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Wanted - two pair cable for 12VDC and RS485

Discussion in 'Electronic Design' started by [email protected], Jun 24, 2005.

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  1. Guest

    I am looking for cable with two twisted pairs: one which can carry
    500mA/12VDC power up to 80 metres and and another which is screened +
    twisted and suitable for a RS485 network. The cable will be used for a
    variety of applications - CCTV control, a small RS485 network
    including a security alarm keypad, lighting control system.

    Is there something suitable available in the UK? (I've looked at Cat5
    cable but would prefer not to use this for various reasons.)

    Belden 1502R (http://tinyurl.com/9pct9) has a 18AWG power pair and a
    22AWG twisted pair. BUT it is only available in the US and can only be
    exported at high cost and in large lengths.

    Would I be better off buying separate cables for power and signal?
     
  2. Why not just use one of the standard multi-core cables?. You can double up
    the pairs for power, and given the cores are twisted pair, and the power
    is DC, there should be insignificant crosstalk between the data and power
    pairs. Something like 9503, and use two pairs for the power.
    The same could be done with 8777, which has individually shielded pairs.
    Realistically if you want 12v at the far end, you would be much better off
    sending 24v, and regulating locally.
    Have you tried FSC?. When I wanted to source some Belden cable in the UK,
    they were able to help.
    They also do their own range of cables, that combine power and data in one
    cable.
    www.fscglobal.com

    Best Wishes
     
  3. Guest

    Thanks, Roger. I spoke to Lee at FSC. He didn't know about combined
    power/data but he recommended a cable with two screened twisted pairs
    of 18AWG at £0.61/metre. I'm guessing that this cable might do the job
    OK
     
  4. The 'silly' thing is that combined data/power cables, seem to have become
    less common. I seen to remember Farnell, doing one with a heavy pair and a
    single twisted pair, but could not find it. There are usually 'thicker'
    versions available for servo applications (have one heavy pair for the
    motor drive power, and usually a shielded pair, or two pairs, for the
    shaft encoder outputs, and/or the zero sensor), but because they are
    designed to be very flexible, they are usually expensive. £0.61/m, sounds
    reasonable if you only want the 80m mentioned.

    Good Luck.
     
  5. Reg Edwards

    Reg Edwards Guest

    Why not use just a single twisted or straight pair of wires. You can
    get your DC power to run over the same pair of signal wires using
    chokes and capacitors in the form of a filter.

    But two separate cables will be the most economic solution. Any old
    stuff will do for the power cable provided it has a low enough DC
    resistance.
     
  6. Guest

    Hmmm. 18awg has a resistance of three ohms for 80m there and back. This
    is voltage drop of 1.5V for 500mA, which is too much for a 12V supply
    voltage. I'm going to check whether my loads really require that much
    current...
     
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