Switch to Bipolar motors #24

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2026-07-10T08:28:53+02:00

osiu97 commented

2026-07-10 08:28:53 +02:00

Current steppers can be converted to bipolar. TMC drivers can be used to drive them then, possibly with stealthchop getting them even quieter

osiu97 added the Kind/Enhancement

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labels 2026-07-10 08:29:10 +02:00

osiu97 commented

2026-07-10 16:05:34 +02:00

TMC drivers are smaller which is nice for the build but might need more pins if UART needed for StealthChop

osiu97 commented

2026-07-16 08:54:44 +02:00

28BYJ-48 Unipolar to Bipolar Hack & A4988 Wiring Guide

Converting the 28BYJ-48 from a 5-wire unipolar to a 4-wire bipolar motor is a straightforward hardware hack. By default, the motor's two internal coils are tied together in the middle by the red wire. To use a bipolar driver like the A4988, you must break that connection so the coils can operate independently.

1. The Bipolar Hack

Remove the plastic cover: Use a small flathead screwdriver or hobby knife to carefully pry off the blue plastic cover on the back of the motor.
Locate the center tap trace: Look at the small printed circuit board where the wires are soldered. You will see a small copper trace connecting the two center solder pads.
Cut the center trace: Use a sharp hobby knife or scalpel to sever this middle copper trace. Make sure there is no copper bridging the gap. (Ensure a deep cut to break electrical contact).
Remove the red wire: Because the common center tap is severed, the red wire is completely disconnected and useless. Snip it off close to the housing to prevent confusion.
Reassemble: Snap the blue plastic cover back into place. Your motor is now a 4-wire bipolar stepper.

2. Wiring to the A4988 Driver

The A4988 driver controls two independent coils (Coil 1 and Coil 2). Because of the hack, your 28BYJ-48's wiring is now paired as Blue/Yellow for one coil, and Pink/Orange for the other.

Wire the motor to the A4988 output pins in this exact order:

2B: Blue
2A: Yellow
1A: Pink
1B: Orange

(Note: If the motor spins in the opposite direction than expected, simply reverse the entire order of these four wires.)

Logic and Power Pins

VMOT & GND: Motor power supply. The A4988 requires a minimum of 8V to operate. You must supply 9V or 12V here — 5V logic power will not work.
VDD & GND: Logic power. Connect this to the 5V or 3.3V pin on your microcontroller.
STEP & DIR: Connect to your microcontroller's chosen digital output pins.
SLEEP & RESET: Jump these two pins together on the A4988 board, otherwise the driver will remain disabled and will not move the motor.

⚠️ Important: Current Limiting (VREF)

Running a standard 5V 28BYJ-48 motor on a 12V power supply is perfectly fine, but only if you limit the current. If you skip this step, the A4988 will dump too much current into the tiny coils and melt them immediately.

Before sending any step commands, you must tune the tiny potentiometer on the A4988. Use a multimeter to measure the VREF voltage (between the metal top of the potentiometer and ground). Turn it down until the VREF reads around 0.1V to 0.15V. This limits the output to roughly 100-150mA, which provides plenty of holding torque for a 3D printed mechanism without overheating the motor.

Reference:

Basic Setup of 28BYJ-48 with A4988 (YouTube): https://www.youtube.com/watch?v=kCDGEezyzfw

# 28BYJ-48 Unipolar to Bipolar Hack & A4988 Wiring Guide Converting the 28BYJ-48 from a 5-wire unipolar to a 4-wire bipolar motor is a straightforward hardware hack. By default, the motor's two internal coils are tied together in the middle by the red wire. To use a bipolar driver like the A4988, you must break that connection so the coils can operate independently. ## 1. The Bipolar Hack 1. **Remove the plastic cover:** Use a small flathead screwdriver or hobby knife to carefully pry off the blue plastic cover on the back of the motor. 2. **Locate the center tap trace:** Look at the small printed circuit board where the wires are soldered. You will see a small copper trace connecting the two center solder pads. 3. **Cut the center trace:** Use a sharp hobby knife or scalpel to sever this middle copper trace. Make sure there is no copper bridging the gap. (Ensure a deep cut to break electrical contact). 4. **Remove the red wire:** Because the common center tap is severed, the red wire is completely disconnected and useless. Snip it off close to the housing to prevent confusion. 5. **Reassemble:** Snap the blue plastic cover back into place. Your motor is now a 4-wire bipolar stepper. --- ## 2. Wiring to the A4988 Driver The A4988 driver controls two independent coils (Coil 1 and Coil 2). Because of the hack, your 28BYJ-48's wiring is now paired as **Blue/Yellow** for one coil, and **Pink/Orange** for the other. Wire the motor to the A4988 output pins in this exact order: * **2B:** Blue * **2A:** Yellow * **1A:** Pink * **1B:** Orange *(Note: If the motor spins in the opposite direction than expected, simply reverse the entire order of these four wires.)* ### Logic and Power Pins * **VMOT & GND:** Motor power supply. The A4988 requires a minimum of 8V to operate. You must supply 9V or 12V here — 5V logic power will not work. * **VDD & GND:** Logic power. Connect this to the 5V or 3.3V pin on your microcontroller. * **STEP & DIR:** Connect to your microcontroller's chosen digital output pins. * **SLEEP & RESET:** Jump these two pins together on the A4988 board, otherwise the driver will remain disabled and will not move the motor. --- ## ⚠️ Important: Current Limiting (VREF) Running a standard 5V 28BYJ-48 motor on a 12V power supply is perfectly fine, **but only if you limit the current**. If you skip this step, the A4988 will dump too much current into the tiny coils and melt them immediately. Before sending any step commands, you must tune the tiny potentiometer on the A4988. Use a multimeter to measure the VREF voltage (between the metal top of the potentiometer and ground). Turn it down until the VREF reads around **0.1V to 0.15V**. This limits the output to roughly 100-150mA, which provides plenty of holding torque for a 3D printed mechanism without overheating the motor. **Reference:** * Basic Setup of 28BYJ-48 with A4988 (YouTube): https://www.youtube.com/watch?v=kCDGEezyzfw

osiu97 commented

2026-07-16 08:55:19 +02:00