48V (PoE) to 12V converter

[Swake]

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

[Richard Crowley]

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$ 3.95

LM2596 Step Down Adjustable 1.5-37V DC/DC Converter



You 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.

[eliocor]

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.html
and their usage:
http://www.soselectronic.com/a_info/resource/c/IEEE802-3at_Class_Programming.pdf

[gxti]

Just search for "802.3af splitter", there's no shortage of pre-made devices. Often they're marketed specifically towards IP cameras, even.

[mariush]

It depends on how much current you actually need on 12v.

Select max input voltage >=48v and go from there

DC / DC Converters - Isolated Board Mount Multi Output : http://uk.farnell.com/jsp/search/browse.jsp?N=225230

[eliocor]

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_Ethernet
http://www.soselectronic.com/a_info/resource/c/IEEE802-3at_Class_Programming.pdf
http://belden.com/docs/upload/PoE_Basics_WP.pdf

[westfw]

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...

[Swake]

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 

[eliocor]

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!

[Richard Crowley]

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.

[German_EE]

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 Ethernet

b) 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.

[rob77]

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.

[eliocor]

Quote from: German_EE on Today at 15:26:13

b) 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.

If you are using real PoE 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.

[eliocor]

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_implementation

As stated in IEC 60950-1:2005 (norm valid also in the USA):

2.2.2 Voltages under normal conditions
In a single SELV CIRCUIT or in interconnected SELV CIRCUITS, the voltage between any two conductors of the SELV CIRCUIT or CIRCUITS, and between any one such conductor and earth (see 1.4.9), shall not exceed 42,4 V peak, or 60 V d.c., under normal operating conditions

The norm dictate also other rules, but they do not interfere with the 2.2.2 paragraph...

[rob77]

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

if 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-kit
it comes with a 48V wall-wart and it has a injector & splitter with selectable 5V or 12V on the "far" end.

[amyk]

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.pdf
http://www.eetimes.com/document.asp?doc_id=1273086

If you want an IC, there's plenty out there too, e.g. http://www.ti.com/product/tps2375

[Jay_Diddy_B]

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 -48V

The 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