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To make it short, I actually have some network equipment (the WRT54GL, my CPE(Customer Premises Equipment, the optical fiber↔ethernet thing), my SIP↔Analog phone converter (PAP2T), and so on) that is powered from power adapters. This causes two major problems:
I have a switch (DLink DES1100-16) at home where all the ethernet wires go. I didn't want to buy a new switch. Plus if I had bought a POE switch, I still wouldn't be able to power the network equipment I wanted over POE as it isn't poe-compliant (you would have to buy some “poe power “sniffers””, some switch mode converters, etc..). You see where i'm going: I would like to make my switch homemade-POE-compliant. As you see, I didn't use the term “POE-compliant”, but I added the word “homemade”. Yep, In real life, “real POE” injects power at around 48-56Volts. All of my equipment runs on 12V. If I would've used 56V, it would have implied some switch mode PSUs (plus you would have to be “802.3af compliant”) in the equipments (you can't have linear ones !), and I don't think it adds much more value.
Well, to make the story short, the voltage delivered via POE by the switch will be 12V. It's much easier like that.
The switch (a DLINK DES-1100-16), is only 10/100Base-T. No gigabit here. That means that pairs 4-5 and 7-8 aren't used. And we will use them to transmit our power !
The POE “norm” defines that pair 4-5 transmits the positive voltage (in our case, 12V). Pair 7-8 will carry the ground.
My switch has 16 ports, so I decided to make 4 “POE-compliant”
So I took the switch apart, and I first measured the resistance between pairs 4-5(blue) and 7-8(brown). I could read 150Ω. As you can see in the picture, each pair is connected to the same cap via a 75-ohm resistor. The capacitor is then connected to the switch ground. You can see the resistors and the cap in the middle-left part of the picture.
PAIR 4-5 -- 75R -| | -- Capacitor -- GROUND PAIR 7-8 -- 75R -|
Theoretically, you could leave this resistance, but that means that each POE-port would consume P = U*I = U²/R = 12²/150 = 1W. In my case, times 4 ports, that's 4W, that's a lot of lost power (plus I don't think the SMD resistors could handle that..) !
So I de-soldered the 4(ports)*2(pairs) resistors.
The PAP2T is a special SIP to analog phone converter. It uses an ethernet connection and a 5V@2A (or so it claims) power connector. The thing really consumes 0.8A@5V. The first thing I thought was to use a simple 7805 to convert to 5V. However, as you know, the 7805 is a linear regulator, and it would need to disipate (12-5)*.8 = 5Watts in heat ! It worked, but that wasn't the solution. So I searched on ebay for simple switch-mode PSUs. And a found some that used the LM2576 chip. Simple and efficient. You just have to select the output voltage via a potentiometer. Simple as that. I just had to integrate the little module to the PAP2T.
A good thing to do would be to add a fuse and a 13v Zener accross. So if you would have had the bad luck of plugging the wrt54gl to a real POE port, the thing wouldn't blow up :p
4-5(+) --- FUSE (1A) --|-----------(+) 13V Zener 7-8(-) ----------------|-----------(GND)