch - rather than running a longer network cable and waste a power adapter/socket. This turned out to be a very neat solution. The shelf it sits on gets a bit of sun in the morning, and this causes the fan on the PoE to switch on. It is clearly audible across the room and it would be quite annoying if I had left the thermostat set to the factory setting of 50 deg C. I increased it to 62 deg C and it is less intrusive.
To change the settings, you have to edit the boot configuration file.
sudo nano /boot/config.txt
Adding the following lines
dtparam=poe_fan_temp0=65000,poe_fan_temp0_hyst=5000
dtparam=poe_fan_temp1=67000,poe_fan_temp1_hyst=2000
Changes the “on” temperature to 65 deg C with a hysteresis of 5 deg, and it speeds up at 67C, with a hysteresis of 2 deg.
Although the PoE HAT works well for this use case, Pi-Hole doesn’t really need a powerful Raspberry Pi, and the combination of a Pi Zero and an Adafruit PoE Splitter with Micro USB Plug (or cheap clone) works almost as well, is about a quarter of the price and is noise-free!
As I mentioned at the outset, the documentation for the PoE HAT is somewhat sparse, but the Raspberry Pi community is huge and, armed with Google, it is possible to find out everything you need to know. The device itself works well and I think is reasonably priced.
The deciding factor as to whether I would buy one would be driven by the application – if you already have an IEEE 802.3af compliant switch, using the PoE HAT results in a compact combination (and a neat one, if you buy a suitable case to allow the fan to do its job). Using a different solution will involve having a number of components connected to each other which is messier, but might end up being cheaper.
If you don’t have an IEEE 802.3af compliant switch, you could buy a PoE injector and use the Raspberry Pi PoE HAT, but it might be cheaper and easier just to buy a non-compliant kit like the tp-link TL-POE200.
In summary, the Raspberry Pi PoE HAT fills a specific niche very nicely, but you should consider the other alternatives before taking the plunge.
