Power Modem and Router from a single DC rail

Discussion in 'The Projects Forum' started by Wiky5, Jan 16, 2016.

  1. Wiky5

    Thread Starter New Member

    Sep 3, 2013
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    0
    Hi all,
    I was wondering if it is possible to connect different devices, which are already connected via ethernet (UTP cables), to the same DC power supply.

    My main motivation for this is on the short term to reduce the number of "wall warts", and in the somewhat longer term to have a large stationary battery to supply power for the modem, router, LED lights, music system and an inverter (pure sine wave FTW!). This setup would allow me to have a much more efficient power conversion from 220v to 12v and some autonomy on the frequent summer blackouts.

    Long story short, will the modem and the router complain if I connect them to the same power supply?
    In case that is a "Yes", does it also mean I could connect a 12v pc to that same rail and the wired network?

    Thank you very much.
    Best regards,
    Willy
     
  2. Wiky5

    Thread Starter New Member

    Sep 3, 2013
    15
    0
    Since Ethernet works with differential signals my first guess is that the UTP cables are in a very high resistance circuit, connected directly to the inputs of differential amplifiers, and therefore there should not be a problem with both devices sharing the same ground...
     
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,546
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    Nope. Ethernet signals are transformer-coupled to the UTP, and usually single-ended out of the drivers and into the receivers. The transformer does the common mode rejection. The cable impedance is 100 ohms (?-working from memory). Overall, Ethernet is not a high-impedance framework.

    ak
     
  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    If all of the devices you want to connect to a common power source have the same input voltage range, then it should work. These devices do not rely on isolated power as part of their signal functioning.

    ak
     
  5. Wiky5

    Thread Starter New Member

    Sep 3, 2013
    15
    0
    Thank you for your answer, AnalogKid.
    I do not recall seeing any transformers or inductors whatsoever when I opened another router, neither on motherboards or PCI NICs, connected to the ethernet jack, hence my question. I'll check again just to be sure...
    Also, I am not talking about PoE.
    Regards,
    Willy
     
  6. eetech00

    Active Member

    Jun 8, 2013
    652
    112
    I think AK wants you to check the wall warts used for each device and check the output voltage and current rating.
    See if you can come up with a universal supply that satisfies each requirement.
     
  7. Wiky5

    Thread Starter New Member

    Sep 3, 2013
    15
    0
    Thank you for your answer, eetech
    Every device works with 12v, yes. My concern is not about the voltages nor the amp ratings of the devices, but rather their capacity to work with non isolated power supplies.

    I've had an unpleasing experience connecting a cell phone via the auxiliary input of a car stereo, and charging it with a 12v-5v dc converter. Apparently the stereo's line in ground was the same as the vehicle's ground, while the 3,5mm audio jack in the cell phone has a "floating" ground at halve its batteries voltaje, so the phone's internal power supply got fried.
    I've checked in some computer motherboards I have lying around and they do not have any kind of transformer between the female 8P8C connector and the ethernet IC...
    Regards,
    Willy

    (In case you're wondering, I'll hack an ATX power supply to give +40A @ 13,8v, to keep a battery charged. While this is not enough for all of the 12V loads I plan on having connected to this, not everything will be drawing full power at once, and any momentary excess of demand will be covered by the battery. Where necessary I'll put step down DC converters. Yes, I know how to build this :). When I do so I might post the guidelines, though I'm not used to this forum/blogging thing...)
     
  8. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,546
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    Pretty much. The rear panels of the devices should tell all more accurately than the wall warts. Each one will state the required power *input*, such as 12 V @ 0.5 A. Note that there probably will be a symbol rather than the letters DC. If all devices require 12 V, then a single (large enough) 12 V source will run all devices.

    As for internal Ethernet signal transformers, they don't look like transformers:
    http://www.pulseelectronics.com/products/lan

    Note that placing the transformers inside the RJ-45 connectors is becoming more common. In that case, you won't see anything that looks even remotely magnetic in nature.

    ak
     
  9. Wiky5

    Thread Starter New Member

    Sep 3, 2013
    15
    0
    Oh, that was unexpected.

    Thanks again for your answer, AK.

    I'll try connecting both wall wart's negative lines with a resistor and check for voltaje differences, just to be 100% sure. It is not uncommon to find non standard stuff in my country... If everything looks normal, I'll make a joint supply with enough capacity for both and then keep on working with the secondary 12v system.

    Because you seem worried about it: yes, they both say they require 12v. The modem needs 1A while the high power router 1,5A, though most probably they both can tolerate voltages far from that value as electronics usually work with 5v or 3,3v or other weird voltaje as 1,116v and necessarily have a voltaje regulator inside (a switcher, I hope!). Those that can tolerate it willl eventually be fed with 13,8v and the others via DC converters. A router I had could tolerate more than 18v and was fed via an unregulated transformer with a full wave rectifier.

    Best regards,
    Willy
     
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