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Understanding and Setting Vref for Stepper Drivers in 3D Printers

Adjusting the Vref (voltage reference) for stepper drivers is crucial when changing or replacing drivers in devices such as 3D printers, laser cutters, and other CNC machines. This guide explains how to calculate and adjust Vref for common drivers like the A4988, TMC2208, and TMC2209.

What Is Vref?

Vref stands for "voltage reference" and regulates the amount of electrical current delivered to the stepper motors. Proper Vref settings ensure the motors receive the right amount of current, preventing issues such as losing steps due to insufficient current or overheating from excessive current. Both the stepper motors and drivers have optimal current ranges that must be respected to maintain performance and longevity.

What You'll Need

Before adjusting Vref, gather the following tools and information:

  • Motor Specifications: Know the rated current for your stepper motors, usually provided by the manufacturer. The RepRap community maintains a detailed database for common NEMA 17 motors.
  • Tools: Digital multimeter, power supply, and a plastic or ceramic-tipped screwdriver (1.5 mm recommended).

A4988 Drivers

The A4988 driver is widely used in desktop machines, including 3D printers. To calculate Vref for the A4988:

Formula: Vref = I * 8 * Rsense

  • I: Desired motor current (typically 10% below the motor's rated current).
  • Rsense: The current sense resistor value on the driver board (e.g., 0.1 Ω).

Example Calculation:

  • Motor rated current: 0.9 A
  • Desired current: 0.8 A
  • Rsense: 0.1 Ω
  • Vref = 0.8 * 8 * 0.1 = 0.64 V

Adjustment Steps:

  • Connect the driver to the controller board without powering it.
  • Power the board via VDD and GND.
  • Set the multimeter to DC voltage (around 2 V).
  • Ground the black probe and touch the red probe to the driver potentiometer to measure Vref.
  • Adjust the potentiometer to reach the calculated Vref value.

TMC2208 & TMC2209 Drivers

The TMC2208 and TMC2209 drivers by Trinamic Motion Control are known for their quiet operation and advanced features like UART mode. In standalone mode, the Vref calculation for both drivers is similar:

Formula: Vref = IRMS * (1.414 * Rsense)

  • IRMS: Root mean square current (motor current divided by √2).
  • Rsense: The current sense resistor value (e.g., 110 mΩ for BigTreeTech stepsticks).

Example Calculation:

  • Motor rated current: 0.9 A
  • Desired current: 0.8 A
  • IRMS: 0.8 / 1.414 ≈ 0.57 A
  • Rsense: 110 mΩ
  • Vref = 0.57 * (1.414 * 0.11) ≈ 0.8 V

Adjustment Steps:

  • Follow the same procedure as for the A4988, but place the red probe on the designated pin instead of the potentiometer.

Summary

Setting the correct Vref is essential for the proper operation of stepper motors and drivers. Use the above methods to calculate and adjust Vref for A4988, TMC2208, and TMC2209 drivers to ensure optimal performance and avoid potential damage. Proper adjustment helps maintain precision and extend the lifespan of your 3D printer or CNC machine.

By following these guidelines, you can become proficient in tuning your stepper drivers, ensuring they operate within safe current margins and contribute to the reliable performance of your equipment.

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