48V (PoE) to 12V converter - Page 1
Forum section: electronics / beginners

author: Swake
author_rank: Frequent Contributor
timestamp: July 31, 2014, 01:33:04 pm
content: I need help finding a 48V (actually power over Ethernet) to 12V converter.Obviously of the switched type because of the efficiency. A buck converter or step down converter.Preferable ready made, or a kit, I don't want to loose time designing a PCB and such.The thing I want to power is a series of foscam cameras.Thanks in advance for the help

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 31, 2014, 02:03:35 pm
content: There seem to be many small, inexpensive DC/DC converters/regulators with 12V output, but they don't go up to the full 48V rated input. I suspect that 48V figure separates the hobby-grade world from the professional (read: high-price) world.For example here is a very inexpensive adjustable DC/DC buck converter... US$ 3.95LM2596 Step Down Adjustable 1.5-37V DC/DC ConverterYou did not state your power requirements (i.e. current).Also, are you talking about full, formal POE where the receiving end does that little negotiation dance to establish how much current it wants? Or are you talking about a simple, informal re-use of the spare wires to send power? If you are providing the power at the source end, you could select a slightly lower voltage and take advantage of those very inexpensive modules. They appear to be available from many places including Ebay, etc.

author: eliocor
author_rank: Unknown
timestamp: July 31, 2014, 02:35:12 pm
content: The requirements for a REAL PoE module are far different from a typical switching regulator...Here you can find some details about some PoE modules:http://www.silvertel.com/products/poe-power-over-ethernet.htmland their usage:http://www.soselectronic.com/a_info/resource/c/IEEE802-3at_Class_Programming.pdf

author: gxti
author_rank: Frequent Contributor
timestamp: July 31, 2014, 02:36:52 pm
content: Just search for "802.3af splitter", there's no shortage of pre-made devices. Often they're marketed specifically towards IP cameras, even.

author: mariush
author_rank: Super Contributor
timestamp: July 31, 2014, 03:35:58 pm
content: It depends on how much current you actually need on 12v.Select max input voltage >=48v and go from thereDC / DC Converters - Isolated Board Mount Multi Output :http://uk.farnell.com/jsp/search/browse.jsp?N=225230

author: eliocor
author_rank: Unknown
timestamp: July 31, 2014, 08:31:25 pm
content: As I previously wrote, real PoE can't be implemented using a mere switching regulator because there must be a handshake between the power supply (switch/...) and your device.Before suggesting a switching power supply, please at least consult the following links:http://en.wikipedia.org/wiki/Power_over_Ethernethttp://www.soselectronic.com/a_info/resource/c/IEEE802-3at_Class_Programming.pdfhttp://belden.com/docs/upload/PoE_Basics_WP.pdf

author: westfw
author_rank: Super Contributor
timestamp: July 31, 2014, 10:49:02 pm
content: What eliocor said. If you're dealing with real PoE, you need a actual PoE extractor, not just a voltage converter.The silvertel modules have been around for a while, come in a wide variety, and are available in small quantities for prototyping, experimentation, and hobbyists...

author: Swake
author_rank: Frequent Contributor
timestamp: August 01, 2014, 09:48:30 am
content: Thanks all for the help. At the time of posting my request I was not aware of the handshaking thing.Searching for 802.3af splitter as suggested returned very interesting products with very acceptable prices (like in <15$).The rated power requirements of the camera's slightly exceeds the maximum amount of power supplied by PoE. It might work.... or not.... depending on environmental conditions for example. I need something reliable....My believe now is to use a decent 12V switched power supply and feed through some RJ45/power splitters. Eventually the 12V regulated slightly higher to compensate for the loss in the long wires, but probably the camera's are ok with less than 12V too. Found some passive PoE splitters... this is what i'm looking for. me happy

author: eliocor
author_rank: Unknown
timestamp: August 01, 2014, 03:13:18 pm
content: Please take care of the current passing in the cable and its dissipation: the reason it was chosen a voltage of about 48-50V was due to the much lower current burden on the S/UTP cable.If you are using a 12V power supply, the losses due to cable resistance will be much higher and your cable will become a heater!!!Nice if you live near north pole... awful if you live near the tropics!

author: Richard Crowley
author_rank: Super Contributor
timestamp: August 01, 2014, 03:17:55 pm
content: At least here in North America, 48 volts is the legal limit of "low-voltage". Anything higher than that is considered the equivalent of "mains power" and requires a licensed electrician to install, etc.Indeed, as eliocor says, sending the "baseband" voltage (12V) over long distances is a difficult thing because of line-losses. That is exactly why they use a HIGHER voltage and then convert it down to the required voltage at the destination.

author: German_EE
author_rank: Super Contributor
timestamp: August 01, 2014, 03:26:13 pm
content: Just as an experiment I tried powering my notebook over the unused brown and green pairs of a network cable, it didn't work so well. I measured 20V at the power supply end but by the time it got to the notebook it was only 16V, not enough to charge the accumulator.Also when doing POE you need to make sure that:a) The remaining pairs are not used for Gigabit Ethernetb) Make really REALLY sure that every networked item doesn't have a short circuit across pins 4,5,7 and 8 or has one or more of these pins wired to chassis. This was done in the past on 10/100 network ports.

author: rob77
author_rank: Super Contributor
timestamp: August 01, 2014, 06:58:17 pm
content: i was using LM2576HVT-ADJ in the past (looks like it's still available) - those are going up to 60V input voltage. and the circuit is fairly simple - just 6 external parts - inductor, catch diode, 2x capacitor and 2x resistor.or there is a fixed 12V output voltage version LM2576HV-12 - just 4 external parts (as above but without the resistors)the non HV versions are going up to 40V input only - so it has to be the HV for 48V input.

author: eliocor
author_rank: Unknown
timestamp: August 01, 2014, 07:19:11 pm
content: Quote from: German_EE onTodayat 15:26:13If you are usingrealPoE switches you don't have to worry about: the negotiation phase is done also to protect the PoE power supply.If the switch will find a short circuit it will disable the PoE power supply on the specific port.

author: eliocor
author_rank: Unknown
timestamp: August 01, 2014, 07:43:18 pm
content: I was remembering well: according to the standard PoE specifications, the voltage range goes from 44V to 57Vdc.http://en.wikipedia.org/wiki/Power_over_Ethernet#Standard_implementationAs stated in IEC 60950-1:2005 (norm valid also in the USA):QuoteThe norm dictate also other rules, but they do not interfere with the 2.2.2 paragraph...

author: rob77
author_rank: Super Contributor
timestamp: August 01, 2014, 07:55:10 pm
content: Quote from: Swake on July 31, 2014, 01:33:04 pmif you need to power only one device and don't want to make your own converter, then you can buy a simple and ready to use power over ethernet kit.i don't think it makes sense to buy a real PoE switch to power a single device over ethernet.something like this one:http://www.dlink.com/us/en/business-solutions/switching/network-and-switch-accessories/poe-injectors-and-splitters/dwl-p200-power-over-ethernet-adapter-kitit comes with a 48V wall-wart and it has a injector & splitter with selectable 5V or 12V on the "far" end.

author: amyk
author_rank: Super Contributor
timestamp: August 01, 2014, 08:56:16 pm
content: PoE negotiation isn't all that complex - you don't need to implement Ethernet data transmission/reception, for example. Most of the intelligence is in the PSE, and for the PD all you really need is a senes resistor, a comparator, and switches to connect either the sense resistor or the load. The load also needs to draw a minimum current, but that's easy enough to do.Here's some schematics:http://powerelectronics.com/mag/606PET21.pdfhttp://www.eetimes.com/document.asp?doc_id=1273086If you want an IC, there's plenty out there too, e.g.http://www.ti.com/product/tps2375

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 02, 2014, 02:30:09 am
content: Hi,The 48V in PoE is negative 48V like the telephone system.This causes a little complication. Most PoE devices require an isolated power supply.Using a H.V. Buck regulator would result in an output referenced -48VThe LTC4267 is an example of a chip that performs the PoE negotiation and is also a power supply controller.Here is an application circuit:Note that the circuit contain bridge rectifiers in case the polarity of the input voltage is inverted.Regards,Jay_Diddy_B