determine necessary current to drive a stepper motor?
This type of stepper does not use much current, in the datasheet, you can see that

Stepper has gear reduction 1:64

Step angle $${360\text{º}\over64\text{º steps}} = 5.625º\text{ pr step}$$

which is full-step.

Resistance per phase is 

$${12\text{V}\over300\text{Ω}} = 40\text{mA}$$

If you need more current than the circuit can provide, then you must use a power driver like this.



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