Stepper Drivers

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A stepper driver is a chip which takes the signals from the microcontroller and determines the electrical current which must be sent to a stepper motor to move a given number of steps in a given direction or hold at a particular position (or sub-position if using micro-stepping)

Pololu A4988 Stepper Drivers


Pololu A4988 stepper driverDriver with labels.jpg

The A4988 stepper motor driver carrier is a breakout board for Allegro’s easy-to-use A4988 microstepping bipolar stepper motor driver and is a drop-in replacement for the A4983 stepper motor driver carrier. The driver features adjustable current limiting, overcurrent protection, and five different microstep resolutions. It operates from 8 – 35 V and can deliver up to 2 A per coil (MAX).

The potentiometers (POTS) on the drivers must be adjusted to control the current passing through the drivers. *See notes on heat below* We want to limit the driver to 1.0 amps without a heatsink and up to 1.5 amps with a heatsink, so we adjust the POT until we measure 0.4-0.6 volts at the marked point. Calculate this for yourself with the formula V(ref) = I(limit) * 0.4

TIP: You'll need a really small screw driver to get the pot to turn.

The ground pin to measure is not circled in the above drawing. It is the pin in the corner closest to the potentiometer screw. Verify this by looking on the bottom at the driver, the silkscreen will say gnd. TOUCHING ANY OF THE PINS SIMULTANEOUSLY (e.g. if the ground from your meter contact two of the closely spaced pins) COULD IMMEDIATELY FRY THE DRIVER. One method to deal with this is to tack a jumper wire to the ground pin with a bit of solder before any power is applied. You can then wrap the other end of the wire around your voltmeters ground lead and place it to the side and free a hand to either turn the pot or contact the marked point.

See the driver shield user guide or A4988 DMOS Microstepping Driver with Translator and Overcurrent Protection for more information.

Pololu offered the following advice on setting the current level:

We usually recommend that you do not use the VREF to set the current limit for the driver as this method is harder to get right compared to adjusting the limiting while measuring the actual coil current with a multimeter or similar tool. When setting the current limiting by measuring the actual coil current, you should have your motor hooked up and powered on. If you have to use VREF to set the current, you do not have to have the motor hooked up, or the output enabled.

Here is some advise to new users experiencing heat related problems:

A 24V power supply is overkill in my opinion and is creating many problems for new users with overheating. By default, the driver boards you received will be calibrated for 1A current at 12V. If you fail to adjust the potentiometer down before using them in a stepper shield with a 24V power supply, you are essentially using them at 1.6 - 2 amps, Very close to the max rated current and you may destroy them if there is no heat sink.

Although the allegro a4988 "Pololu" drivers are rated to 35V, Using them at anything above 15-18V creates tremendous excess heat. My advise is to turn down your 24V power supply to 18V. Then adjust your driver current limit pots down before you plug them in. Finally, tune the pot *VERY SLOWLY* to achieve smooth stepper operation. You can tell that the setting is correct by the sound of your steppers and the fact that your chips will be very warm to the touch during continuous operation. The sound should be smooth and quite buzzing. If your chips are so hot you cannot touch them then you are running too high voltage / too much current.

I have seen many different suggestions about tuning the pot by measuring the vref voltage and calculating the current (including the instructions linked above). Again, in my opinion this is totally unnecessary. Just start low and slowly work your way up for smooth operation. No measurements required. Note that it is not necessary to achieve any specific current setting, regardless of what your steppers are rated for.

In addition, if you really have a boner for using 24V, go right ahead. Just be sure to start with the pot VERY low and adjust VERY VERY VERY slow just like outlined above.