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Figuring out current draw

inavitech

So I'm working on a project with an unknown and not sure how to figure out what I need to know. My customer wants new USB chargers and I'm trying to figure out if the circuit will handle the load of the new chargers. The current charger is a single USB A. Input is 28VDC, output is 5VDC at 3 AMPS, 15 WATTS. Not sure what formula to use to compute current draw on the input side. My new chargers consume 1.25AMPS each.

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  • There really isn't a formula that is going to give you the answer that you seek.  "P = (E * I) + losses" isn't much help.
    If you measure your input current draw, you can figure out what those losses are but even circuit simulators aren't going to be able to predict what you will really lose across the device.  Your load will also likely exceed 1.25A from time to time.  28V to 5v sounds like adding USB charging to industrial equipment.  If you put a USB outlet there... people will use it for all sorts of things.

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  • There really isn't a formula that is going to give you the answer that you seek.  "P = (E * I) + losses" isn't much help.
    If you measure your input current draw, you can figure out what those losses are but even circuit simulators aren't going to be able to predict what you will really lose across the device.  Your load will also likely exceed 1.25A from time to time.  28V to 5v sounds like adding USB charging to industrial equipment.  If you put a USB outlet there... people will use it for all sorts of things.

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