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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.

  • 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.

  • If you are using a switching regulator to convert 28V to 5V the current on the input would be 3A x 5/28 /efficiency.  For example if the switching regulator efficacy is 85%... it would work out to.... 3A x5/28/0.85= 0.63A. 

  • in reply to dang74

    And you need to take account of switch on surges - it's often very hard to get good information from suppliers..

    In cases like this you need to choose the closest replacement you can find and do some testing and measuring in the actual application.

    MK

  • Hello, when designing also consider long term eg is this running 24/7 will components need extra heat dissipation 

    will the average monkey be pushing and twisting the USB connector can you add extra support for the usb socket or have the socket on fly leads  

    is there any consideration for cable going short circuit, is the supply clean or is it automotive and has has spikes etc

  • That's the thing, just can't find any formulas to do the computations. Guess I'm going to have to wait until the aircraft comes in and pull one of the outlets they want to replace and see if the data plate has any good information to help determine what the circuit is drawing off the branch and go from there.

  • in reply to inavitech

     Go the simpler route.  First:  Decide that you want the same output capacity as the existing system.  If the system that you are replacing is old, the odds are greatly in your favor that new switching supplies will be more efficient - unless you purchase them on Ali Express or eBay.  Buy a new one.  Load it.  Measure it.  Multiply that by the number of outlets you are relacing.  Determine if that draws less than the current system.

    If you are trying to incorporate that with a load demand analysis (what they really use), the only reliable method is a load study.  Whoo-hoo!  YOU get a data logger!  And YOU get a data logger. etc.

  • in reply to inavitech

    If this is on a plane won't it need the full certification stuff ?

    MK