# Multiple A4988 in parallel causing voltage drop

Hi

I am trying to connect three nema17 with three A4988 motor drivers.
The drivers are in parallel configuration, current limit is set to 0.32A on each.
The external power supply is providing 12V and automatically provides the required current. The power supply can provide 36v at 3A.

Using an arduino uno or an arduino mega2560 we have successfully run two drivers in this configuration. The voltage remains at 12v and the current is 0.6A.
However when we add the third a4988, the voltage drops to 7V. Although the current drawn by the circuit is 1A (0.32A by each driver). all three motor run but the performance is bad. We don’t understand why the voltage is dropping…
What could be the possible cause of that? …please advise thanks !

Hello.

Do you have a datasheet or any other specifications you could provide for your stepper motors? Do you have a current limit set on your power supply? How did you set the current limit on your drivers?

Also, when determining the amount of current needed for a stepper, you should be aware that bipolar stepper motors have two coils which can consume power at the same time and the current limit you set is per coil.

-Nathan

hello,

I set the current limit for the A4988 drivers using the Pololu youtube tutorial. and the current limit is set to 0.32A for each motor.
The current limit on the power supply is 1.2A as the drivers are in parallel so the current shouldn’t be exceeding 0.96A.

The circuit works perfectly fine for two drivers and two motors but as soon as we use three drivers, the voltage drops from 12V to 7V. The current is steady at 1A. We do not understand why the voltage drops.

here is the motor that we are using
http://www.phidgets.com/products.php?category=23&product_id=3340_0

here is the datasheet that i found for the motor 42STH38-1684MB-12.

For current-limiting stepper drivers like these, the driver behaves like a switching regulator and the input power (minus some efficiency losses) is roughly equal to the output power; the input current will not be the same as the output current unless you are using a low voltage supply. Also, as I mentioned before, each stepper motor drives two coils and the current limit is set per coil. The current supplied to both coils depends on the microstep position, but it is between 100% and about 142% of the single coil current limit.

What voltage did you set VREF on the driver boards to? Can you post pictures of your power supply and test setup here? Some that show the voltage and current display of the power supply before and after connecting the third driver would be helpful.

-Nathan