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The A4988 supports such active current limiting, and the trimmer potentiometer on
the board can be used to set the current limit. One way to set the current limit is to put
the driver into full-step mode and measure the current running through a single motor
coil while adjusting the current limit potentiometer. This should be done with the
motor holding a fixed position (i.e. without clocking the STEP input). Note that the
current you are measuring is only 70% of the actual current limit setting, since both coils
are always on and limited to this value in full-step mode, so if you later enable
microstepping modes, the current through the coils will be able to exceed this
measured full-step current by 40% (1/0.7) on certain steps; please take this into
account when using this method to set the current limit. Also, note that you will need
to perform this adjustment again if you ever change the logic voltage, Vdd, since the
reference voltage that sets the current limit is a function of Vdd.
Note: The coil current can be very different from the power supply current, so you
should not use the current measured at the power supply to set the current limit. The
appropriate place to put your current meter is in series with one of your stepper motor
coils.
Another way to set the current limit is to calculate the reference voltage that
corresponds to your desired current limit and then adjust the current limit
potentiometer until you measure that voltage on the VREF pin. The VREF pin voltage is
accessible on a via that is circled on the bottom silkscreen of the circuit board. The
current limit, I
, relates to the reference voltage as follows:
MAX
or, rearranged to solve for VREF:
R
is the current sense resistance; original versions of this board used 0.050 Ω current
CS
sense resistors, but we switched to using 0.068 Ω current sense resistors in January
4
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