Adjust Stepper Driver Current

Adjust Stepper Driver Current

Before you can adjust your stepper driver current you need to know

  1. What is the Vref (reference voltage) formula for the driver?
  2. What is the Rcs (current sense resistance) value for the driver?
  3. What is the rated current/phase of the motor?
  4. How many motors are connected to the driver? If more than one connected how will the motors be wired?

The first three questions can be answered by using the information contained within the products data sheet that will come with the product or can be sourced online. Question four is down to the printer design your are following. Some printers use two stepper motors for the Z axis or maybe you are multi-printing. Either way if you are running two or more motors on one driver the way you connect the motors will effect the reference current you are about to set on the drivers. If you wire two motors in series then connect them to one driver the Vref will be set to the current rating of one motor. If you wire two motors in parallel then connect them to one driver the Vref will be set to twice the current rating of one motor.

The two drivers we can use on our Rumba control board are the A4988 and DRV8825. Both of these drivers can be used with our chosen Nema 17HS4401 stepper motors. These motors have a rated phase current of 1.7 Amps. If we were going to use the A4988 driver which is stated to have a 50 mOhms Rcs value

The formula for the A4988 driver is Vref = 8   x   I max   x   Rcs

Therefore                                             Vref = 8   x   1.7   x   0.050   = 0.68 Volts       So you would trim the driver to 680 mV

If we were going to use the DRV8825 driver

The formula for the DRV8825 driver is Vref = Current Limit   /   2

Therefore                                             Vref =           1.7     /    2       = 0.85 Volts        So you would trim the driver to 850 mV

To adjust the drivers you will need a 1.5mm straight or small Phillips screwdriver and a multimeter (right hand pic demonstrate clipping alligator to screwdriver). A ceramic screwdriver can be purchased cheaply on line and is a much safer way to adjust the drivers.

  1. Ensure you motor cables are disconnected from the drivers and switch on board
  2. Set your multimeter to 2 Volts dc and switch on
  3. Clip the positive alligator clip to the shaft of the screwdriver
  4. Place the negative probe on the negative terminal of the boards main power connector
  5. Carefully place the screwdriver into the adjustment screw of the trim pot (potentiometer)
  6. While reading your multimeter rotate the screwdriver to achieve your Vref setting. Clockwise to increase or counter clockwise to reduce the current limit

Some useful links

Setting Driver Current Limit Video

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