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Thread: POE Circuit

  1. #1
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    POE Circuit

    So i have developed a POE Board for the Teensy 4.1 and it works great provided i force the POE to ON at the Switch or use a dumb power injector.

    It will also work if i set it to auto at the switch however it then shuts off because my power use is so low it thinks there is a problem

    Basically the board plus sensors and the Teensy 4.1 are consuming about 54ma and i need the consumption to be at least 200ma

    I need some help with a circuit that can dump that excess power as heat.

    I'd appreciate any pointers.

    Thanks

    Tom

  2. #2
    Junior Member
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    Feb 2021
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    Lights are always a fun way to waste power (LEDs can be had that consume as much as 30ma, and there are light bars, too).

    Of course you could also use a power resistor.

    Given the power over ethernet is usually a very high voltage (compared to what ICs use), 200ma seems like an unreasonable draw requirement.

    Where did you get that number?

  3. #3
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    Thanks Gary.

    SO yeah the POE incoming is 48 Volts however there's a DC Buck converter in my design that down converts it to 5V and is capable of drawing up to 3A. However I'm not drawing anything close to that.

    Average utilization under full load the POE Switch reports 54ma of consumption on the port.

    The 200ma number was straight up pulled out of thin air based on what i'm seeing from other POE devices on my network.

    For example a Cisco 3800 Series Access point consumes on average 148mA at 48 Volts PoE Out voltage of 48 V and PoE Out Power of 7 W

    My PoE Circuit is only consuming 54mA at 48 Volts 2.4 W

    the switch thinks that the device is malfunctioning if it's not drawing at least 100mA I figured if i had a circuit to work from i could tune the draw accordingly.

    Alternatively i can leave the port to Forced On and it will work fine that way to but then i have some danger ports on my network that if i plug in a non poe device it could damage it.

    If i were to use a power resister i'm not exactly sure where to place it. Would i put it in circuit ahead of the buck regulator ?

  4. #4
    Junior Member
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    I would think you'd want to load the POE with the resistor, but something isn't right.

    I know Wikipedia isn't exactly the best for accurate information, but here's what the POE page says:

    "To retain power, the PD must use at least 510 mA for at least 60 ms at a time. If the PD goes more than 400 ms without meeting this requirement, the PSE will consider the device disconnected and, for safety reasons, remove power."

    This seems much more reasonable than having to draw a continuous 200ma of POE.

    At 48V, 10ma is 0.48 watts. But at 5V, that's 98ma.

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