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Electronics for CNC apparates

Last update: 31/07/2015

If someone needs to power small bipolar stepper motors, there are a lot of small integrated controller/drivers producted by Allegro and other manufacturers. Among them the Allegro A3987 and the Allegro A4983 are really interesting because they can drive bipolar stepper motors upto 1,5-2A using microstepping (upto 1/16).


Unfortunately the package of the A4983 (28 pins QFN) is too particular to be soldered in a home environment while the Allegro A3987 even if really small (4.5x7.8mm) can be still soldered, using normal manual devices, having a traditional 24 pins TSSOP package.
Both need very few external components and are particularly indicated to be controlled by microprocessors or in small cnc machines.
The Allegro A3987 follows the traditional Allegro configuration (internal translator, external current control resistors, pins to select the step mode, external enable pin). Characteristics:
  • upto 50V;
  • 1.5A/phase max; 
  • 3-5V logic;
  • automatic selection of the decay mode;
  • reduced motor noise;
  • PWM current control;
  • VRef 0-4V;
  • microstepping upto 1/16.
The current value is set according to the following formula:
ITRIP (max)  = VRef/ (8xRsense). Therefore, using a 0.25ohm sense resistor,the value of VRef can be set to:
3.0V = 1.5A    2.5V = 1.25A    2.0V = 1A    1.5=0.75A

allegro a3987





BOM Part                        Value

C1                                    100uf   
C2,C3                             .1uF     
C4                                    0.22uF    
IC1                                   LM340MP-05    
JP1                                  PINHD-1X2      
R1,R2                              0.25          
R3,R4,R5,R7,R10,R11   10k     
R6                                   2k      
R8                                   5.6k   
R9                                   47k     
SJ1,SJ2                          jumpers
SV1, SV2                        MA04-1       
Main IC                          A3987
 L  FULL STEP        








 The PCB has not been proposed because too many modifications are possible accordingly to the main power supply.
Please remember that the max voltage input value for LM340 is 22-24V. If the voltage is higher remove it from the schematic and use an external 5V supply for the logic of the A3987 (VDD) or insert a circuitery to low the voltage value before the LM340. 
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