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Current rating of an RJ45 connection

Discussion in 'Electronic Design' started by Joerg, Aug 29, 2012.

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  1. Joerg

    Joerg Guest


    Have to send 12V/500mA across a connection. So far there is only an
    RJ45. Looking at datasheets I saw current ratings of 1.2A and 1.5A which
    sounds like a lot.

    Is this kosher? I know PoE sends up to 600mA across but I've always felt
    uneasy about it when they went up from the (older?) 350mA.
  2. Jamie

    Jamie Guest

    it's not the wire you need to worry about it's the cheap crimp style
  3. Guest

    I think I remember PoE as something like ~400-500mA per pair, but
    that's gotta
    be including everything from connectors, wire size and how many cables
    that are
    usually packed together in a large installation

  4. CharlieB

    CharlieB Guest

    Amphenol make a range of connectors rated at 1.5A and I've successfully used them in NRTL listed POE powered products drawing >800mA

  5. Guest

    Don't forget distance. PoE is one of the few times I've had to worry about
    "maximum power transfer". BTW, 400-500mA sounds right but my memory is a bit
    fuzzy on that, too.
  6. Nico Coesel

    Nico Coesel Guest

    RJ45 isn't the best connector. In my experience you need to reseat the
    connector after a few years for it to make good contact. IMHO sending
    a lot of current through it may make things worse. BTW PoE always uses
    two parallel wires to carry the supply to the device whether the power
    is phantom fed or through unused pairs.
  7. Guest

    sure distance matters when you need to figure out how much power you
    have left at the
    of the cable, but it doesn't affect current rating

  8. Joerg

    Joerg Guest

    Thanks, Nico. Also thanks to all others. One of the things concerning me
    is the fact that datasheets derate to zero milliamps at slightly above
    70C. Got that info this morning and it was quite the surprise. This is
    from the connector we are using:

    The unit will sit in full sun occasionally and can become as hot as the
    inside of a car under that condition.
  9. Joerg

    Joerg Guest

    You can get 22AWG TP for that. Usually only two-pair but that would
    suffice here.

    I found 1.2A as a low value at 25C, so far. But the surprise was a
    rather fast derating to almost zero at 70C.

    The worst is a BFC on the source side plus a BFC on the other side and
    the only inrush limiter is a short cable and an RJ45 connection ...
    bzzzt ... *PHUT*

    But we'll avoid that here.

    Thanks! I have bookmarked it. In this case we'd be ok because there
    would be essentially only two insertions. One at production test and
    another at site installation. Both non-powered.
  10. We regularly see plastic boxes supposedly rated for 85 degC badly
    deformed after being in a car in Australia.
  11. Joerg

    Joerg Guest

    True, but the 577mA of 24AWG suffice in this case.

    We just did a test run. Stays under 60C. Whew ...

    10,000uF across RJ45 pins? Wow! That is brazen.

    We'll have a generator with uC triggered starts as needed to replenish a
    large battery. So always juice. But the battery will not directly feed
    the system, there's a current limiter.

    That's probably the same feeling people get when a request comes in via
    collect call, "Dad, I'd need another $500, soon" :)
  12. Jasen Betts

    Jasen Betts Guest

    I've seen 85 degC capacitors (I assume it was the cap) quit in similar circumstances
    When I parked I had to unplug the blinker unit and put it under the
    seat so that it would stay cool. I never got round to replacing the
    cap with a 105 degree part.
  13. SoothSayer

    SoothSayer Guest

    All the Gold utilized has changed all that.

    Hundreds of billions of normal air connections made every day (all
    around the world), and years later still fine. And there is nobody
    wandering around flexing all the lines either. Gold does not oxidize at
    any rate you would understand. Obviously. Platinum, practically never.

    Your experience must have been with tinned 'stingers' on CB radios.

    Cat 5 and 6 say for the last 15 years, has been just fine as far as
    contact issues go.

    And half an amp is trivial.
  14. legg

    legg Guest

    This could simply mean that a change in colour or surface finish of
    the casing is required. Different colours and materials have different
    albedo qualities.

    White is three times better than grey, order of magnitude+ better than

  15. Joerg

    Joerg Guest

    Not sure if the difference is that stark in real life. I own a white car
    and a couple of weeks ago during really hot summer days I had a black
    Volkswagen rental. I didn't feel that much of a difference when coming
    back to the parked car after severak hours.
  16. Joerg

    Joerg Guest

    Most of this civilian computer stuff isn't really geared towards harsh
    life. Even garden tools aren't. I had a shovel with plastic handle where
    the top plastic layer had softened, discolored and became soft. I didn't
    notice until the stuff was stuck to my paws.

    I am surprised this BFC ever made it through a design review. Or maybe
    the ECO was just rubber-stamped and robo-signed :)

    One of the reasons I don't do fixed bid work :)
  17. Guest

    mythbuster tried it, think they saw a difference of 10'F
    between black and white car

  18. legg

    legg Guest

    Really. So then you've got nothing to worry about.

    Solar contribution to thermals ia roughly a kilowatt per square meter
    in the direct overhead case.


    This message has been purged of all expletives or any sentences
    containing them.
  19. Joerg

    Joerg Guest

    Sure, but there is also re-radiation and reflection. Except maybe for a
    Hell Angels Harley or a low-rider rap mobile, vehicles are normally not
    painted a dull "primer black". It's more metallic-anthracite plus a
    shiny clear coat on top.

    But I didn't use any :)
  20. legg

    legg Guest

    Oh, yeah - don't get fahrenheit and centigrade mixed up. This tends to
    happen in anecdotal conversation with Americans.

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