# How do I set the a4988 driver to handle a .4A motor?

The vref calculation that I have found online is as follows:

I_TripMax * 8 * .1Rs

I_TripMax is the current going through one winding of the motor. The current/phase on the motor’s datasheet is the current for both windings (or .4A).

I have read that the current rating shown on the datasheet of the motor is 70.71% of I_TripMax. If this is true, do I take the current rating of .4A and divide it by .7071?

if I_TripMax is .4A / .7071 = .57A then the Vref calculation should be .57A X 8 X .1 = .456… This doesn’t seem right. What am I missing?

Hello.

The correct calculation to use to set the reference voltage on our A4988 drivers depends on which current sense resistors your A4988 driver has. You can find that equation, along with pictures that help you identify which resistors you have and how to modify the equation, under the “Current limiting” section of the A4988’s product page.

The 70.71% value you noticed in the datasheet is the percentage of I_tripMax current that is allowed to flow through each coil when the driver is in full step mode. (This value reaches 100% in the microstepping modes, so if you are planning to use microstepping, you should not need take that 70.71% value into account.) We have written an example of how to deal with that value at the end of the “Current limiting” section of the A4988’s product page. If there is anything in there that you do not understand, let me know specifically what you are having trouble with and I can see if I can help.

-Jon

I think I have that issue figured out know… Thanks for the response. I do have another question regarding the DRV8825. I am using Marlin for a 3D printer and was wondering if I need to double the steps because of the 1/32 stepping? Also, I have heard that the DRV8825 skips steps at certain speeds. If I run the DRV8825 in 1/16, will it eliminate any of this? What would be a benefit of running it at 1/16?

Thanks,
Rick

I recommend talking to the manufacturer of your 3D printer since how well that driver performs in each microstepping mode depends on the particular system it is used in.

-Jon