Power Distribution Question

Question: Should I distribute power using DC or AC across RJ45 cable which I am also using for serial communication?

Details: I'm working on some circuits that interface with my pc. The plan is to have several boards with microcontrollers and sensors that communicate back to the pc. They will all be daisy chained together with RJ45 cable and use rs485 to communicate (and then rs232 back to the pc). The total distance may be up to 500 ft or so. My proof of principal is working. I built it on a project board and have a PIC16F688 with a temperature sensor, a light sensor, and some op amps and can transfer the data to my pc.

The next phase is to build the circuits. My question has to do with supplying power it it all. I plan to use a 12.6V center tapped transformer, diodes, and voltage regulators to supply power, including the negative and positive rails for my op amps. I'm going to start by putting the transformer and rs232 -> rs485 conversion on one board, then the first PIC and sensors on a second board, a third PIC and sensors on another board, etc. each connected with RJ45 cable. The question is, how to get power to the other boards. Do I convert to DC on the first board and distribute it over the RJ45 or should I send the plus and minus AC over the RJ45 cable and convert to DC on each subsequent board?

I'm thinking my concerns are voltage drop if I use DC and interference if I use AC but I'm not sure what other design trade offs I might be missing. I would like to get some opinions.

Thanks,

paul

Sending DC down Cat5 is done regularly to power CCTV equipment, Large wifi access point, audio/video baluns, etc. It's all about how much current you need at the other end.

Another idea is to distribute the unregulated +- Voltage, and put local regulator chips on each board. That way, any voltage drops dont matter, as long as voltage at any given board stays above the dropout voltage of that particular regulator.

Please be aware of voltage drop, especially in Cat5 cable, with its pretty small gauge wires! If you have a run that's say 500 feet, I would start out the voltage at like 18, so it would drop down to about 12 at the other end.

Here's a nice cat5 voltage drop calculator:

**broken link removed**

Thanks gents. I was thinking like imix500 that it could be done with DC and I think the current is fairly low with the PIC chips but the op amps seem to draw a little more, especially as I add more sensors the current will likely continue to build up. And I also was thinking about the unregulated voltage like Mike ML suggested. But I can see from the site suggested by Birdman Adam that maybe I need a higher voltage to start with.

For Birdman Adam, were you suggesting that I go with the unregulated AC and convert on each chip like Mike ML suggested?

Do you need a bipolar power supply?

What about distributing AC and converting it into both positive and negative voltages at the other end?