"PoE" - Page 1
Forum section: electronics / projects

author: Isaac Newton
author_rank: Contributor
timestamp: April 27, 2013, 04:53:36 pm
content: I'm thinking in using a Ethernet cable to power and serve as the bus for a RS485 system. I've seen some modules from Freetronics that provide PoE by injecting in one of the twisted pairs a 7 to 24V DC voltage that is then regulated in the target device in the bus.Well, why not just inject 5V to the twisted pair and take care of the noise after? (Assuming that the modules connected to the bus have a low power consuption, like just a mcu and some indicator LEDs and stuff).

author: Isaac Newton
author_rank: Contributor
timestamp: April 27, 2013, 04:55:37 pm
content: I'm thinking in using a Ethernet cable to power and serve as the bus for a RS485 system. I've seen some modules from Freetronics that provide PoE by injecting in one of the twisted pairs a 7 to 24V DC voltage that is then regulated in the target device in the bus.Well, why not just inject 5V to the twisted pair and take care of the noise after? (Assuming that the modules connected to the bus have a low power consuption, like just a mcu and some indicator LEDs and stuff).

author: c4757p
author_rank: Super Contributor
timestamp: April 27, 2013, 05:03:01 pm
content: How long are the cables? You might want to use a higher voltage and step it down at the end to reduce loss. They're not exactly thick wires.

author: 4to20Milliamps
author_rank: Regular Contributor
timestamp: April 27, 2013, 05:06:44 pm
content: 22 gauge wire is pretty small so over any substantial distance, or if you have much of a load, you will end up with quite a bit lower voltage at your destination.You will have to take voltage drop into consideration of your design.http://www.active-vision.com/CAT5-Voltage-Loss-Calculator-s/160.htm

author: c4757p
author_rank: Super Contributor
timestamp: April 27, 2013, 05:07:36 pm
content: Are you aware that you double-posted this?

author: Unknown
author_rank: Unknown
timestamp: April 27, 2013, 05:09:39 pm
content: Posting a thread twice is disrespectful to the people spending their time trying to help you. They may be wasting time drafting a reply to a question that may already be answered in the other copy. You should ask a moderator to close one of them.

author: Isaac Newton
author_rank: Contributor
timestamp: April 27, 2013, 05:13:44 pm
content: I think I posted this topic in the wrong section... Anyway, I'm aware about the losses issue, but do you think that 32 max low power modules (according to the max485 datasheet) would suck all this power to make a huge difference in the voltage drop? The maximun length of cable suported by the rs485 standards is 1200m with a 9600 boud rate but I wont use not near the half of this length...

author: Isaac Newton
author_rank: Contributor
timestamp: April 27, 2013, 05:16:37 pm
content: I tried to delet the other post, but I couldn't! Sorry...

author: Isaac Newton
author_rank: Contributor
timestamp: April 27, 2013, 05:18:20 pm
content: I tried to delet this post, but I was not alowed to.Moderator: If you have a post you want to delete but can't, use the Report to Moderator button.

author: c4757p
author_rank: Super Contributor
timestamp: April 27, 2013, 05:51:54 pm
content: Do the computations! I'm sure the datasheet lists the power draw, and you can measure or look up the wire resistance.

author: peter.mitchell
author_rank: Super Contributor
timestamp: April 27, 2013, 07:04:22 pm
content: Bah just shove 30-60v down a pair and stick a buck converter on the other end.

author: notsob
author_rank: Frequent Contributor
timestamp: April 27, 2013, 11:39:49 pm
content: Be careful there are 2 PoE systems, the real one uses 48 volt supply to make at least 40V available at the end of the cable to power devices. The "hobby" field is using 24 volt.

author: ve7xen
author_rank: Super Contributor
timestamp: April 28, 2013, 12:09:24 am
content: PoE works because Ethernet is AC coupled. I don't think RS-485 is. It is a differential bus, but 20+V of common mode will probably not make the receivers or transmitters very happy. If you've got spare pairs, sure.

author: Alana
author_rank: Frequent Contributor
timestamp: April 28, 2013, 01:09:42 am
content: Ok but you have 8 wires/4 pairs and both ethernet and [if i remember correctly] RS485 uses 1 pair for RX and one for TX. That leaves 2 spare pairs that are signal-free and can be used for POE.Back in my amateur networking days usual POE injector connected blue and brown pairs on the cable side to power socket with those 4 pins unconnected on device side. Worked like charm :d

author: notsob
author_rank: Frequent Contributor
timestamp: April 28, 2013, 01:44:55 am
content: Be careful there 100M uses 2 pairs 1G uses all 4 pairs

author: Hardcorefs
author_rank: Regular Contributor
timestamp: April 28, 2013, 04:46:42 am
content: Quote from: Isaac Newton on April 27, 2013, 04:53:36 pm5v won't work, also you will need to use a large current.Shove a larger voltage at a smaller current then buck convert it locally.

author: westfw
author_rank: Super Contributor
timestamp: April 28, 2013, 07:43:56 am
content: So you're thinking of using cat5 CABLE for some rs485 stuff, with NO actual "ethernet" anywhere, and want to use the spare pairs in the cable to carry power?Sure, that'll work. HOW low power? cat5 cable is 24ga, with about 2.6ohms per 100feet, or 0.13V drop per 100feet at 50mA. Put a couple dozen such nodes on your bus, and you'll start to see some serious drops, along with potential heating...There are two depressing things about PoE on actual ethernet. The first is that SO many people are just running random voltages over the spare pairs, and expecting things to work. The second is that REAL PoE is so expensive that it isn't competing...

author: Isaac Newton
author_rank: Contributor
timestamp: May 08, 2013, 09:45:13 pm
content: I'm thinking in using this method to make a home automation system where the modules would be mounted were the wall mounted switches go. Since I don't want tho brake the walls and and stuff, I'm thinking in using this kind of "PoE" to power the modules. Having individual AC-DC converters on the module would be too expensive and kind of a overkill. The modules would just be based ond a simple MCU like a pic or something and have the rs485 transceiver and a simple LED feedback and some buttons. Just to be safe and don't play with Murply's Law, I think it's better to put something like 24 to 40V in one of the pairs and buck it down to the modules...

author: C
author_rank: Super Contributor
timestamp: May 08, 2013, 10:46:38 pm
content: The POE standard does not use a one pair for DC+ & DC-, The standard uses two pair with one pair for DC+ and one pair for DC-Ethernet uses transformer coupling, and if the standard only used one pair for DC+ & DC- it would be putting that Dc voltage across that transformer winding, which would be really BAD. By using two pair you have two transformers with two windings at different potential an not a problem.You should also use two pair for power when you use RS485. By using a pair for each voltage you remove the problem of putting a high voltage across the RS485's receiver or transmitter differential pair. You still have the possible error of violation of the RS485's common mode range.Using two pair would also reduce the voltage drop as you would have two 24 gauge wires handling the current.C