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Posts: 608
Registered: ‎09-17-2013
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providing over 40mA to load

[ Edited ]

I found myself needing more than 40mA for my load, and the mport can't. However, the poe can provide up to 0.5A. I'm a bit concerned about the wire getting chopped and shorting the poe supply. Since the mport is seperated by a good distance,  will be using a separate cable to power the load. This still works for the load even if it is next to the mport, thought the supply cable to the mport isn't "protected."

 

Basically connected the load in parallel to the poe supply (pins 4+5 and 7+8), and added a simple current limiter. This way I can prevent the load from drawing over a certain level even in short circuit conditions. Though it is a tivial circuit, I'm posting a copy here, in case it anyone finds it useful.

 

The way to adjust the current is Iload = (Vd + Vd - Vb)/R10. In this example you can think of it as .7/3.9 or 180 mA. However, you can fine tune it a bit by adjusting Vd, through resistor R12 (i used a 20k pot). The less current you provide the diodes, the lower the voltage and the lower Iload will be (.5 + .5 - .7)/3.9 or about 75 mA.

 

It isn't a great circuit, but it is a quick and dirty solution for this use.

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