The A5 and A3S printers use a MKS GEN L motherboard, which features replaceable standard “stepstick” format stepper drivers that are responsible for powering and controlling the printer motors. These small modules can be replaced with better quality drivers if desired, or simply swapped out if they are damaged.
The stepper drivers must be set to the correct power level, to ensure the right amount of power is provided for the motor size being used. On each stepper driver, there is a small variable potentiometer (resistor) that sets a reference voltage to tell the driver how much current to deliver to the motor. This potentiometer can be turned with a small screwdriver.
The reference voltage, or VREF, can be measured with a multimeter from the metal potentiometer top, to a ground point on the motherboard. The maximum VREF that a stepper driver is rated for, is typically determined when using a heatsink that is cooled by a fan. In the JGAurora A5 and A3S, there is no internal active cooling inside the main chassis, so care must be taken to avoid the drivers overheating. When stepper drivers overheat, they are not usually damaged, but they will auto-shut down until their temperature drops back down to a suitable working level.
Different models of stepper driver, have different maximum current ratings. To work out the current output, each stepper driver also has a specific formula that allows the output current to be calculated from the reference voltage. Make sure you are using the VREF suggestions for your specific type of stepper driver!
|Example of Ground (GND) point:|
|Example of VRef measuring points (only for A4988):|
The voltages below are recommended for stock A4988 stepper drivers only and are calculated with the formula below:
VREF = Rated Motor Current(2 amps) * 8 * 0.05 = your VREF value to set on the driver.
If you have upgraded your stepper drivers, then it is still important to tune your stepper driver properly!
The motor current must be sufficient to prevent stalling, but too high a current can cause the TMC2XXX to shut down due to overheating. Cooling of the TMC2XXX stepper drivers with a fan is ESSENTIAL for reliable operation. They may be quiet, but they are certainly run hot!
The voltages below are suggested for TMC2130 stepper drivers only (November 2019):
The TMC2XXX stepper drivers have an internal 5 volt voltage regulator that generates their 5V reference voltage from the motor supply voltage (24V). Therefore, the reference voltage can only be set when the 3D Printer is powered on, so that the 24 volt power is available.
The 5V reference voltage is compared to the voltage output of the trimpot (mini-potentiometer) which is used to set the maximum motor current.
If you have the TMC2130 chip, and have wired it for SPI communication (and same with the TMC2208 via UART), you can control the motor current digitally through gcode commands. However, the maximum motor current is still limited by the potentiometer, so make sure you have that turned up sufficiently, before you try using digital control. See this page for more information.
Good info on tune vref for tmc2130 on the voron page here
TODO - good discussion here regarding appropriate vref.