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Possible uses for Power over Ethernet
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. "
Actually, the company who only wired two pair can get PoE for switches that provide PoE, but not external injectors. There are two standards for power, if the power is injected off the switch it uses dedicated pairs, if it is injected on the switch it uses the data pairs (phantom power).
UL considers the average human (for testing safety purposes) to be about 500 Ohms. Since most PoE is around 12v, you end up with about 24 milliamps across a 500 ohm load.
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.
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.
How does the Slashdot Effect happen given that no slashdotters ever RTFA?
Yes you can. The power is running on the same wires as the data. This is roughly the same technology as Broadband Over Power. The power is 60Hz but the data is MHz...or in your case, GHz. Little to no crosstalk.
earlier this year I converted our warehouse of 250,000 square feet to 802.11b WiFi using Cisco 1200 series AP's. Our cost per AP was 1900.00 (CDN) using power over ethernet. This cost includes contractors, electricians, cat5e, fiber, and even antennas. Im not here to toot any horns for 802.11b or Cisco or anything... but our cost per AP would have been WAY higher if it weren't for power-over-ethernet technology.
You do realise that RF is nonionising radiation, right? And that *ionising* radiation is required to cause the mutations in DNA that lead to cancer, right?
Just checking.
p
In Korea, long hair is for old people!
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.
The spec addresses issues like "devices that can't handle it." You just have to RTFS.
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.."
Invalid Checksum. Retrying.
60 Hz. was selected as a national standard because Tesla worked out the numbers and George Westinghouse went along with him, and the rest of the country ended up going along with Westinghouse. Westinghouse was concerned about power transmission and 60 Hz. was selected for efficiency with the generator and transformer technology of the day. Remember Tesla's proficiency with those.
... your heart will also stop dead if the current happens to pass through it. A lot of lives were spared over the years because we didn't go with Edison on that one. The other reason to rejoice is that a DC power distribution system would have required power plants plastered all over the place since transformer operation (and hence high-voltage landline transmission) would have been impossible. Today we could probably do it with high-powered DC-AC inverters, but that technology was way beyond Westinghouse and Edison.
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.
The higher the technology, the sharper that two-edged sword.