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Could someone explain this?

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Hi, i have an 2A water pump and a 1A powersupply. I wanted to limit the current to 1A that is going to the pump beacause it was pumping to fast (and the powersupply is only 1A).

So i calculeted 12/1=12ohms and 12*1=12W. And the nearest one i found in my personal stock was 10ohms 5W. (i will go and buy a 10W one soon)

And when i measured the current with only the resistor and it was 1,05A but when i had both the pump and the resistor the current was 0,61A. With only the pump the current was 1,23A.

So that the pumps resistance should be 12/1,23=9,7ohms, then I measured the resistance of the pump and it was 4,5ohms.

So could someone explain this to me? :) And how many ohms resistor should i have to get the current to 1A, should it be 12-4,5=7,5ohms or 12-9,7=2,3ohms?


Active Member
4.5 Ohms is the off-state resistance of the pump which increases during run time because of generation of back-EMF in motor armature. And you want to limit the on-state current to 1A so use the on-state resistance in your calculation i.e. 9.75 Ohms. The final resistance value would be 12 - 9.75 = 2.25 Ohms.
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