I have the DRV8825 board and I purchase a lot of them… i’m using them in a product I manufacture. In some instances I have a need where I want to leave the drivers Enabled so that the motor will hold position when not in use, however I do not want them to hold with full current potential. I would like them to hold with some limited current say 50% for now. There are vertical applications where I want the motors to be enabled so they do not fall under gravity but they don’t need to hold with their entire 2.8a current as that will just make the motors stinking HOT and they start to squeal.
Is there any example circuit or implementation anyone can think of to vary the current (Vref) externally or remotely? Would just limiting the VMOT voltage work?
The most straight forward way to do that with the DRV8825 is probably to change the voltage at the VREF via. The schematic diagram we provide for the driver board shows the potentiometer circuit used to set VREF. If you have a microcontroller with a digital to analog converter (DAC), you can just provide a voltage to the VREF via that corresponds to the voltage you wish to output. Otherwise, you might use something like a pulldown resistor between the VREF via and a GPIO pin on the microcontroller to lower the voltage.
The Toshiba TB67S249FTG stepper driver IC has an Active Gain Control (AGC) feature that changes the amplitude of the current in the stepper motor coils based on the torque required by the motor and we have a few different breakout boards for it in different form factors that might be interesting for you. Also, you might look at our MP6500 Stepper Motor Driver Carrier, Digital Current Control board, which has digital inputs for controlling the current.
Limiting the VMOT voltage would probably not work well. If VMOT is below the voltage required to run the coils of your stepper at the current limit you set, microstepping will be distorted. Also, many common stepper motors have a coil voltage below the minimum input voltage for VMOT, so VMOT cannot be lowered enough to limit the current more than the current limiting feature of the driver.
