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Converting 28BYJ-48 Stepper to Bipolar for Microstepping


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This mini-project is as a result of the focussing on my lens + FW + ASI1600MM-Cool widefield imaging rig being too coarse with 28BYJ-48 stepper motor and usual driver board.

The ubiquitous little 28BYJ-48 stepper motor is connected internally as a unipolar stepper motor with the centre tap on the windings connected together.  This prevents it from being used as it is with bipolar motor drivers such as the A4988 which provide micro-stepping.  These drivers need the two windings to be separate as it uses them in an H bridge.  However, there is a relatively simple modification that can be applied to the 28BYJ-48 to break the link between windings for anyone capable of handling small things and pliers or wire cutters and a craft knife.

The motor is held together by 4 tiny lugs on the casing which are bent inwards to secure the output shaft plate.  Using strong small pliers or wire cutters these little lugs can be prized outwards to release the end plate.  Then the innards of the motor can be extracted carefully from the round casing complete with wires and blue plastic piece.  Sliding the plastic bit up the wires reveals the coil connections as shown below.  Next job is to cut the track on the PCB that joins the two windings and cut off the red wire, which is no longer used.  Finally the motor can be reassembled.  For use as a bipolar motor the coils are yellow-blue and orange-pink.

One further point to note...  Each half winding is rated at either 5v or 12v as printed on the back of the motor so when the whole of each winding is used this doubles the voltage rating.  Also, the A4988 or similar has a current setting and this is used to control the power to the stepper motor, so the supply voltage can be more than twice the voltage printed on the case.  Thus a 5v motor can be run on 12v or even 13.8v as often used in observatories and provided by a nominally 12v car or pleasure battery.

With this modification and A4988 stepper motor controller with micro-stepping, finer control can be provided from a 28BYJ-48 stepper motor used in a remote focussing system.

Here are some photos :-

  1. Coils and connections
  2. Link severed
  3. Other parts of the motor
  4. Plastic part pushed back into position and assembly put back in case
  5. Rotor replaced
  6. Gearbox replaced but not yet aligned - care must be taken to engage the teeth of the rotor pinion with the first gear in the train.

Focus motor mods 01.jpg

Focus motor mods 02.jpg

Focus motor mods 03.jpg

Focus motor mods 04.jpg

Focus motor mods 05.jpg

Focus motor mods 06.jpg


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