Power over Ethernet (PoE) basics and beyond

https://www.edn.com/poe-basics-and-beyond-what-every-engineer-should-know/

by voxadam • 9 months ago

262 205 9 months ago

cyberax2• 9 months ago

What about PoE for 10G Ethernet? I see that there are some vendors (e.g. Ubiquity) that are offering devices with it, but I don't see it in the standards?

VanTheBrand3• 9 months ago

As of 802.3bt (PoE++) the standard includes support for “all standardized copper link speeds of up to 10GBASE-T.” The previous standard 802.3at (PoE+) added gigabit support.

So any 10GbE (and 2.5GbE) PoE/PoE+ devices out there are technically not to spec (lots of these on Ali Express) but I believe the the Ubiquiti 10GbE stuff is all at least PoE++. [1]

(They do have their own non spec labeled PoE+++ products though, which are really just “802.3bt Type 4” but they added another plus because that probably sounded better.) [2]

[1] https://store.ui.com/us/en/products/us-xg-6poe, https://store.ui.com/us/en/category/accessories-poe-power/co...

[2] https://store.ui.com/us/en/category/accessories-poe-power/co... , https://help.ui.com/hc/en-us/articles/115000263008-PoE-Avail...

zamadatix2• 9 months ago

I think every major vendor has had 10G PoE switches: Arista, Aruba/HPE/Juniper, Cisco, Extreme, and Fortinet for sure. The problem is there is little use case for 10G + PoE in the enterprise and even less for consumers. Ubiquity likes to tout it for the 10G APs... but, realistically, most are worried about airtime with APs, not 10G wired throughput from a single one when they have a thousand.

As a result, it tends to be relegated to the "high end switch which has every feature those one-off customers demand but costs an arm and a leg as a result" model/family. E.g. the only ones I ever sold were to a hospital that wanted to have select switches have 10G for radiology workstations but also wanted to still be able to plug 1G APs in without having to think about the port types. Radiology was covering the cost, so they didn't care it was a waste of money.

VanTheBrand3• 9 months ago

I find it useful in Broadcast Video Production (that’s where I end up using it most) and yeah with Wifi7 supporting > gigabit speeds I’ve seen some Wireless Access Points supporting it (though 2.5GbE Poe++ is more common there and practically speaking enough)

zer00eyz3• 9 months ago

My question is this: what would you plug into POE that would need 10gbe?

wmf3• 9 months ago

A multi-radio WiFi 8 AP?

mrheosuper• 9 months ago

an all nand-flash NAS ?

fc417fc802• 9 months ago

What if you need a small switch somewhere due to the runs otherwise being too long?

MBCook• 9 months ago

I had no idea it could supply 90 watts or so. At that point you could power some pretty powerful terminals.

The new 14” MacBook Pro comes with a 70 watt charger. An M4 Air only gets a 35 watt adapter.

Basically seems like enough power is available to run something pretty powerful.

cyberax2• 9 months ago

I have a WiFi 6 AP that can saturate a 2.5GB link when I test it with two devices. So far, for me the peak speed for an individual device was around 1.6GB

I have not yet tested WiFi 7 APs, but they are supposed to be even faster. The use-case for me is video editing over WiFi (I do have a 10GBe Thunderbolt adapter but hey, I like wireless).

varenc5• 9 months ago

Ubiquiti sells Wi-Fi access points with 10GbE uplink ports that are PoE powered: https://store.ui.com/us/en/category/all-wifi/products/e7

In theory at peak throughput the access point might use close to 10 gigabit. But definitely more than 1G/2.5G.

kkapelon• 9 months ago

Another smaller switch closer to the location of your devices.

toast0• 9 months ago

Should work just as well as PoE for 1G ethernet. All four pairs are active data, but you should be able to add a DC bias between pairs and tap it off the transformer on the other side, just like 1G

brcmthrowaway• 9 months ago

1. Why are IEC/Edison cables so thick if Ethernet can carry the equivalent power?

2. How does PoE compare to Powerline Networking?

otterley• 9 months ago

No Ethernet cable that I’m aware of is capable of carrying anywhere near 15A of current. Even type 4 (90W assuming 57V) is 1.5A.

gwbas1c3• 9 months ago

And don't forget that, with ethernet, every device needs its own run of cable, its own port at the router, ect, ect. Normal power cables support multiple devices.

skinner927• 9 months ago

1. Assuming IEC refers to cables we plug a desktop PSU into mains/wall: IEC can carry up to 1800w vs 100w PoE++

2. Powerline networking is considerably slower and less reliable than CAT5/6. Additionally, building code for running power lines is much more strict than low voltage CAT5/6

dlgeek• 9 months ago

Nerdsniped: You're describing a IEC 60320 C13 cable - they're technically only spec'd for 10A, which means you're looking at ~1200W, not 1800.

(However, UL will list them for the full 15A -> 1800W, and I'm sure plenty carry that. And for that matter, I suppose you can get twice that in Europe on 240v...)

skopje5• 9 months ago

PoE is awesome. My custom home security system is all CCTV PoE with a gstreamer backend running on four-core fanless linux box. Way to go. Complete control. No batteries, no wares spying on me, no personal data getting scraped by big guys. (Cloud connectivity sucks because I have segmented mp4s and jogging through them hurts but I only care for events after they happen, not while they happen.)

skulk• 9 months ago

Got any recommendations on what cameras to get? The market is absolutely flooded with cheap shitty cloud-connected all-in-one cameras making it hard to find good, simple products.

stargrazer• 9 months ago

reolink, also look at the frigate nvr software, they have a list for decent recommendations

lights0123• 9 months ago

Any of those that mention ONVIF or RTSP will do if you put them on a LAN without internet access

Cyberdogs7• 9 months ago

I have built out several Amcrest systems. You have the many options for recording and access, that will allow remote access without going to the cloud.

foobarkey• 9 months ago

The cheapest (~15 USD bullet, 20 USD dome) PoE cameras on AliExpress (focal length is pretty much the most important parameter to look at, depending on the fov you want) hooked up to a Unifi NVR. Skip all the vendor manuals, setup steps, and apps - adopt them directly to Unifi Protect.

I put them on separate vlan where they get no outbound network connectivity.

For cases where you want things like facial detection or license plate detection (automatic doors/gates) get a Unifi AI though and those things cost, but for normal perimeter/room monitoring the cheap ones are very good

mlsu2• 9 months ago

I got a lot of 6 Axis cameras from eBay, it was around $200. I think they took them off a school or something but they were in great shape. They look great and have no spyware etc because it’s an industrial company. I recommend getting some industrial surplus because they tend not to have all the bloatware and have significantly better weatherproofing, casing, etc, even if the optics are the same as the consumer units.

skopje5• 9 months ago

https://www.cctvsecuritypros.com/

Blue-line domes, the $240 ones. Four of them. I'd get more but do not know how to make outside routing look neat. i have one bullet and i don't like it and don't use it, i prefer the wide-angle domes with ir.

throw0101c3• 9 months ago

Depending on your paranoia level, there are lists of National Defense Authorization Act (NDAA: i.e., non-Chinese) camera OEMs:

* https://www.a1securitycameras.com/blog/non-chinese-security-...

Some names: Axis, Avigilon, Bosch, Vivotek, Hanwha Techwin (SK), Acti (TW), Motorola, Mobotix.

craftkiller• 9 months ago

I'd recommend checking out "Project Farm" reviews. They do actual tests and comparisons of products rather than the current trend of reading off marketing copy and shilling sponsored products. I've seen some of their reviews on cameras and the difference in clarity across brands is shocking. Not that important if you just want to know "my package was stolen", but very important if you want to read their license plate.

Here is one such review: https://www.youtube.com/watch?v=HYUY61ZFZAs

shadowpho3• 9 months ago

Poe is great for many things, but it’s not as efficient as direct connection (for both low and high power.)

rjdj377dhabsn• 9 months ago

Why's that?

foofoo12• 9 months ago

That's pretty much stating the obvious. X is great for many things, but it’s not as efficient as Y.

Life is a balance between inefficiency and inconvenience. Throwing that statement in without actual numbers is just derailing the conversation.

yardstick• 9 months ago

Do you upload events to a remote location? How is your storage box secured against theft? That’s my biggest concern for doing local cctv- if you are robbed, they’ll grab anything that looks of value.

skopje5• 9 months ago

yes that is something you need to decide for yourself i'm ok with it. i push my segments up to an s3 bucket but yes if they find the box before the rsync i lose. oh well. there's much more valuable stuff in the house they'll probably go for first. i suspect junkies aren't that smart.

dheera2• 9 months ago

Except when it isn't awesome. There are multiple PoE standards. Passive PoE, active PoE, PoE+, PoE++, PoE+++, 802.11af, 802.11at, 802.foo, blah blah.

If they had just stuck with 12VDC and buildings had 12VDC wall sockets everywhere, everything would have been fantastic.

I also had a PoE HAT for a RPi that smoked it. Never doing PoE again. 48V and 3.3V electronics probably don't belong within 10cm of each other.

varenc5• 9 months ago

I use a PoE "extractor" to power my RPi over PoE and it works great. The extractor does the negotiation and safely gets the normal 48V PoE power, then converts that to 5V outputted on a USB-C cable that powers the RPi. Extractor also has an Ethernet[1] passthrough port that goes into the RPi as well. A bit basic, but seems relatively error proof.

[1] https://x.com/varenc/status/1961587127931867466

buccal• 9 months ago

240V AC and 5V DC manage to live close in a charger without problems. Problems with quality does not depend on voltage. I love the concept of PoE with one exception that it requires constant 1W or similar load to work even if it is not needed for low power device.

skopje5• 9 months ago

I never had to know the difference. I have four cctv cams on ~100ft of cat5 each. didn't have to think about it, just plugged them in and they worked.

ssl-33• 9 months ago

If they had just stuck with 12VDC and buildings had 12VDC wall sockets everywhere, everything would have been fantastic.

Huh? We used to have low-voltage AC and DC powered cameras in the world (and we still do, too).

Those are awful in implementation because buildings, whether or old or new, don't have 12VDC sockets everywhere -- or at all, really.

Nor should they have 12VDC sockets for cameras; they're unnecessary.

I've run my share of siamese coax for low-voltage-fed analog cameras, and also separate power for low-voltage Ethernet-connected cameras, and I'm completely over those concepts.

With proper-fucking IEEE POE, we have standards and it only takes one cable to make it work properly instead of more than one.

If a switch isn't up to the power demands of a particular camera, then: No big deal. I can upgrade or supplement that switch without rewiring even more of the building than was already necessary to get Ethernet going.

(Structured cabling for the win.

Passive POE: Not even once.)

eqvinox10• 9 months ago

There are multiple PoE standards.

No, there aren't, not in the way you imply. There is the IEEE 802 PoE standards, which are all compatible (save for not enough power), and designed to carefully negotiate and especially never break non-PoE devices. And there is bullshit (sorry) like "Passive PoE" that is ironically an active violation of the IEEE specs, can break pretty much anything, and you shouldn't buy so the likes of Ubiquiti and Mikrotik finally get the wallet vote and stop f*cking doing. Unfortunately, the proper PoE PD logic is a few dollars of extra expense.

Yes, there is a slightly higher risk of killing devices due to faults in the PoE supply logic. I have the official PoE HAT for a RPi 4. I have to say it is somewhat poorly designed due to space constraints; the isolation between 48V and 3.3V should be better. I'm not even sure the RPi PoE HAT is spec compliant.

But I don't think you can/should blame this on PoE.

numpad03• 9 months ago

The (negative)48VDC voltage comes from regular telephones. So did the RJ45(8P8C) plug. It's not an invented thing.

throw0101c3• 9 months ago

Except when it isn't awesome. There are multiple PoE standards.

There are three: IEEE 802.11af, at, and bt.

af can deliver up to 12W at the powered device (PD), at delivers up to 25 W, and bt either 51W (Type 3) or 71W (Type 4):

* https://en.wikipedia.org/wiki/Power_over_Ethernet#Standard_i...

Any device you purchase should list the IEEE standard it supports and how much power it may draw.

benoliver9992• 9 months ago

What switch do you use? I have PoE wifi APs throughout the house, but I bought an Aruba switch and it's super noisy tbh. Fine for me because it's in the basement, but I couldn't recommend it

skopje5• 9 months ago

some netgear 8-ch poe switch. i don't recall. it's been on and running for about 8 years with no issues, way up near the ceiling of my garage, covered with dust. its plugged into a wrt1900 router i bridged via wifi to my main router.

ssl-33• 9 months ago

At one of our offices (it is not a large office), I have a 24-port Netgear POE switch running the show. If it has a fan inside (it may! there's cutouts for fans on the sides of the chassis but I have not looked inside), I've never heard it.

It fit the price-performance curve for our needs several years ago when we eventually outgrew the previous Netgear POE switch...that was also apparently fanless, and that I installed in 2007.

IIRC, it is a GS724TP. It's running a dozen cameras and some access points, and almost all of the rest of the ports are filled up with computers and printers and other Ethernet stuff. No issues at all to to report so far.

(A used enterprise switch with serious fans may be cheaper and/or more featureful and/or more reliable, but do we need that kind of noise at home? We sure don't need it at that small office.

I've also installed some fanless Cisco POE switches with big heatsinks (and dual power supplies, each fed from different sources) on some high-dollar projects where ultimate reliability was kind of a big deal, but... sheesh.

If one of these installed Netgear switches dies in one of these low-risk environments, I'll just patch things up for now and get a replacement coming under warranty.)

benhurmarcel• 9 months ago

If one of those cameras is outside, did you consider the case of someone plugging in his laptop on that ethernet cable? He'd be on your local network.

teddyh• 9 months ago

Zero-trust networking.

matt-p• 9 months ago

You would normally put CCTV on its own vlan for this reason.

transpute• 9 months ago

MACsec, https://forum.openwrt.org/t/macsec-802-1ae-with-802-1x-eapol...

tehlike4• 9 months ago

Vlan is the answer.

bityard3• 9 months ago

If someone can roll up to your property undetected and get physical access to the network, then the security cameras aren't even doing their job.

nucleardog2• 9 months ago

I did. Implemented a "simple" solution (simple for anyone who is going to be setting up their own IP camera system and NVR):

Cameras are on their own VLAN. Port isolation is enabled so they can't connect to each other. Only connectivity allowed to/from that VLAN is from the cameras to the router for NTP, and from the NVR to the cameras.

So if you plug in you can... check the current time on my router. Maybe see how many other cameras are on that segment? Likely not going to get very far given you're already caught on camera, an alert's been fired, and pretty soon I'm going to be making a call to the police.

rmunn• 9 months ago

Practical question for HN: How do you all label your PoE cables so that you don't accidentally plug the powered cable into a socket that wasn't expecting 48 volts on those pins and fry something? (I know the power injector is supposed to only deliver power when it's safe, but if all your devices work exactly as they should all the time, then I'd like to buy that bridge in Manhattan you're selling).

Do you buy Ethernet cables of different colors and say "Yellow is reserved for PoE, all yellow cables should be assumed to have power on them"? Or do you slap a "48V" label on both ends of the cables you're going to use for PoE and the label is what warns you that this cable should only go into the PoE receiver, and not into an unpowered device? Or do you just not label your PoE cables any differently, and trust that the injector will never malfunction at the same time that you plug the PoE cable into the wrong device?

leoh2• 9 months ago

What every engineer really should know!

supertrope• 9 months ago

Always buy standards based equipment. 802.3af, 802.3at, 802.3bt. You can label cables and jacks with red lettering (“Passive PoE. will fry your laptop port. Really!”) but it only takes one mistake to let the magic smoke out.

VanTheBrand3• 9 months ago

I completely avoid passive PoE. Not worth the risk. On the standardized active stuff I’ve never had any issues even when I’ve plugged it broken cables to unpowered devices.

Kirby64• 9 months ago

Unless you’re using the “passive” PoE variants (ubiquiti sold these for awhile, for instance) that always has voltage on the pins, there is no risk. Negotiation is mandatory for the actual IEEE variants. Just use those and don’t worry about it.

sodaclean8• 9 months ago

Even with passive PoE you're fine as long as everything is A, B, or any mix thereof.

Apparently, some mag-jacks have the center taps for each pair commoned via 75ohms to ground through a capacitor... so I could be wrong.

doubleg72• 9 months ago

Well the entire point of standards is so things work exactly as they should every time. I haven’t seen any issues with standards based poe

hackmiester3• 9 months ago

All 21,000 ports I administer have 802.3 standard PoE enabled at all times. Incidents of inadvertent powering are at zero. I think this is just a non problem.

sodaclean8• 9 months ago

Because of how ethernet works (differential signaling + signal transformers), PoE is effectively a wire at 48v connected to nothing if the device doesn't support it.

The only issue arises if somebody wires a patch cable completely wrong (neither A nor B), and manages to put one leg of passive PoE's +24v pair matched to one leg of the 0v pair. Which, will promptly smoke the signal transformer... assuming short circuit protection doesn't cut power first. This is why we killed passive PoE.

generuso• 9 months ago

From what I have seen, Ethernet ports always have a small isolation transformer for each twisted pair, between the connector and the PHY. Usually four of such transformers are combined in one small magnetics package. The insulation in the transformer is specified to withstand over a kilovolt of lightning induced voltage -- that's one of the purposes of such galvanic isolation.

The data travels as the differential voltage in each of the twisted pairs, and is transmitted magnetically by the transformer to the secondary winding. The power is applied between different pairs, and in each pair appears as a common mode voltage. This is all stopped by the transformer, and in devices designed to support PoE, the PoE circuits tap the mid-point of the primary windings to access the supplied voltage.

So at a first glance, it seems that if 48 volts is applied between the twisted pairs to a non-PoE device, this voltage would simply be blocked by the transformer. But since there is a widespread concern about this, there must be more to the story -- maybe somebody who actually worked with these circuits can explain why this is more complicated than it seems at first?

Edit: Found an answer. It seems that at least some of the designs of non-PoE Ethernet jacks terminate the common mode signals to a common ground though 75 Ohm resistors. In this case, if the voltage were applied between the twisted pairs, the resistors would dissipate far too much power and would burn out. So there is definitely a concern with the dumb PoE injectors and at least some non-PoE devices. https://electronics.stackexchange.com/questions/459169/how-c...

sodaclean8• 9 months ago

Never noticed the 75ohms before- but it'd be 150ohms for passive PoE. (Through one pair and to another, two 75's)

Theres fixes, but passive PoE was a pretty dirty hack- so negotiation got added.

varenc5• 9 months ago

With modern 802.3 spec compliant PoE: I don't worry about it. At least with all the switches I've used. Never ever had it send power to some device that wasn't expecting it.

This is a bit analogous to USB-C PD power supplies, which can supply 12V/24V, but only do this when devices ask for it. I don't worry that my laptop's USB-C power supply will go rogue and send 24V to my earbuds.

jonathanlydall• 9 months ago

Many years ago I was using 12v passive homemade PoE at my house, manually had injection wires in on switch side of cable and manual break out on far end into barrel connector plugged into an AP.

Once I accidentally plugged the cable into a laptop and the port didn’t work until I powered the laptop off and on again, but no lasting ill effects on laptop at all.

thesuitonym2• 9 months ago

Network engineer here: It's not an issue. If the device isn't expecting PoE, it doesn't negotiate PoE, and no power is transferred.

stego-tech4• 9 months ago

PoE is a godsend that should really be in more consumer devices and households, alongside structured wiring. An AppleTV, Chromecast, or NVIDIA Shield can easily fit within the envelope of PoE+, as can many enterprise-grade switches and WAPs (see UniFi as an example). Converting AC to DC once at the switch is more efficient (in resources and often, but not always, power) than including bulky PSUs for every device, while simplifying the ease of setup for end users (in theory).

Whenever possible, I opt for PoE. It’s a damn shame it’s limited to a niche userbase given its myriad advantages.

dfc2• 9 months ago

In my head enterprise grade switch has 48 ports with some >10g SFPs for uplink. What does enterprise grade mean to you? And what enterprise grade switches are poe powered?

hackmiester3• 9 months ago

Arista 710P for instance. I don’t see what port count has to do with it, it runs the same OS and has the same capabilities as all their other switches. Cisco has a Catalyst 9k like this too.

greycol2• 9 months ago

It's such a wide field that it's hard to pin down. I agree if your thinking about what a business that isn't just a handful of people needs then we'd be looking at the kind of switch you're thinking of if it has standard office workers. But soon as you start talking about businesses that are manipulating central data (which keep in mind would probably include most primary business, design workers or anyone working with media not just software people) you're talking about a wide gamut of devices that you wouldn't really (at least traditionally) call consumer grade.

Mikrotik website has a good selection of them and if you look at the other hardware types it'll be interesting in getting an idea of weird things you don't see in normal offices.

https://mikrotik.com/products/group/switches

Apart from obviously larger bandwidth options like 28qfsp 100gb (I'm unaware if mikrotik does them but 400gb is normal in some circles) there's things like reverse POE switches, media converter switches, and all sfp+ switches.

Poe++ exists and you can use switches with it to power poe+ switches that will power poe switches. Or they can be used to power laptops or NUCS directly.

hdgvhicv• 9 months ago

So my arista 710p-16p isn’t entries because it’s not 48 ports? Or my Nexus 9300 24p? I’ve got some old juniper 4300 32F ports handling 1g fibres (moderns arista switches aren’t great as the 25g SFPs won’t autoneg at 1g on fibre as they don’t support clause 37)

My standard campus switches are 722s with 48 ports and 25/10 SFPs, but there are use cases when smaller switches make sense.

wmf3• 9 months ago

Probably USB-C wall outlets will end up solving this instead.

greycol2• 9 months ago

Which is ok if done right, but if they're anything like the usb-a ones there'll be plenty that are continuously pulling much more power than they need let alone the danger of uncertified ones.

For those thinking about adding one they've grabbed off amazon and installing themselves, please do a bit of hunting and reading rather than just buying the first word soup brand cheapest ones. Also remember installing uncertified electronics in your walls is a good way to void your insurance if they're the cause of disaster and turn it into a legal battle even if they're not.

userbinator• 9 months ago

Besides being reversible, USB-C is a horrible connector. Tiny contacts, no positive retention, and a massively overengineered standard that should've been broken up.

stego-tech4• 9 months ago

I thought so as well when USB-C was first seeing widespread adoption, but now I’m not so sure. High-end PoE deployments can reach 90W of power down the line, and even HDBASE-T can support 100W of power down the line. Combined with the 8P8C connector both use (which is easy to field repair or replace and has positive retention), and I’d much rather see more structured ethernet runs and outlets with PoE/HDBASE-T for all but the most demanding or performant kit.

beala3• 9 months ago

I don’t follow. What part of this would a USB-C wall outlet solve? This would just be swapping a 120/240v cable for a USB cable, right? PoE reduces the number of cables, among other advantages.

numpad03• 9 months ago

They have shorter operational life than most computer keyboards.

morshu90013• 9 months ago

Might be because it's scary. User plugs passive poe into something not expecting it, magic smoke

ianburrell3• 9 months ago

Passive PoE is evil, standard PoE is safe since does negotiation.

labcomputer• 9 months ago

Theory and practice and all that, but that shouldn’t happen regardless.

A correctly-designed Ethernet interface is galvanically isolated at both ends to avoid ground loops, differing grounds, and other nasties over long distances.

ianburrell3• 9 months ago

There are finally PoE adapters that give USB-C power and USB Ethernet. Those should allow home theater devices to be powered and use wired Ethernet. But the ones I have found are expensive, but should drop if there is demand.

stego-tech4• 9 months ago

I already use splitters throughout my stack and devices, but that’s additional material and complexity cost compared to native PoE.

addaon• 9 months ago

I also haven’t found any bidirectional ones yet, even though the hardware (except maybe the negotiation chip if it’s pure PDE) should be able to support it. Would be really nice for development of PoE devices to just hook up one dongle to my laptop.

scottlamb• 9 months ago

Converting AC to DC once at the switch is more efficient (in resources and often, but not always, power)

Can you expand on "often, but not always, power"? Here's my guess:

* It's more efficient for the small stuff: little wall warts aren't very efficient I think in part because there's some no-load consumption for each. The switch pays that no-load cost once for many devices and has like an 80-plus gold or better PSU, hopefully. And then I think even cheap buck converters are like 95% efficient; they have some no-load consumption too but I think less than the wall warts? And even though this goes over 2 (or 4) tiny wires, at 48V–56V, the current is low enough that power loss is not bad because those wires are just for one small device, and P=I^2R.

* It's less efficient for the big stuff: that P=I^2R starts to suck for the PoE case, and in the non-PoE case they're more likely to have efficient AC->DC conversion on their own. 90% efficient beats 90% * 95% efficient.

speleding• 9 months ago

A power supply can operate most efficiently if its power output is close to what it was designed to supply. Typically, a PoE switch has a large power budget to take into account the myriad devices that might be connected to it.

If you have one small PoE device connected to a large PoE switch then it would be less efficient compared to a non-PoE switch and a small separate power supply for the device.

sodaclean8• 9 months ago

It's 2 or four pairs. So at minimum you have two 26awg (spec is 24awg, and the slim-line cables are labeled "data only") wires- which is not as big a deal as you made it out. Spec comes out to about an amp per pair.

toomuchtodo• 9 months ago

It’s supported by the latest revision of Glinet’s Comet kvm over IP hardware, which was a cool upgrade imho.

https://www.gl-inet.com/products/gl-rm1/

somerandomqaguy• 9 months ago

Doesn't really make any sense to in that example.

Where ever you're putting the TV you have to put in regular power anyways, so it's fairly tidy to just put the device's power cable parallel with the TV's power cable. WiFi will handle communication. On the other hand, NEC and CEC requires minimum of 2 inches gap for communication wiring to electrical so you're now you've got that minor complication.

POE makes sense mostly when it makes sense to combine communication and power cabling. Corded phones, wifi access points, security cameras, small touch screen modules, etc. Not saying what you're doing can't work, but the added expense of installing parallel CAT6 everywhere doesn't seem worth it.

RulerOf• 9 months ago

An AppleTV, Chromecast, or NVIDIA Shield can easily fit within the envelope of PoE+

I ended up buying a PoE extractor and barrel plug adapter for my Roku, and another extractor for my HDHomeRun.

It annoyed the heck out of me that they had PoE running to them and still had to be plugged into a separate transformer.

stego-tech4• 9 months ago

I feel your pain. Prior to my network rebuild at home, I had splitters powering the EdgeRouter, Philips Hue Bridge v2, a Raspberry Pi, and was tempted to add more for the various small (<8-port) ethernet switches scattered about. UniFi expanding their switch lineup to include PoE-powered gigabit and 2.5G switches was a godsend, as were PoE HATs for the RPis’. Only the Hue Bridge remains on a splitter, because Signify won’t make one with native PoE.

benoliver9992• 9 months ago

All our desk phones at work are poe and they have pass through ports to allow connecting a PC to the LAN. Really neat to just have one cable that gives you a powered phone, and Ethernet access.

Gibbon1• 9 months ago

I have a mild obsession with the idea that there should be a PoE lighting standard. There is a two wire automotive ethernet standard that can deliver 50-100W. Which is enough to run a couple of LED or fluorescent lights.

beala3• 9 months ago

I remember when USB first came out and there was a ton of novelty in powering all sorts of random stuff out of your computer’s USB port like fans and mug warmers. This has me wondering what sort of whacky stuff I could do with a PoE extractor/splitter. An ethernet desk lamp would be fun. I have 24 PoE ports on my switch and I’m only using three of them for PoE. Time to get creative.

wolrah5• 9 months ago

I have a mild obsession with the idea that there should be a PoE lighting standard.

Ubiquiti did this for a while, the product line was called UniFi LED and IIRC it didn't get much further than a few panel lights intended for drop ceilings and a wall mount dimmer switch.

IIRC the justification was that because it was low voltage it could be installed by anyone instead of potentially requiring an electrician and you then also got the ability to dynamically adjust grouping, switch behaviors, etc. if for example your floorplan changed.

priceofmemory• 9 months ago

My entire basement is currently lit by Poe++. Poe ethernet cable to metal utility box. Metal utility box contains Poe++ to 12v adapter. 12v adapter output wires screwed into utility light socket base. 12v led lightbulb screwed in to led lightbulb socket.

gwbas1c3• 9 months ago

My Chromecast uses USB C for its network. I got a power brick (official Chromecast) that also takes an ethernet cable and then provides a wired ethernet connection.

That being said, a quick Google search for "poe usbc" yields some devices that are much more expensive than the power brick I bought, but in theory would let you run a Chromecast from a poe ethernet port with wired ethernet.

radicality2• 9 months ago

I bought last year a device like that I think - was from some company named TexasPoE I think, and it took Ethernet cable, and output has usb-c with 1GbE and power. I sometimes use it with my iphone or iPad and do get the wired Ethernet connection and charging

numpad03• 9 months ago

I agree sentimentally but apparently it's not considered the brightest idea to run data and real amounts of power together. I think that's why PoE is always treated like alcoholic beverages.

mmaunder• 9 months ago

Jeez top post on HN and there's a full overlay ad to "download a mac extension". This deserves a summary post to save others the click. Here's the "what every engineer should know" without the spam:

PoE (Power over Ethernet) sends both DC power and data through the same twisted-pair Ethernet cable, allowing devices like IP cameras, wireless access points, and VoIP phones to run without separate power lines. The power is delivered by Power Sourcing Equipment (PSE) — either an endspan (built-in PoE switch) or a midspan (PoE injector placed between a non-PoE switch and the device). The powered device (PD) negotiates power via detection and classification before voltage is applied, preventing damage to non-PoE gear. IEEE 802.3af (Type 1) provides up to 15.4 W at the source, 802.3at/PoE+ (Type 2) up to 25.5 W delivered, and 802.3bt (Type 3/4) extends that to roughly 60–90 W using all four wire pairs. Engineers need to understand not just wiring, but also cable category limits, pair usage, power losses over distance, and heat dissipation — especially at higher power levels. Modern PoE designs must consider standards compliance, thermal management, and efficiency, as power density rises with new generations of PoE technology.

pwg• 9 months ago

With UblockOrigin in default deny all javascript mode there was no overlay ad.

IncreasePosts• 9 months ago

Running a browser with javascript disabled is great, I recommend it to everyone. More often than not, you get a better experience - more responsive, your battery lasts longer, fewer ads. And, if the site breaks for some reason, you just allow list the site and reload.

willis9362• 9 months ago

Other stuff engineers should know:

- PoE endpoints should have isolation barriers, factor this into cost and size estimates

- Don't skimp on TVS

- ideal diode full bridge rectifiers are really cool and you should use them (in more power entries than just PoE)

dynode• 9 months ago

Awesome I’m just doing a POE design now.

- Was having a conversation today about isolation and grounding for POE (product has a metal case). Do you have a reference? Or standard?

- TVS ahead of the bridge right?

- Do you have a part recommendation or reference design for ideal diode POE?

eqvinox10• 9 months ago

- Was having a conversation today about isolation and grounding for POE (product has a metal case). Do you have a reference? Or standard?

Huh. I'm not the GP poster but interesting question. AFAIK there is no proper ground reference on the LAN cable. I'm not sure I've ever seen a metal case… wait, I do, outdoor wifi APs have metal cases sometimes.

If you find out, report back ;D

- Do you have a part recommendation or reference design for ideal diode POE?

I've done a PoE device (802.3at, 25W) and just went with TI's reference design; the higher power ones use ideal diodes, sometimes there's multiple circuit variants.

(It's not worth mucking with the PoE design for small-scale builds; the reference design might be a bit more expensive but you get that money back on way less trouble to deal with.)

willis9362• 9 months ago

The chassis net should be passed through the cable shield and the power isolated to force the current return through the cable in case power ground is bonded to chassis (which is commonly done, not sure why, I prefer a 1MOhm standoff). While not perfect this link is concise and provides handles to the relevant specs.

https://www.brainboxes.com/faq/power-isolation-in-poe-ethern...

Yeah, TVS before any other silicon junction. It's nice to throw a single-use medium or slow blow SM fuse before the TVS to open circuit in device faults.

This is going to be individual preference. I like the density and low design risk of fully integrated solutions like Microchip's PD70224. As long as you spec your FETs appropriately you can't go wrong with TI or AD options (VDS of at least 100 V, ID of at least as much current as you want to cram through with healthy headroom, RDSon that makes you happy, VGS that's compatible with the datasheet charge pump, size and cost that doesn't make you weep). When in doubt, stay very close to the datasheet's design.

I just saw that the PD70224 is not recommended for new designs. What an awful day to have eyes.

Oh, it's been superseded by the PD70288. Much lower RDSon, but a huge 8x8 package. The charge pump is mysteriously gone and there is now a UVLO of 24V. This is more PoE-specific, which is less generally interesting to me.

If only someone would sell me an ideal diode full bridge rectifier IC with integrated FETs, OCP circuit breaking, UVLO, OVLO, a fault flag, control input, and current monitor, I'd never buy a different power entry IC.

zxcvgm• 9 months ago

I use the FDMQ8205. It's an old part, a little pricey, but keeps the board footprint low. It also has a sufficiently high UVLO, so it acts like regular diodes during the classification phase and you don't need to factor those in to the Rcls values.

eqvinox10• 9 months ago

ideal diode full bridge rectifiers

For the less electronically inclined, an "ideal diode" surprisingly does not contain plain diodes, it refers to actively controlling MOSFETs to function as diodes.

They're more efficient and quite amazing in PoE applications in particular!

sodaclean8• 9 months ago

Amazing indeed- since PoE is DC.

jwr• 9 months ago

Don't skimp on TVS

This. I know from a friend (ahem) that if you do, you will discover problems in production deployments, when it's too late to fix things.

Animats• 9 months ago

Not clear on how multiple sources on the same cable work. Is that allowed? Is there a power break at midspan, or does power flow through? How are the regulators coordinated?

eqvinox10• 9 months ago

There are never multiple sources; a midspan PoE injector breaks the power flow and injects only its own power supply.

It's done through the center tap on the Ethernet transformers. Midspans have another set of those transformers and inject the power on the PoE PD facing side. Whole pair(s) carry common-mode DC current, so basically your green pair could be ±48V and the orange pair 0V. If any, the upstream switch's injected power would just end at the other coil of the Ethernet transformer in the midspan. However, the midspan also doesn't pass through PoE negotiation, so the switch won't turn on power to begin with.

RandomBacon• 9 months ago

Question for anyone who might have a suggestion:

I have a Ring home security system. I would like to get an offline home CCTV that only records when the alarm is set (either in Home or Away).

A quick internet search does not show an API. I'm not sure Ring has a device that I could wire a relay (if that's the right thing) to.

dang• 9 months ago

[stub for offtopicness]

davydm• 9 months ago

came for the poetry, left with an engineering lesson

leoh2• 9 months ago

I'm just, like, vibe-coding, man. Why do I need to know about PoE?

scuff3d• 9 months ago

Not gonna lie, I'm kind of disappointed this isn't a Path of Exile tutorial for engineers.

elromulous• 9 months ago

"every engineer"? Should a chemical or nuclear energy engineer have to know about PoE?

ThrowawayTestr• 9 months ago

Sure. It's really useful if you're setting up a home security network.

JoshGG• 9 months ago

POE. Purity Of Essence ?

varenc5• 9 months ago

you're a hero

sema4hacker• 9 months ago

POE switches can have more jacks than they can provide power for. I have five pan-tilt-zoom (PTZ) cameras but learned they can need up to 40 watts each, preventing plugging them all into a particular POE switch I had. Chaining a couple of switches solved the problem, be beware how much power your devices need to consume.

ifwinterco• 9 months ago

Is it actually likely that all 5 would be drawing full power at once though?

It's normal in household wiring (at least here in the uk) for circuits to be somewhat undersized based on the concept of "diversity" (i.e. it's highly unlikely every socket on the circuit will be drawing 13A simultaneously)

publicmail• 9 months ago

I would expect power consumption to be fairly consistent for something like a surveillance camera that emits a stream 24/7.

Underphil• 9 months ago

This is true of any electrical system, no?

ww520• 9 months ago

It is also good on delivering power to AES50 snake stage box to microphones that needs fathom power.

red3692• 9 months ago

I recently needed to buy a new power supply for my Mikrotik router (hAP ac²), so I decided to just buy a Ubiquiti PoE injector instead.

It feels magical to have the PoE injector tucked in a cupboard with the optical network terminal, and outside Narnia, the router has only one cable going to it. Also, the Ubiquiti PoE injectors are particularly satisfying. Powered by standard AC cables, and a nice simple design. Now that I've experienced this magic, I'm not going back!

However, as much as I love the hAP ac², it only accepts passive PoE. I don't love passive PoE - it scares me! Unfortunately, it seems like most (all?) Mikrotik routers only accept passive PoE.

Does anyone know of a good alternative when it comes time to replace my router? I would have liked it to be Ubiquiti, but I don't usually read positive things about them around here.

bombcar2• 9 months ago

You can get the inverse of an injector that takes active PoE and makes a barrel plug out of it.

red3692• 9 months ago

Thanks for reminding me of that! I considered that when I was finding it hard to find a passive PoE injector.

It's not quite as tidy, for example, with the router sitting visibly on a desk, but it's close. And you're right that it could work for anything.

nucleardog2• 9 months ago

It feels magical to have the PoE injector tucked in a cupboard with the optical network terminal, and outside Narnia, the router has only one cable going to it.

Last time I remember feeling like that was the day I unplugged a RB5009 and it... just kept running. Was standing there holding the power cable in my hand, clearly unplugged, and the router was sitting there still happily blinking away. Like, this clearly can't be possible but I'm staring right at it and it's happening.

Took me a minute, but eventually realized the Starlink box that provides power to the dish _also_ provides power on the local side for their provided router as well, and apparently it was happily powering mine now.

mns• 9 months ago

PoE actually saved me, or at least helped me simplify some things. My router is in one place, while the distribution box where all the cable conduits lead to every room in another place. The distribution box (where also the optical cable originally comes from to the router) has no power plugs. I wanted to have Ethernet plugs in every room (especially for any fixed devices), so I managed to find (quite difficult in Germany) a PoE powered switch for the distribution box with no plugs available, get a PoE injector to power it and from there on to wire and install Ethernet plugs in all rooms.

le-mark• 9 months ago

Can anyone comment on what VEX robotics kits do with POE? Their controller box connects to motors via cat5 (they call them “smart cables”)which carry’s power, pwm, and encoder data. I don’t think it’s carrying ethernet but could be? Any sources for more info would be appreciated!

lawik• 9 months ago

It seems to be RJ11 and they might be spec:ed slightly differently (thicker leads?) to power motors. I get the impression it is mainly to control what can be used in competitions.

This was what I found from skimming around: https://www.robotevents.com/V5RC/2018-2019/QA/35

I doubt it is Ethernet at all, so it wouldn't be Power-over-Ethernet. Just some useful connectors and wires making for an appropriate cable. Also seems like you can make your own perfectly fine. Or they might melt. I suppose try it.

gwbas1c3• 9 months ago

Is POE really relevant in the consumer space? All of my laptops get power and wired data through their USB C port. Does it make more sense to just send data over USB C?

For example, my Chromecast gets power and wired ethernet through its USB C port. (I have an official Chromecast power brick that I plug an ethernet port into.)

qwertyuiop_• 9 months ago

You wouldn’t have to worry about POE in the device category you mentioned.

sota_pop• 9 months ago

As an EE, once upon a time had a rep pitch PoE lighting fixtures. Cool idea, like allowing each individual fixture to be addressable/ controllable on a BAS, but they just had too many gotchas like coupling lighting (and by extension life-safety systems) to the server/network, additional voltage drop considerations, etc. Otherwise and for network gear, I really like PoE!

viggity• 9 months ago

I worked for a PoE lighting company for a couple of years. Yes, the ability to program your lights based on time of day, occupancy sensors etc was all nice for the end consumers. But the big advantage was that since it didn't use mains power, the owner of the building didn't have to hire a union electrician at eleventy gillion an hour to move some light panels around in a drop ceiling.

parliament32• 9 months ago

Something I've never understood:

The above figure shows a PoE injector with auto negotiation, a safety and compatibility feature that ensures power is delivered only when the connected device can accept it. Before supplying power, the injector initiates a handshake with the PD to detect its PoE capability and determine the appropriate power level.

If PoE requires negotiation, and the device requires PoE for power.. how does it perform the handshake without being powered/booted first?

_flux• 9 months ago

Presumably lower voltage/current is provided first?

bokohut• 9 months ago

Quite the mix of comments in 2025 from something that has existed and been foundationally critical in many unseen systems.

I designed and built my first POE system in 2004, at my own house as a dogfooding POC, and that system stills works to this day. Since that time I have built and installed many more without issue that continue to move along doing what they were intended to do, protect life and property via recording activity privately. My own home footage has been called upon several times by law enforcement and was critical in convicting at least one home break in crew.

The benefit of install is simple to comprehend for those with significant experience in the electrical field, run one small wire for data and power and ensure the POE supply is on a battery - done. Additionally I add those using WiFi for security are laughed at daily as losses pile up, web search MLB player home break-ins, as running a hardline cannot be jammed but many foolishly put all their assets solely behind WiFi security. Also these surveillance systems require no external cloud by design so no one is watching remotely, unlike the Fed and State viewing your Ring cameras for years and now which recently partnered with Flocker. No one cares more about you than you so if someone is selling you security ask yourself what it is you are actually paying for.

In closing, as we move into a new era of technological efficiency forced by rising energy prices and costly electrician labor hours, one is going to witness an uptake of POE adoption in more and more nontraditional places. It is already happening and its moment will come as more recognize the cost benefit to this greatly simplified power delivery method with integrated battery backup.

What you cannot see matters most!

outofmyshed• 9 months ago

Passive PoE getting a lot of stick in the comments, but for one-off point solutions, it’s fine as long as you know what you’re doing.

I have it to run a couple of MikroTik devices in awkward places and not having to run wall warts and flimsy DC cables around the place is very handy.

My next switch upgrade will be a proper PoE+ one, but it’s not justified yet.

Power over Ethernet (PoE) basics and beyond

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