Power over Ethernet, how to step down at a reasonable cost? - Page 1
Forum section: electronics / projects

author: lk
author_rank: Regular Contributor
timestamp: November 23, 2012, 08:18:50 am
content: Greetings,I want to set up some IP cameras and I don't want to break the bank in the process, the way I planned to this is to use POE. But problem is that most cameras that are POE enabled are very expensive, and I don't want to spend that kind of money ? so why not build a DC-DC buck converter myself, so I can power some of the cheaper cameras that need either 5v or 12v input, from a POE switch or injector.Currently I'm looking at the MC34063 DC-DC converter, its fairly simple to work with, and it would fit the bill, apart from the problem with it is that the POE spec goes as high as 57 volt, this must be related to battery voltages I believe.My very simple approach would be to use a series of diodes to drop the voltage, I don't know how clever this is, but I would only need 1A tops on the 12/5 volt side.I would love to hear some comments on this, and remember I try to keep cost down and design as simple as possible.-lk

author: kripton2035
author_rank: Super Contributor
timestamp: November 23, 2012, 08:32:51 am
content: simple but efficient approach :http://www.tuxgraphics.org/electronics/200903/hobby-poe.shtmlyou put the voltage you want on a poe cable but you will have some voltage drop along the line specially if it is a long one.so insert a higher voltage is a good practice, then a step down voltage controller as the mc34063 is a good choiceI'm making a poe injector with a 24v transformer and a cabling like the one in tuxelectronic on a board with 4 inputs and 4 outputs.I will use also a mc34063 on the target boards to step down to 3.3v

author: bitwelder
author_rank: Super Contributor
timestamp: November 23, 2012, 09:10:16 am
content: I also think that (unless you have plenty of IP cameras to feed) a DIY approach should be enough.In that regard, you could make your own 'injectors' by taking e.g. a standard RJ-45 dual socket (that comes with a complete box), drill a hole on the side for a power jack and then internally plug the two socket together so to copy two twisted pairs and using the remaining two pairs on the output socket to inject the power.

author: peter.mitchell
author_rank: Super Contributor
timestamp: November 23, 2012, 10:05:22 am
content: oh, and put red tape on it and write "POE" just to be safe.

author: lk
author_rank: Regular Contributor
timestamp: November 23, 2012, 11:49:43 am
content: Kripton, what you have going there sounds like what i need, so i don't have to deal with the silly voltages on the "IEEE" POE, it could certainly solve my problem. The ideal solution would still be the possibility to attach to my already existing switches and get the power form there, even with the extra work that is needed so that the equipment turns on power on the ports.And generally i would like to make something without loose wires-lk

author: ptricks
author_rank: Frequent Contributor
timestamp: November 23, 2012, 01:29:54 pm
content: Easiest switcher I have found is LM2576.It requires 1 small heatsink, 1 inductor, 1 diode, and your capacitors. Efficiency is good and they can handle up to 3A. Voltage inputs can go as high as 60VDC for the LM2576HV . Package is TO-263. If you can keep the supply under 42V then you can use the regular part not the HV part and save a few $$. The regular sell for about $3 and the HV parts are about $5-6http://www.ti.com/lit/ds/symlink/lm2576hv.pdfYou can also get pre-built versions on ebay for about $7 , they usually dont heat sink the part so if you are using a lot of current this might not be ideal.http://www.ebay.com/itm/DC-DC-3-30V-to-4-35V-LM2577-Boost-DIY-Mobile-Phone-Power-Supply-Module-/221152539615?pt=LH_DefaultDomain_0&hash=item337db7f3df

author: free_electron
author_rank: Super Contributor
timestamp: November 23, 2012, 02:44:03 pm
content: Real poe requires a lot of stuff. The devices actually request power... There is data being transferred.Roll your own. Use the two spare pairs in the ethernet cable. Apply 12 volts there, small switcher at point of load.

author: lk
author_rank: Regular Contributor
timestamp: November 23, 2012, 05:47:42 pm
content: Greetings, seems like the best option is to go with making my own setup, but i dont think 12 volts will do it, even at 30 meters and 1 amp, i have already a 4 volt dropthis is what made the 48 volts which 56 seem so tempting.ptricks, i have looked at the LM2576HV it would be perfect apart from the fact that I'm cheap, and want to keep the cost down-lk

author: gxti
author_rank: Frequent Contributor
timestamp: November 23, 2012, 06:29:09 pm
content: Have you looked at commercial PoE splitters? Probably much cheaper than a camera with builtin PoE. You can also get "fake PoE" injector/splitter pairs even cheaper than the real stuff but still get the voltage boost/buck.

author: Monkeh
author_rank: Super Contributor
timestamp: November 23, 2012, 06:58:11 pm
content: Just how much power do you think your camera will take?

author: AndyC_772
author_rank: Super Contributor
timestamp: November 23, 2012, 07:23:46 pm
content: Don't forget that you'll need an 803.3af compliant PD chip in your product, in order to identify itself to the PSE and actually turn on the power. PoE isn't just applying 48V across the Tx and Rx pairs, there's more to the protocol than that. Linear Technology has quite a good range of devices which implement PoE.

author: lk
author_rank: Regular Contributor
timestamp: November 23, 2012, 08:05:17 pm
content: AndyC_722: I realize that POE is just not turned on as it would result in magic black smokebut as far i can see from the application notes i would need a resistive load of 19k to 27k with 120nf or less parallel capacitance. After this a constant current load, indicates the load class, i have yet to find this as i cant get my hands on the IEEE standard documentation. I'm sure all this can be done with discrete components.-lk

author: Bloch
author_rank: Unknown
timestamp: November 23, 2012, 08:20:14 pm
content: Discrete Components Enable Bold PoE Solutionshttp://powerelectronics.com/mag/606PET21.pdf

author: kripton2035
author_rank: Super Contributor
timestamp: November 23, 2012, 08:23:53 pm
content: cheap ebay step down modules (supports 22v on input)http://www.ebay.com/itm/DC-To-DC-Converter-Buck-Step-down-Voltage-Module-12V-To-5V-3-3V-LED-Power-Modul-/180830601650lm2576 are still more expensive.

author: kripton2035
author_rank: Super Contributor
timestamp: November 23, 2012, 08:30:03 pm
content: my homemade poe injector :

author: lk
author_rank: Regular Contributor
timestamp: November 23, 2012, 09:18:22 pm
content: Bloch: Thank you, this was exactly what i was looking for, i especially like the example schematics, makes it seem less daunting.Kripton: i might do it the same way as you have, its just that the 48 volt are really needed if you want to avoid the voltage drop on the longer cables, and it could in principle be up to 90ish metershowever i still need to find a way to drop the POE voltage a tiny bit so that my mc34063 can handle the level.-lk

author: jabramo
author_rank: Regular Contributor
timestamp: November 23, 2012, 10:20:19 pm
content: You can get something like this on ebay for less than 5 bucks shipped.

author: ptricks
author_rank: Frequent Contributor
timestamp: November 23, 2012, 11:30:59 pm
content: Quote from: lk on November 23, 2012, 05:47:42 pmIf you want to do long distance then consider using low voltage AC, with a voltage drop of about 1.68 volts you could send 12VAC over 100 meters and still be able to get over 14VDC out.Rectify and filter at the endpoint and you could use cheap LDO 7812 parts and the power waste from 14VDC is low, about 1watt..

author: SeanB
author_rank: Super Contributor
timestamp: November 24, 2012, 05:18:50 am
content: If you use a transformer send 60VAC down, and at the other end use a standard small transformer. If you need 12VAC use a 120/24VAC unit or a 220/48VAC unit. Lower iron losses and still a standard part. For the send end use a 60VAc secondary or use 2 back to back, one giving 12VAC and the second used in reverse being the same transformer as on the far end.This was very common on TV distribution amplifiers where the head end would be up to 200m away from the mains power, and you could get 60W of power at 220VAC to run the head end units while keeping the voltage in the coax below it's voltage rating. A little lossy but easier than running a mains cable up to a rooftop.

author: westfw
author_rank: Super Contributor
timestamp: November 24, 2012, 05:50:30 am
content: I'm not sure what exactly you mean by "very expensive", or whether you already have an actual PoE-providing ethernet switch. But "real" IEEE conforming Device-side PoE buck converters can be had starting at about $8 each (in 1s.)http://www.silvertel.com/poe_products.htm#Ag9000http://www.semiconductorstore.com/cart/pc/viewprd.asp?idProduct=41612I always figured PoE was a hack to make Ethernet Switches more expensive/profitable, by holding up the carrot of "cheaper wiring." Can't have those professionals needing to figure out where TWO wires from their IP phone go...

author: lk
author_rank: Regular Contributor
timestamp: November 24, 2012, 07:27:52 am
content: westfw, those PDs are in the right price range, but what i had in mind still was something with an 2 ethernet jacks, one coming from the POE switch with 48 volts, and the 2 datapairs, and one what would connect to my camera with only the datapairs, and then and output powerjack that could power my camera. all on a nice PCB without any rouge wires. this would all fit into a little "junction box" like this one from our premier place to buy cheap Chinese stuff in denmark (sorry its in danish)http://www.harald-nyborg.dk/p1408/forgren-daase-75x75mm-staenktaetThis is what i do for my analog cameras.I hope this makes more sense nowjabramo: i have ordered a pair of those, but they don't help with dealing with voltage drop on long linesand its an easier solution, i would just prefer to be able to utilize the standard IEEE802.3afAnd if someone is interested, an electrician in Denmark is properly around 100USD an hour, plus materials. And i can easily get a POE enabled switch for that amount of money here, even without having to buy it in China. And of course the POE enabled cameras are expensive as well, so that's why i wanted some way to use the cheaper "IP" cameras, and therefore started this thread-l-lk