martinrowan
Posts: 76
Joined: Wed Aug 26, 2015 11:00 am
Location: Berkshire, UK
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New POE+ HAT - USB Current limits

Sat May 29, 2021 3:14 pm

Hi,
The new POE+ HAT claims to support an output current of up to 4A.
The original PoE HAT implements the 802.3af standard, and can deliver a guaranteed minimum of 13W to the Raspberry Pi. This is enough to power a Raspberry Pi 4 at maximum load, but not quite enough to power the hungriest USB peripherals at the same time.

The PoE+ HAT implements the 802.3at standard. When used with a compatible switch or injector this means it can deliver up to 25W, as you can see from this comparison table.
- From launch blog post.

However, the Raspberry Pi 4 still has a current limit of 1.2A shared across all 4 ports, so a maximum of 6W. So how are am I meant to be able to use heaver USB loads and utilise this potential of up to 25W?

I've tested with the latest POE+ HAT and my original POE HAT, and both hit current limits when the combined load on the USB port approach 1.2A.

Thanks Martin

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rpdom
Posts: 18878
Joined: Sun May 06, 2012 5:17 am
Location: Chelmsford, Essex, UK

Re: New POE+ HAT - USB Current limits

Sat May 29, 2021 3:58 pm

The original HAT provided 13W. That is 2.6A. That was fine for the Pi 3B+, but the 4B is rated as up to 3A including that 1.2A. So give it only 2.6A and there isn't enough to provide the full 1.2A for USB on a fully loaded 4B.

The new HAT will provide more than 3A, so it can ensure the 4B can supply the full 1.2A to the USB. (It's probably good for a 5B too, but that is just a guess).
Unreadable squiggle

GlynH
Posts: 40
Joined: Thu Oct 24, 2019 3:43 pm

Re: New POE+ HAT - USB Current limits

Sun Jun 06, 2021 8:39 pm

I have a question about the new 4A PoE HAT vs the older v2 HAT

How many watts input might be required from the PoE switch into the newest HAT to comfortably power a Pi 4?

The reason for me asking is because I am looking at a Cluster Case that can hold up to 8 Pi's and there is room for a compact, 8-port Gigabit PoE Switch with SFP to fit into the case but a switch of this spec severely limits the choice to something like the TP-Link TL-SG2210P.

This is where it gets interesting. The spec states 15.4w per port which sounds more than adequate but then it also states a total PoE Power Budget of 58w which means only 3 maybe 4 ports at a push out of 8 would be capable of outputting at full 15.4w power with .

I currently run 3 Pi 4's with the PoE HAT v2 and looking at my main switch I see the following quota;

Pi1 4.1w / 78mA
Pi2 4.2w / 80mA
Pi3 7.4w / 140mA

The demand for Pi3 is higher due to this being the only Pi with a USB dongle plugged in but the next Pi will have a Meteo-Pi HAT which I am hoping to use for powering a Davis VP2 console but I'm not certain of the current requirement for that.

I know the new PoE HAT is capable of utilising a maximum of 25.5w vs the standard 15.4w of the earlier device but that doesn't mean it will so should I expect a similar power consumption when using the new PoE HAT vs the v2?

Thanks & kind regards,
-=Glyn=-

martinrowan
Posts: 76
Joined: Wed Aug 26, 2015 11:00 am
Location: Berkshire, UK
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Re: New POE+ HAT - USB Current limits

Tue Jun 15, 2021 5:07 pm

Hi Glyn,
In my testing, an idle Pi 4 with the original PoE HAT used 3.2W the new PoE+ HAT in the same configuration used 5.2W. These were measurements reported by my UniFi PoE Switch.

You might be interested in my blog post: Raspberry Pi PoE+ HAT Review – Part 2: Problems with this extra power consumption also increases the heat. Depending on the configuration of your case for your 8 Pi's this may or may not be an issue.
Thanks Martin

wickpi
Posts: 1
Joined: Fri Jun 18, 2021 5:15 am

Re: New POE+ HAT - USB Current limits

Fri Jun 18, 2021 5:28 am

I'll add a reply here to mention that while I have the same issue, I found that using a 5v redundant PSU (rpi official), when already plugged into PoE (even at low voltage, 12V), I did not experience the same coil whine.

Without _some_ voltage over PoE, this hat sounds horrible - to the point I'd rather not use it & just buy a simple fan hat, however with PoE + redundant, it seems OK (not sure about safety of this however, due to apparent lack of zener protection)

GlynH
Posts: 40
Joined: Thu Oct 24, 2019 3:43 pm

Re: New POE+ HAT - USB Current limits

Sun Jun 20, 2021 4:17 pm

martinrowan wrote:
Tue Jun 15, 2021 5:07 pm
Hi Glyn,
In my testing, an idle Pi 4 with the original PoE HAT used 3.2W the new PoE+ HAT in the same configuration used 5.2W. These were measurements reported by my UniFi PoE Switch.

You might be interested in my blog post: Raspberry Pi PoE+ HAT Review – Part 2: Problems with this extra power consumption also increases the heat. Depending on the configuration of your case for your 8 Pi's this may or may not be an issue.
Thanks Martin
Hi Martin,

Thanks for the reply and link...you put a lot of work into your reviews and I know I am not alone in saying a big thank you for that!

It would seem your findings are similar to mine.

My DrayTek switches report;

Pi1: PoE v2 running Pi-hole - 4w/75mA
Pi2: PoE v2 running Virtual Radar Feeder with USB Dongle - 7.6w/143mA
Pi3: PoE v2 running Home Assistant - 4.1w/78mA
Pi4: PoE+ running nothing but a virgin Buster OS at the moment - 6.2w/117mA
Pi5: Still in the box! As are the other four PoE+ HATS while I wonder whether to keep them or not! :lol: Seems like I might have to as the v2 seem to be out of stock everywhere which seems to pour water on one of the Pi Foundations answers to Jeff Geerling where they casually flip him off with the comment; "If lower power consumption at idle is important, you should stick with the original PoE HAT."

Easier said than done if they have been discontinued and can't be obtained from anywhere! :? :roll: :evil:

A little over a week ago I posted on the Pi Foundation website Announcing the RPi PoE+ HAT page https://www.raspberrypi.org/blog/announ ... i-poe-hat/ my own thoughts & findings https://www.raspberrypi.org/blog/announ ... nt-1558002 reproduced below;

------snip------
Hi there RPi Foundation,
Great news on the uprated PoE+ HAT…thank you very much.
I’ve just bought 5 of them to replace the older v2 where the fans are noisy and for the extra power.
I have some issues that I would like to share with you if I may. There could be some user error here but hopefully if there is you can point that out! :)

1. The fan position on the new PoE+ HAT has changed so now the ventilation slots in the new Uputronics GPS HAT for example obscure the fan. May be an issue with other HATS?

2. The extended tall 4-pin header now cannot be used to connect/stack as the 4-pin header on the PoE+ HAT has no pass-through unlike the older v2 HAT and when using the extended/tall 40-pin header it lifts the PoE+ HAT above the 4-pin header on the Pi? I suppose another 4-pin header of the right size might resolve that but it is annoying.

3. There is no camera ribbon cable pass-thru’ slot on the PoE+ HAT unlike the earlier version?

4. When you fit the supplied screws for the spacers the Pi4 no longer fits into the official RPi case as they impact the locating posts! Possibly the same on other cases?

5. Also with the official RPi case you can no longer clip the top lid on as there is now no gap at the back of the PoE+ HAT for the plastic lugs to locate unlike the notch on the earlier boards!

Apart from that all good!

Thanks for listening!
------snip------

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