# 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
3
0
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
1,541
409
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