Figuring out current draw

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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kmikemoo 29 days ago +1

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…
michaelkellett 28 days ago in reply to dang74 +1

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…
michaelkellett 23 days ago in reply to inavitech +1

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

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dang74 29 days ago

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.
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dang74 29 days ago

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.
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michaelkellett 28 days ago 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
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