This product is a carrier board or breakout board for Allegro’s A4988 DMOS Microstepping Driver with Translator and Overcurrent Protection; we therefore recommend careful reading of the A4988 datasheet (1MB pdf) before using this product. This stepper motor driver lets you control one bipolar stepper motor at up to 2 A output current per coil (see the Power Dissipation Considerations section below for more information). Here are some of the driver’s key features:
This carrier has reverse power protection on the main power input and built-in 5 V and 3.3 V voltage regulators that eliminate the need for separate logic and motor supplies and let you control the driver with microcontrollers powered at 5 V or 3.3 V. We also sell a smaller, higher-performace version of the A4988 carrier without voltage regulators.
Like nearly all our other carrier boards, this product ships with all surface-mount components—including the A4988 driver IC—installed as shown in the product picture.
Some unipolar stepper motors (e.g. those with six or eight leads) can be controlled by this driver as bipolar stepper motors. For more information, please see the frequently asked questions. Unipolar motors with five leads be used with this driver.
The A4988 stepper motor driver carrier with voltage regulators comes with 0.1″ male header pins that can be broken into smaller strips and soldered in for use with solderless breadboards or 0.1″ female connectors. You can also solder your motor leads and other connections directly to the board.
|Minimal wiring diagram for wiring a 5V microcontroller to an A4983/A4988 stepper motor driver carrier with voltage regulators (full-step mode).|
The driver requires a logic supply voltage (3 – 5.5 V) to be connected across the VDD and GND pins and a motor supply voltage (8 – 35 V) to be connected across VMOT and GND. The logic voltage can be supplied from an external source, such as that powering the logic of the rest of the system, or by jumpering the output of the 5 V or 3.3 V voltage regulator outputs to VDD. There are also surface-mount pads that allow VDD selection to be made by making a solder bridge across the appropriate pads. Note that the driver’s regulators can also be used to power other electronics in the system, such as the controlling MCU.
The motor power supply should be capable of delivering the expected currents for the stepper motors being used (peaks up to 4 A).
Four, six, and eight-wire stepper motors can be driven by the A4988 if they are properly connected; a FAQ answer explains the proper wirings in detail.
Warning: Connecting or disconnecting a stepper motor while the driver is powered can destroy the driver. (More generally, rewiring anything while it is powered is asking for trouble.)
Stepper motors typically have a step size specification (e.g. 1.8° or 200 steps per revolution), which applies to full steps. A microstepping driver such as the A4988 allows higher resolutions by allowing intermediate step locations, which are achieved by energizing the coils with intermediate current levels. For instance, driving a motor in quarter-step mode will give the 200-step-per-revolution motor 800 microsteps per revolution by using four different current levels.
The resolution (step size) selector inputs (MS1, MS2, and MS3) enable selection from the five step resolutions according to the table below. MS1 and MS3 have internal 100kΩ pull-down resistors and MS2 has an internal 50kΩ pull-down resistor, so leaving these three microstep selection pins disconnected results in full-step mode. For the microstep modes to function correctly, the current limit must be set low enough (see below) so that current limiting gets engaged. Otherwise, the intermediate current levels will not be correctly maintained, and the motor will skip microsteps.
Each pulse to the STEP input corresponds to one microstep of the stepper motor in the direction selected by the DIR pin. Note that the STEP and DIR pins are not pulled to any particular voltage internally, so you should not leave either of these pins floating in your application. If you just want rotation in a single direction, you can tie DIR directly to VCC or GND. The chip has three different inputs for controlling its many power states: RST, SLP, and EN. For details about these power states, see the datasheet.
To achieve high step rates, the motor supply is typically much higher than would be permissible without active current limiting. For instance, a typical stepper motor might have a maximum current rating of 1 A with a 5Ω coil resistance, which would indicate a maximum motor supply of 5 V. Using such a motor with 12 V would allow higher step rates, but the current must actively be limited to under 1 A to prevent damage to the motor.
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 to measure the current running through a single motor coil without clocking the STEP input. The measured current will be 0.7 times the current limit (since both coils are always on and limited to 70% of the current limit setting in full-step mode). Please note that changing the logic voltage, Vdd, to a different value will change the current limit setting since the voltage on the REF pin is a function of Vdd.
Another way to set the current limit is to measure the voltage on the REF pin and to calculate the resulting current limit (the current sense resistors are 0.05Ω). The current limit relates to the reference voltage as follows:
Current Limit = VREF × 2.5
So, for example, if the reference voltage is 0.3 V, the current limit is 0.75 A. As mentioned above, in full step mode, the current through the coils is limited to 70% of the current limit, so to get a full-step coil current of 1 A, the current limit should be 1 A/0.7=1.4 A, which corresponds to a VREF of 1.4 A/2.5=0.56 V. See the A4988 datasheet for more information.
Note: The coil current can be very different from the power supply current, so you should 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.
The A4988 driver IC has a maximum current rating of 2 A per coil, but the actual current you can deliver depends on how well you can keep the IC cool. The carrier’s printed circuit board is designed to draw heat out of the IC, but to supply more than approximately 1 A per coil, a heat sink or other cooling method is required.
This product can get hot enough to burn you long before the chip overheats. Take care when handling this product and other components connected to it.
Since the input voltage to the driver can be significantly higher than the coil voltage, the measured current on the power supply can be quite a bit lower than the coil current (the driver and coil basically act like a switching step-down power supply). Also, if the supply voltage is very high compared to what the motor needs to achieve the set current, the duty cycle will be very low, which also leads to significant differences between average and RMS currents.
|Schematic diagram of the md09a A4988 stepper motor driver carrier with regulators.|
Note: This board is a drop-in replacement for the original (and now discontinued) A4983 stepper motor driver carrier with voltage regulators. The key difference is that the newer A4988 offers overcurrent protection that the A4983 lacks; it is otherwise virtually identical to the A4983.
|Size:||0.7″ × 1.4″|
|Minimum operating voltage:||8 V|
|Maximum operating voltage:||35 V|
|Continuous current per phase:||1 A2|
|Maximum current per phase:||2 A3|
|Minimum logic voltage:||3 V4|
|Maximum logic voltage:||5.5 V4|
|Microstep resolutions:||full, 1/2, 1/4, 1/8, and 1/16|
|Reverse voltage protection?:||Y5|
Yes. To avoid damaging your stepper motor, you want to avoid exceeding the rated current, which is 600 mA in this instance. The A4988 stepper motor drivers let you limit the maximum current, so as long as you set the limit below the rated current, you will be within spec for your motor, even if the voltage exceeds the rated voltage. The voltage rating is just the voltage at which each coil draws the rated current, so the coils of your stepper motor will draw 600 mA at 3.9 V. By using a higher voltage along with active current limiting, the current is able to ramp up faster, which lets you achieve higher step rates than you could using the rated voltage.
If you do want to use a lower motor supply voltage (under 8 V) for other reasons, consider using our DRV8834 low-voltage stepper motor driver carrier.
Yes, you do! Setting the current limit on your stepper motor driver carrier before connecting your motor is essential to making sure that it runs properly. An appropriate current limit also ensures that your motor is not allowed to draw more current than it or your driver can handle, since that is likely to damage one or both of them.
Setting the current limit on our A4988, DRV8825, DRV8824, DRV8834, and DRV8880 stepper motor driver carriers is done by adjusting the on-board potentiometer. We strongly recommend using a multimeter to measure the VREF voltage while setting the current limit so you can be sure you set it to an appropriate value (just turning the pot randomly until things seem to work is not a good approach). The following video has more details on setting the current limit:
Measuring the current draw at the power supply does not necessarily provide an accurate measure of the coil current. Since the input voltage to the driver can be significantly higher than the coil voltage, the measured current on the power supply can be quite a bit lower than the coil current (the driver and coil basically act like a switching step-down power supply). Also, if the supply voltage is very high compared to what the motor needs to achieve the set current, the duty cycle will be very low, which also leads to significant differences between average and RMS currents: RMS current is what is relevant for power dissipation in the chip but many power supplies won’t show that. You should base your assessment of the coil current on the set current limit or by measuring the actual coil currents.
Please note that while the A4988 driver IC is capable of supplying 2 A per coil, the chip by itself will overheat at lower currents. The carrier board PCB helps draw heat away from the IC, but we have found that it generally requires a heat sink to deliver more than approximately 1 A per coil (the Black Edition A4988 carrier has a four-layer PCB that lets it deliver up to around 1.2 A per coil without a heat sink), but this number depends on factors such as ambient temperature and air flow. For example, sealing three A4988 driver carriers in close proximity in a small box will cause them to overheat at lower currents than a unit by itself in open air.
The answer to this question depends on the type of stepper motor you have. When working with stepper motors, you will typically encounter two types: unipolar stepper motors and bipolar stepper motors. Unipolar motors have two windings per phase, allowing the magnetic field to be reversed without having to reverse the direction of current in a coil, which makes unipolar motors easier to control than bipolar stepper motors. The drawback is that only half of the phase is carrying current at any given time, which decreases the torque you can get out of the stepper motor. However, if you have the appropriate control circuitry, you can increase the stepper motor torque by using the unipolar stepper motor as a bipolar stepper motor (note: this is only possible with 6- or 8-lead unipolar stepper motors, not with 5-lead unipolar stepper motors). Unipolar stepper motors typically have five, six, or eight leads.
Bipolar steppers have a single coil per phase and require more complicated control circuitry (typically an H-bridge for each phase). The A4983 and A4988 have the circuitry necessary to control a bipolar stepper motor. Bipolar stepper motors typically have four leads, two for each coil.
|Two-phase bipolar stepper motor with four leads.|
The above diagram shows a standard bipolar stepper motor. To control this with the A4983 or A4988, connect stepper lead A to board output 1A, stepper lead C to board output 1B, stepper lead B to board output 2A, and stepper lead D to board output 2B. See the A4983/A4988 datasheet for more information.
If you have a six-lead unipolar stepper motor as shown in the diagram below:
|Two-phase unipolar stepper motor with six leads.|
you can connect it to the A4983 or A4988 as a bipolar stepper motor by making the bipolar connections described in the section above and leaving stepper leads A’ and B’ disconnected. These leads are centre taps to the two coils and are not used for bipolar operation.
If you have an eight-lead unipolar stepper motor as shown in the diagram below:
|Two-phase unipolar stepper motor with eight leads.|
you have several connection options. An eight-lead unipolar stepper motor has two coils per phase, and it gives you access to all of the coil leads (in a six-lead unipolar motor, lead A’ is internally connected to C’ and lead B’ is internally connected to D’). When operating this as a bipolar stepper, you have the option of using the two coils for each phase in parallel or in series. When using them in parallel, you decrease coil inductance, which can lead to increased performance if you have the ability to deliver more current. However, since the A4983 and A4988 actively limit the output current per phase, you will only get half the phase current flowing through each of the two parallel coils. When using them in series, it’s like having a single coil per phase (like in four-lead bipolar steppers or six-lead unipolar steppers used as bipolar steppers). We recommend you use a series connection.
To connect the phase coils in parallel, connect stepper leads A and C’ to board output 1A, stepper leads A’ and C to board output 1B, stepper leads B and D’ to board output 2A, and stepper leads B’ and D to board output 2B.
To connect the phase coils in series, connect stepper lead A’ to C’ and stepper lead B’ to D’. Stepper leads A, C, B, and D should be connected to the stepper motor driver as normal for a bipolar stepper motor (see the bipolar stepper connections above).