RS-485 + AC/DC Power on CAT5/6

Discussion in 'Analog & Mixed-Signal Design' started by Michael Murton, Jul 11, 2017.

  1. Michael Murton

    Thread Starter New Member

    Aug 7, 2016
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    Doing some preliminary design of a project that will be using RS-485 for serial communications over moderately large distances (10-100m, possibly more) and I am trying to come up with the best way to power the remote devices. I would like to use CAT5/6 and RJ45 plugs because they are readily available. The RS-485 would be half-duplex, so it would only require one pair, and I don't need it to be particularly fast. I am not able to find a power transmission solution that I like though, here are the ideas I've had so far:

    48V
    I have considered 48VDC through the remaining pairs. This is a good option to combat voltage drop over longer distances, since the highest voltage I will need to supply at a remote device would be 12V for the RS-485. This would require more complex/expensive voltage regulators at the remote and host devices as I need to have 12V and 5V or 3.3V for logic devices. Using 3 wires each for power and ground would mean I need to balance higher loads across all 3 wires, again increasing complexity. This doesn't seem to satisfy enough of my design criteria, so I kept looking.

    Mixed 12V and 24V
    I also considered having one pair supply 24VDC and ground to power the remote devices, and another pair supply 12VDC and ground for the RS-485 transceiver. This would make voltage regulation simpler, but increases concerns about voltage drop, especially for the 12V. This also didn't seem like the right solution.

    Isolated AC
    It occurred to me that I could use lower voltage (18-24V say) AC to be able to get the power farther and avoid the voltage drop suffered by DC. I would have a transformer at the host reducing and isolating the 120VAC mains down to something more reasonable, and transmit that down a single pair of the CAT5/6 cable. Each remote device would have a small rectifier and SMPS to get 12V/5V/3.3V as required.

    Now we are into things that I know less about, so here is what I'm hoping you will be able to help me with:
    Having serial data in the same cable as AC raises some concerns though, would 60Hz AC impart much interference into a differential signal like RS-485? If so, are there easy ways to combat this?
    Is there an easier way to do any of this that I might have missed?


    Thanks in advance for your help,
    Mike
     
  2. MrChips

    Moderator

    Oct 2, 2009
    19,132
    6,148
    Don't use AC.
    Use 12VDC and buck converters at the remote device.
    RS-485 uses 5V.
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
    18,847
    5,890
    Not sure how it would fit in with your plans, but by far the best cable I have found for long distance transmission of RS485 etc, is Belden-9891 10 base 5 Ethernet cable, it has individually shielded pairs, with OA braid shield, it also has one pair at a slightly larger gauge for power.
    Max.
     
  4. Papabravo

    Expert

    Feb 24, 2006
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  5. Michael Murton

    Thread Starter New Member

    Aug 7, 2016
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    Ah yes... Missed that little detail. RS-485 drivers are rated for a maximum bus voltage range of -7V to 12V, not that the bus should idle at 12V. That solves that then.

    Yes, almost. I will probably run 14-15V down the line since I will have some devices that will be running 12V components.

    Thanks
    Mike
     
  6. Michael Murton

    Thread Starter New Member

    Aug 7, 2016
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    Both of these cables look superb, however, they are both overkill and overbudget for the design I have in mind.

    Thanks,
    Mike
     
  7. Papabravo

    Expert

    Feb 24, 2006
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    I did not see price as a requirement in your original post.

    Maybe they are overkill. It's always better to under promise and over deliver. If the one you select falls short, you have a handy alternative. The power pair on the Thick cable has excellent characteristics for delivering usable voltage from a 24V supply located at one end. 500 m will have a substantial IR drop at at any significant current.
     
    Last edited: Jul 11, 2017
  8. Michael Murton

    Thread Starter New Member

    Aug 7, 2016
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    You're right, I did not mention that. I also neglected to mention that this project will be for my own use. I am looking to design the system to be modular and easily expandable, without too much complexity.
     
  9. dendad

    Distinguished Member

    Feb 20, 2016
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    815
    Have a look too at these...
    http://www.mornsun-power.com/html/product/RS-485-Transceiver-Module.html
    Isolating the RS485 could simplify your design I think. These are a great device and if you isolate the network it does remove a lot of noise problems. Even it you do it at one end. Then feed +ve the power via a center tapped inductor down the signal wires and the -ve down another pair or shield.
     
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