Question about POE and 12v devices

Discussion in 'Networking & Security' started by DouglasteR, Jan 7, 2013.

  1. DouglasteR

    DouglasteR Limp Gawd

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    Hi there my friends :p

    I have a simply yet worrying question !

    Lets say i have a passive POE Injector of 48v, and an IP camera (that uses 12v POE and 12v input directly), when i use it by POE, it will use only 12v of the 48v supplied or i should use a poe adapter to downscale the voltage ? Because im worried that it may kill the camera.

    Thanks for the help =*
     
  2. goodcooper

    goodcooper [H]ardForum Junkie

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    absolutely depends on the camera... if you're asking if you CAN kill the camera, you most definitely can...

    whether or not it will work will depend on the camera...
     
  3. DouglasteR

    DouglasteR Limp Gawd

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    Humm, thanks for the help. Im definitely worried :(
     
  4. BigBadAl

    BigBadAl Limp Gawd

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    Voltage is pushed/forced - think the SPEED of water out of the end of a hosepipe.

    Current it drawn/pulled - think AMOUNT/VOLUME of water out of the end of the hosepipe.

    Voltage is there, that`s what you get, without a `device` to lower the voltage, the 48V from the POE will rush into the camera, it`s an almost guarantee the magic smoke will escape!

    Current is supplied, it is there to be drawn on, it is available to the devices to take if they need it.
     
  5. DouglasteR

    DouglasteR Limp Gawd

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    Looks like im lucky that all my 4 ip cameras are capable of stepping down 48v :p

    Thanks guys, really appreciate it.
     
  6. jjeff1

    jjeff1 Limp Gawd

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    There is some confusion here.

    When people use the term POE, they mean 802.3af POE, which is a standard. A device supports it or it doesn't. When using POE, the power source (switch or injector) will negotiate with the endpoint device to determine how much power is needed and other controls. You can even plug in a non-POE supporting device into a POE port, no harm done. Without the POE negotiation, no power is actually sent.

    Now, some devices, like this D-Link thing here:
    http://www.dlink.com/us/en/support/product/dwl-p200-power-over-ethernet-adapter-kit

    Use the same concepts of 802.3af to inject power into the wire at one end, and then "remove" it at the other, with a separate 12 volt power jack. But this isn't 802.3af, and you shouldn't attempt to use the injector without the receiver.

    There is another POE standard, called 802.3at, which supports more current (watts) than 802.3af. It's backwards compatible.

    Any time you're doing POE, you really want devices that support one of the standards, and hopefully you can get a managed switch as your power source, which makes troubleshooting much easier.
     
  7. DouglasteR

    DouglasteR Limp Gawd

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    Humm, so first the POE senses the impedance on the line, and then send the actual needed voltage ?

    This explain a lot.

    Also, do POE cameras get hot ? very warm i mean (to the touch)

    Thanks jeff
     
  8. jjeff1

    jjeff1 Limp Gawd

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    It's quite a bit more complicated than simply measuring impedance, it's enough to know that there is a handshake between the power device and the endoint that occurs before any power is actually delivered. The voltage never changes, just the wattage.

    Anything that uses power will heat up. Your camera might simply be inefficient or it might have heaters in it if it's designed to work outdoors in cold weather environments.
     
  9. DouglasteR

    DouglasteR Limp Gawd

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    Very nice jjeff, thanks for the answers.

    Also, The Dahua representative said that when the IP Camera or other POE devices says : "12v - POE" it actually means that it use 12v from the local power supply or 48v poe.

    This settles it.


    THanks for the help guys.
     
  10. jjeff1

    jjeff1 Limp Gawd

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    Yep. if you look at the spec sheets for those cameras, they list power source as "DC12V/AC24V, PoE(802.3af)"

    So you can use standard POE, or a 12VDC or 24VAC power supply.
     
  11. goodcooper

    goodcooper [H]ardForum Junkie

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    actually, that particular dlink device does run on 802.3af... have 4 of them hooked up to a POE switch (didn't use the injector part) running 4 12 volt devices in hard to reach areas... if you check the specs it takes a full 48v