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Electronics for CNC apparates

Last update: 31/07/2015

Almost always the working current of a stepper motor is set by a simple voltage (Vref) applied on a pin of the driver/controller i.c. This voltage, usually, is taken from the +5V necessary to the logic of the same i.c. and it is set by a voltage partitioner built using a couple of resistors. We'll try now to calculate the values of such resistors:




partitore tensione  

In the simplest form of a partitioner we'll find only a couple of resistors R1-R2 that will set the Vref according to the following formula:


Vout = Vin * (R2 / (R1+R2))  
then, as an example, assuming Vin=5V R1=3k R2=6K we'll have 1.66V on Vout.


Such solution could be perfect if we were going to create a controller for a specific motor (and with a specific current/phase). We could set, therefore, the Vref to a fixed voltage.  


Unfortunately, usually we must consider that a controller/driver could be connected to different motors with different currents/phases. In this case, therefore, we must be able to vary the Vref according to the motor connected to the driver. 

The solution consists interposing in the middle of the two resistors a trimmer, a variable resistor: 

partitore con trimmer  

Using the above mentioned formula we 'll know that the voltage on R1-R3 will be:



Vr1 = Vin * ((R3+R2)/(R1+R2+R3) while the voltage on R3-R2 will be:

Vr2 = Vin * R2 / (R1+R2+R3)


example: Vin=5V, R1 e R2 = 1K, R3 = 5K  quindi


Vr1 = 5*(6k/7k) = 4.28V

Vr2 = 5*1k /(7k)= 0.714V


The Vref therefore can vary between 0.71 and 4.28V


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