Possible uses for Power over Ethernet 385
jsailor writes "Power over Ethernet allows devices to draw power from the Ethernet cable they use for
networking. Power is provided by the LAN switch (end-span) or an intermediary device (mid-span). The current spec. is 802.3af and was covered on slashdot before. It provides approximately 13W at the end of a 100 m cable and is commonly used for IP phones, wireless access points, and increasingly security cameras. The technology saves costs associated with running power to the odd locations access points find themselves in and allows IP phones to be moved around with out carrying a power brick. The industry is considering a new standard that
would provide up to 39W to a network device. Bizarre uses include electric
razors. "
Re:Seems Kinda Weird / Wired (Score:2, Informative)
So for new construction, it's probably not a big deal. But for adding new devices to an existing facility, it could be a lot easier/cheaper.
39 W is Enough for Whole Village in India (Score:1, Informative)
Admittedly Indian culture is somewhat better than Chinese culture [phrusa.org], but we are really scraping the bottom of the barrel here.
Re:Short circuit (Score:4, Informative)
Re:13W could be dangerous... (Score:4, Informative)
For that to have a severely negative effect, it would need to cross your heart. Most of the current will likely go around your skin (you are your own faraday cage) so you most likely would never even feel it.
Plus, you would have to actually come in contact with it...which is pretty easy to avoid.
Re:13W could be dangerous... (Score:4, Informative)
Your assertion that ...used to them being safe as phone lines... begs the question*. Phone lines are not intrinsically safe, and the central office can easily provide several watts of power at 90VAC for ringers.
*Look, ma! Someone on slashdot who knows what 'begs the question' means!
In the US, at least, to meet Part 68, telephone gear must also handle line-crosses to 600Vac without creating a hazardous situation.
Re:Gigabit ethernet? (Score:4, Informative)
Lower cost per AP (Score:5, Informative)
Power over USB (Score:2, Informative)
I want PoE, but it's stupidly expensive. (Score:3, Informative)
Right now it triples the price of a switch [hp.com]. (Compare the 2626 and 2626-PWR, for example.)
So no.
Re:Easy enough, (Score:5, Informative)
Just checking.
p
I've used PoE in my small business. (Score:2, Informative)
I've done some "playing" with that switch and I see it somehow detects that the ethernet device is able to handle PoE and then turns that on. You plug in normal ethernet devices it does not supply power to them. I'm sure this is specified in the 802.3af specification - something I've not had time to read! But I somehow doubt that this switch would cause any problems with old ethernet devices. Even if you are worried, you can log into the switch and turn the power on/off on a per-port basis. We even tried inseting a single port power injector down stream from this switch - and it then would not supply power to that port - and the injector was. So it all "just works" or at least from what I've found so far!
Re:Seems Kinda Weird / Wired (Score:5, Informative)
Transformers are not used to convert AC to DC. Transformers only convert AC to a different voltage AC. The rectifier portion of the average brick (the part that does convert to DC) is very tiny. Often it's only four diodes and a capacitor.
So basically, if you needed a transformer to power a device from AC, you're just completely screwed if you try to power it from DC, unless it was regulated at the right voltage beforehand. Since we're discussing PoE, that would be a giant no.
Switch-mode power supplies are just as efficient with DC as with AC. They are very small and lightweight, and that's what you'll find in most 802.3af-powered devices. However, if you want to keep discussing alternative forms of local power distribution, those transformers also become very small and lightweight if you change the operating frequency from 50/60Hz to, say, 100kHz.
Re:Gigabit ethernet? (Score:1, Informative)
BTW I work for a manufacturer of 802.3af compliant equipment.
Re:13W could be dangerous... (Score:4, Informative)
That 13W isn't always there. The device has to be POE enabled. The hub supplying power senses the device. It then measures a resistance across one of the pairs looking for a very specific resistance. That's what specifies IF PoE is wanted, and then there are different current limits you can request. The hub end is required to limit the current supplied and also monitor for faults (and if so disable the power).
The spec isn't just some yoyo hooking up an ac adapter to a supposedly unused pair and saying "it works.."
Re:Electric razors? (Score:2, Informative)
Actually, there is a standard 12V outlet design used throughout the world. It's called a car cigarette lighter socket, and it can carry 10-20A without melting, unlike Ethernet cabling, which would probably catch fire quickly.
-b.
Re:13W could be dangerous... (Score:5, Informative)
Your professor is a bit confused about the electric chair business: it was Thomas Edison who claimed publicly that Westinghouse's 60 Hz. system was much more dangerous than Edison's own direct current power system. This was strictly a marketing ploy: he and Westinghouse were going head-to-head in an all out corporate war and Edison wanted to win, badly. He had no scientific basis for his claims. In other words, he lied, publicly and repeatedly. He even went so far as to have a major correctional institution that was building a new electric chair facility install Westinghouse generators in order to "prove" how dangerous alternating current could be. The reality is that Edison was way off base: direct current is substantially more risky than alternating: for example, if you grip a pipe charged with 120 VAC, you will get a nice shock but will be able to release your hold. The jolt might cause your heart to fibrillate but most likely you'll survive. Grab that same pipe with 120 VDC and your muscles will lock and you won't be able to let go
The only thing that saves us from instant death the first time we walk across a carpeted room in dry weather is our epidermis. That layer of dead tissue makes an excellent electrical insulator. Otherwise, the first static spark you drew touching a doorknob would stop your heart. Remember, the insides of your body are an ionized, highly-conductive mess: a hundred-odd pounds of adulterated salt water. If you stuck a couple of pins in each index finger, and put those pins across a flashlight battery, you would probably die. Your bloodstream would conduct that tiny current flow directly through your heart. But touch those same terminals with the outer layers of your skin intact: no problem.
And I'm not making this up: if you've ever been in a hospital burn unit, you would see that everything in those rooms is heavily grounded, and extreme precautions are taken against static discharge or any other electrical artifact reaching the patient. It's amazing. I worked in a lab at a major teaching hospital / university for a while, and I noticed that there were these odd metal plates with heavy-duty green leads hanging out of them, sticking out of the walls and floors. I asked, and was told that the lab space was a converted burn unit. People that have had significant areas of their skin burned off are fatally susceptible to even minor electrical discharges.