So I have designed an Ethernet PoE module, I have not actually made the module yet.
The idea is to use passive PoE @ 12V (so not ieee compliant).
It is based on the w5500 chip for Ethernet communication and can send a regulated 5V to an arduino.
I have made the schematics so far, and I was wondering if someone would be so nice to check them through, that there are no obvious mistakes
I am thinking especially about the voltage converter part (part2), as the rest is pretty much from the documentation of the w5500. The module should be able to handle up to 400mA load, and powered over Ethernet with a 12V 600mA supply.
Thank you!
PS: I'll update this thread with the PCB when I get there
The converter looks good, and I like the inductors for isolation. I see a problem with the POE portion. At 400 mA you will experience a voltage drop of about 5V per 100 Feet (200 feet round trip) using 28 AGW cat wire. You can use the voltage drop calculator at: "Voltage Drop Calculator" to validate this. Play with the calculator and you will see why they set the voltage much higher then 12V for POE. This response is to help you get started in solving your problem, not solve it for you.
Good Luck & Have Fun!
Gil
I did check the voltage drop before, and yes I just realized my mistake - I forgot to count for the round trip, so a 30 meter cable is actually 60 meters of cable resistance!
But still I think I'm good because the two pairs will be used (5&6 and 7&8), as you can see in the HY931147C connector - I'll make sure they are also both used at the other end
Worst case scenario is:
cheap Ethernet cable with aluminium instead of copper => about 21 Ohm resistance for a 100 feet cable
400 mA => voltage drop of 4.2 (with 2 pairs) => voltage of 7.8 for the NCP1117 => all good
And the case of my project, we will probably only need 200 mA and a 50 feet cable, so all is good
But ironically my new problem is the heat that will be generated by the NCP1117 if the voltage drop is small.
Let's say the voltage arriving is 11V, at 400mA, that means the NCP1117 will generate (11 - 5) * 0.4 = 2.4W of heat!
I guess an easy way to handle it is to simple drop the voltage source to 9V if I am using a short cable, that will make it (8 - 5) * 0.4 = 1.2W of heat.
I chose the DPAK package as it can handle more heat by itself, it can handle 1.2W with a copper pad of 1cm2.
Make the solution easy, simply use a SEPIC converter, they can be gotten for less then $3.00. Some can use an input of about 3V to 36V, they are not all the same. The conversion will be at least 85% so you no longer have a heat problem and can use short to long wire, you can do the calculations. This response is to help you get started in solving your problem, not solve it for you.
Good Luck & Have Fun!
Gil
Thanks again Gil. I looked at it and kept the LDO thinking I will not need that much current anyways. I put a big fat copper pad underneath hoping that will help reduce the heat anyways
ToneArt:
Thanks again Gil. I looked at it and kept the LDO thinking I will not need that much current anyways. I put a big fat copper pad underneath hoping that will help reduce the heat anyways
