DC ~50v (POE) to 5v current draw calculation

Discussion in 'The Projects Forum' started by Exie, Dec 15, 2014.

  1. Exie

    Thread Starter New Member

    Jan 22, 2008
    Hi Folks,

    Forgive me for a such a simple question, but I'm not very good with this stuff.

    I have a device, it got my multimeter to measure it, and it draws about 1500ma (1.5Amp) @ 5v.

    But I'm thinking of powering it from a POE Ethernet switch. To make that work, I'm thinking about buying an SKE10C-05 which looks pretty straight forward. My question is, how do I work out how many milliamps will it draw from my POE switch.

    The spec says at full load, the converter will draw 270ma, but I presume that will be less if my device is only drawing 1500ma, do I just work it out on percentage terms and guesstimate ~200ma ?

    I'm sure there is a proper calculation to work this out.
    I guess I should also take into account some loss (heat) with the conversion process too ?
  2. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
    I think that your guesstimate is accurate. Power in will equal power out minus whatever is wasted in the converter. The converter is 80% efficient (let's call it 75% to account for manufacturer's optimism) so 50V@200mA = 10W in minus 2.5W (25%) lost in the converter = an output power of 7.5W (5V@1.5A).
    Last edited: Dec 15, 2014