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Using Cheap Asian Electronic Modules Part 22: by Jim Rowe Three Stepper Motor Drivers Want to build your own 3D printer or CNC machine? You will need multiple stepper motors to control it, and a way to drive them. Or maybe you have some stepper motors from old printers or disc drives and want to reuse them. Here are three of the most common stepper motor driver modules and how to use them. T his article assumes you understand the basics of how stepper motors work. If you want an introduction to this type of motor then read our primer in the January 2019 issue (siliconchip.com.au/Article/11370). The first driver module we’re looking at is also the largest, at 60 x 55 x 28mm, including the finned heatsink for the driver IC. It’s based on the ST Microelectronics L298N dual H-bridge driver chip and is currently available on eBay for less than A$3.80. The “N” on the end of the chip version signifies that it’s in a 15-pin Multiwatt Power package, intended to be mounted vertically on a heatsink. ST Micro also make a similar version (L298HN) intended to be mounted horizontally, and a version in a PowerSO20 SMD package (L298P). Fig.1 shows a simplified block diagram of what’s inside the L298. It has two full H-bridge drivers (using bipolar power transistors) and so can drive both stator windings of a standard two-phase bipolar hybrid stepper motor. Each bridge has an enable input and two logic control inputs, and both bridges have their negative supply connections brought out separately, to siliconchip.com.au allow for an external current sensing resistor (RSA and RSB, shown in red). The L298 can operate from supply voltages from 6-46V and can handle up to 2A per bridge. The inputs are TTL compatible. This makes it the most rugged of the driver ICs we’re looking at here, especially when it’s fitted to that 23 x 25 x 15mm finned heatsink. Fig.2 shows the full circuit of the L298N-based driver module. In addition to the L298N chip itself (IC1), there’s regulator REG1, which provides a 5V supply for the logic circuitry from the stator supply voltage Vms, if no separate 5V supply is available. REG1 is enabled simply by leaving the jumper shunt in place on the “5V_EN” header. There are also eight MDDM7 fastswitching silicon diodes to ensure that all four outputs of IC1 are protected from damage due to back-EMF spikes from the motor stator windings, at the end of each current pulse. The upper diodes prevent the outputs from swinging more positive than one diode forward voltage drop above the supply voltage (Vms), while the lower diodes prevent them from swinging below ground by more than one diode forward drop. Australia’s electronics magazine Note that there are no current sensing resistors fitted between the Sensea (pin 1) and Senseb (pin 15) pins of IC1 and ground. Instead, these pins are brought out to the two pairs of header pins (CSA and CSB) at the right-hand end of the 6x2 pin DIL header, just below IC1 in Fig.2. This allows you to connect in current sensing resistors if you wish, or just short both pins to ground (by leaving the jumper shunts in place) if you do not need current monitoring. The other four pairs of header pins (U1-U4) allow you to disconnect the four 10kW pull-up resistors between the control inputs of IC1 and +5V. Four of the five indicator LEDs (LEDs1-4) show when each of the four logic inputs is high, while the fifth (LED5) is a 5V power-on indicator. This module is quite flexible but it has one significant shortcoming: it is purely a dual H-bridge stepper driver, lacking any built-in indexing controller. ST Micro make a matching controller chip for use with the L298, called the L297. This can control the L298 for full- or half-stepping, wave microstepping and clockwise or anticlockwise rotation. It can also sense the voltFebruary 2019  35 Fig.1 (right): block diagram of the L298N IC, which is shown as part of the module above, attached to the heatsink. ages across the current sensing resistors CSA and CSB, and use PWM to control and regulate the stator winding currents. However, the L297 chip costs around $16 – nearly four times the price of the L298 module itself. Instead of using an L297 controller chip, you can use software running in your Arduino, Micromite or some other micro. Developing this can be a bit of a challenge but it is by no means impossible. By the way, the L298N module isn’t restricted to driving a stepper motor. It can also be used to drive a pair of conventional brushed DC motors – one from each of the two H-bridges. All you need to do is feed one input of each bridge with a PWM (pulsewidth modulated) pulse stream. You could drive one input for clockwise rotation and the other for anticlockwise rotation. Fig.2: complete circuit diagram of the L298N-based stepper driver module. CSA and CSB can be fitted with two currentsensing resistors if needed, otherwise they can just be shorted to ground. 36 Silicon Chip Australia’s electronics magazine siliconchip.com.au DRV8825-based module The next module is much smaller and combines a stepper motor controller and driver, both within the Texas Instruments DRV8825 chip. The module measures just 20 x 15 x 16mm, including the stick-on heatsink; and is currently available from eBay suppliers for around $2 each. The DRV8825 chip packs a lot into a 28-pin SSOP (SMD) package, as you can see from the internal block diagram, Fig.3. There are two full H-bridge drivers, labelled MOTOR DRIVER A and MOTOR DRIVER B. These use N-channel power Mosfets and can operate with a supply of 8.245V, with a drive capability of up to 2.5A (for each channel) at a supply voltage of 24V. Each driver has provision for connection of current sensing resistors at the bottom of each bridge (Isena and Isenb). The block above the motor drivers is a charge pump used to develop the gate drive supply for the upper Mosfets in each bridge. Then at upper left, there’s a 3.3V regulator, which can provide the current reference voltages for the two bridges (AVref and BVref). The DRV8825 also includes its own stepper control logic/indexer block, shown at lower left. This has STEP and Fig.3: block diagram of the DRV8825 IC. DIR logic inputs for basic motor control, plus three MODE control inputs (MODE0, MODE1, MODE2) which determine the stepping mode. A total of six different stepping modes are available: Full-stepping, half-stepping, quarter-stepping and three different microstepping resolutions (8/16/32 microsteps per full step). The microstepping is performed us- ing PWM current control together with synthesised sine and cosine waveforms. Internal feedback from the Isena and Isenb pins allows the PWM circuitry to regulate the motor winding currents at the same time. The chip supports fast, slow or mixed current decay modes. The SLEEP input allows the internal circuitry to be shut down for very low current drain between active motor drive periods. There are also ENBL and RESET inputs, both of which have internal pulldowns. And there’s a FAULT output, which goes low if the device detects an over-temperature or over-current condition. Fig.4 shows the full circuit of the DRV8825-based stepper driver module, and there’s little in it apart from the DRV8825 chip (IC1). The 10nF capacitor between pins CP1 (1) and CP2 (2), and the 100nF capacitor connected between the Vcp pin (3) and the motor voltage line Vma are needed so that the internal charge pump can develop the high side gate drive voltage for the two internal H-bridge drivers. The chip’s Isena and Isenb output current sensing pins are connected to ground via 0.1W resistors, to allow the regulation circuitry to operate. Trimpot VR1, shown at upper left, allows the maximum current level in each Fig.4: complete circuit diagram of the DRV8825-based stepper driver/controller module. While this circuit is less complex than the L298N-based module shown in Fig.2, it doubles as a controller and driver instead of only being a driver. siliconchip.com.au Australia’s electronics magazine February 2019  37 The DRV8825 (left) and TB6612FNG-based module (right) shown slightly enlarged. Note the stick-on heatsink for the DRV8825, which would likely be required when driving large stepper motors with windings that pull 1A or more. motor winding to be set to any desired level, by setting the voltage at the AVref and BVref pins. The DRV8825 data sheet advises that there is an op-amp with a gain of five times in the feedback circuit from the Isena and Isenb pins, so the relationship between the maximum motor winding current, the sensing resistor values and the Vref voltage (set by VR1) is quite straightforward: Imax = Vref ÷ (5 × Rsense) So with the 0.1W sensing resistors used in this module, the maximum winding current (Imax) will be equal to Vref × 2. As a result, VR1 can easily set the maximum current level up to 2.5A. For example, setting VR1 so that Vref = 1.0V will give a maximum winding current of 2A. As you can see, despite its tiny size, the DRV8825 has a surprising range of capabilities, including a very flexible built-in indexing controller to simplify controlling a stepper motor from a micro. TB6612FNG driver module The third stepper motor driver module is based on the Toshiba TB6612FNG chip. It’s slightly larger than the DRV8825-based module, measuring only 20.5 x 20.5 x 11mm, including headers. Currently, it’s available from various suppliers on eBay for around $1.65 in one-off quantities. Fig.5 shows a simplified block dia- gram of what’s inside the TB6612FNG, which comes in a 24-pin SSOP SMD package. It’s basically a pair of Hbridge drivers, each driven from a control logic block. So in many ways, it’s rather like the L298N, except that the H-bridges use LDMOS power transistors rather than bipolar power transistors. The TB6612FNG is rated to operate at a maximum motor supply voltage (VM) of 15V, and to deliver output currents of up to 1.2A average or 3.2A peak, for each channel. But it also needs a logic circuit supply voltage (Vcc) of between 2.7V and 5.5V, and there is no on-chip regulator to derive this from the motor supply. So this must be supplied externally. Fig.5 (left): block diagram of the TB6612FNG driver IC. Fig.6 (above): complete circuit diagram of the TB6612FNG-based module which is only a driver module and does not have any control circuitry. 38 Silicon Chip Australia’s electronics magazine siliconchip.com.au Note that although the ground connection of each H-bridge is brought out to a pair of device pins (3 & 4, 9 & 10), these pins are all linked together inside the device. You therefore can’t individually monitor or control bridge currents. You’d have to use a single resistor, and it would develop a voltage corresponding to a vector sum of the two bridge currents. By the way, like the L298N, the TB6612FNG does not include any indexing/control circuitry ahead of the control logic. So it too needs external indexing hardware or software to drive a stepper motor. On the other hand, it’s suitable for driving a pair of brush-type DC motors, using PWM input signals to control motor speed and the Ain1/Ain2 and Bin1/Bin2 signals to determine rotation. Fig.6 shows the actual circuit of the TB6612FNG based driver module, and clearly, there is very little in it apart from the main chip itself (IC1). There are just three bypass capacitors on the supply lines and two 8-pin SIL headers (CON1 and CON2) to make the input and output connections. It couldn’t be much simpler. Trying them out Since the driving schemes of the L298N and TB6612FNG are quite similar, we’ve decided to concentrate on demonstrating how to use the L298N and DRV8825-based modules. And we’re going to demonstrate driving one from an Arduino and one from a Micromite. You should not have difficulty adapting our examples to different combinations of the modules and controllers if it turns out that you’d prefer to use some other pairing. First, let’s start by driving the L298N-based module from an Arduino. While this module lacks its own indexing controller, the Arduino IDE comes with a library called “Stepper” which has functions to perform indexing. That makes hooking up controller chips like the L298N (or the TB6612FNG) quite easy. Fig.7 shows how we connected the L298N module to an Arduino Uno and a typical bipolar stepper motor. The connections between the Uno and the module inputs are the defaults for the Stepper library, so it’s important to follow these carefully. The stepper motor windings are each connected to either the MOTOR A or MOTOR B output terminals, while the Vms and GND terminals are connected to the motor power supply. All the jumper shunts are left in place on the module. Also, note that the module’s centre GND pin needs to be connected to one of the GND pins of the Arduino. That’s because there is no other connection between the two GNDs, and the control signals would otherwise not work correctly. The Arduino IDE Stepper library comes with some example sketches written by Tom Igoe. We adapted one of these to make it easier for our readers. It’s called “SCstepper_oneRevolution. ino”, and you can download it from the Silicon Chip website. It directs the stepper motor to rotate in one direction by a full revolution, then reverse and rotate back by a full revolution. The number of steps required for a full revolution needs to be added to the sketch before you run it. The correct figure for many motors is 200, so that is the default. If you find this sketch interesting, you’ll find another three sketches in the “Examples” folder of the Stepper library folder on your PC (if you have installed the Arduino IDE). These will all work with the setup shown in Fig.7, performing different functions. Microstepping with the Micromite We decided to drive the DRV8825based module from a Micromite because with its inbuilt indexer, it’s a little easier to program “from scratch”. Fig.8 shows how we connected the module between the Micromite and a bipolar stepper. The main STP and DIR inputs of the module are driven from pins 10 and 9 of the Micromite, with the SLP and RST inputs both driven from pin 16. Similarly, the ENBL input is driven from pin 22, while the M0, M1 and M2 mode control inputs are driven from pins 21, 18 and 17 respectively. Fig.7: wiring diagram to connect the L298N-based driver module driving a 4-wire bipolar stepper motor with an Arduino or compatible board. Note that the module’s ground connection needs to be wired to the Arduino’s ground connection otherwise the control signals would not work properly. The program is available from the Silicon Chip website. siliconchip.com.au Australia’s electronics magazine February 2019  39 The three screengrabs of the example microstepping program for the DRV8825 running on a Micromite. From left to right there is the main menu at power-up, the SET FUNCT sub menu (which determine how the drive pulses should be sent) and then the SET MODE sub menu (which is used to select the stepping mode). On the output side, the motor windings are connected to the A1, A2, B1 and B2 pins, while the motor supply is connected to the Vma (+12V) and GND pins. The two GND pins are also connected together, and on to a GND pin on the Micromite. This is done to ensure that both the module and the Micromite have a common ground. An electrolytic capacitor of at least 100µF must be connected between the Vma and GND pins of the module, as shown in Fig.8. This is to provide a low impedance reservoir from which the module’s H-bridges can draw current pulses – without any impedance from inductance in the power leads. The USB-UART bridge module at top centre in Fig.8 is to program the Micromite from your PC, as well as to provide the Micromite with 5V DC. Note that while the DRV8825 module comes with a tiny (9 x 9 x 5mm) finned heatsink which can be attached to the top of the DRV8825 chip using an adhesive patch, it is unnecessary when driving a small stepper motor from a 12V supply. Presumably, it would be required if the module is driving a reasonably large stepper motor with windings drawing over 1A from a 24V supply. In our test, the winding current was only about 330mA and even without the extra heatsink, the DRV8825 became only barely warm. The module PCB provides copper patches on both sides under the chip, linked by an array of vias. So it already has a useful amount of heatsinking. After studying TI’s datasheet and application notes, I was able to write a Micromite program to control a step- per via the DRV8825 module. This program is named “DRV8825 stepper driving program.bas” and you can download it from the Silicon Chip website. When loaded onto a Micromite with LCD BackPack, at power-up it will present you with the main screen with six touch buttons, shown in Screen 1. The buttons are labelled SET FUNCT, SET MODE, < DIR, DIR >, START and STOP. Pressing SET FUNCT loads the SELECT FUNCTION screen shown in Screen 2. This lets you choose from one of five functions: SINGLE (send a single step pulse each time), CONTIN (send a large number of step pulses), 1/2 REV (send pulses for a half revolution of the motor), FULL REV (send pulses for a full revolution), and FWD-REV (send pulses for one full revolution in one Fig.8: wiring diagram for the DRV8825-based driver module connected to a 4-wire bipolar stepper motor and Micromite. The 100µF electrolytic capacitor is required to provide a low impedance supply for the module’s two H-bridges. 40 Silicon Chip Australia’s electronics magazine siliconchip.com.au direction, followed by pulses to make the motor return in the opposite direction to its original position). The sixth button on this screen is labelled RETURN, allowing you to get back to the main screen without changing the existing selection. If you press the SET MODE button on the main screen, you’ll be presented with the SELECT STEPPING MODE screen shown in Screen 3. This allows you to select one of the six stepping modes provided by the DRV8825: FULL STEP, HALF STEP, 1/4 STEP, 1/8 STEP, 1/16 STEP or 1/32 STEP. Touching any one of these buttons selects the desired mode and switches you back to the main screen. The two red buttons on the main screen are used to select the direction of motor rotation. And touching the START button at lower left should result in the motor performing the selected function, using steps of the mode you’ve selected. The STOP button allows you to stop the motor at any time. This program demonstrates a fair number of possibilities when it comes to using the Micromite to control a stepper motor using the DRV8825 module. Some useful links on each of the modules are listed below: www.st.com/en/motor-drivers/l298 www.ti.com/product/DRV8825 siliconchip.com.au/link/aama Low-cost stepper motors currently available Currently, there are quite a few new stepper motors available via eBay and other online sources. Here’s a sample of those we found in the standard NEMA sizes, together with their price range: NEMA 11: around $11 each NEMA 17: $12-22 each or five for $38-69 NEMA 23: around $50 each There were also many small nonNEMA steppers available at much lower prices. For example, a 28BYJ-48 5V unipolar stepper motor bundled with a ULN2003 driver module was around SC $3.22 each. siliconchip.com.au DID YOU MSS OUT? Is there a particular project in S ILICON C HIP that you wanted to read – but missed that issue? Or perhaps a feature that really interests you? Grab a back issue . . . while they last! The SILICON CHIP Online Shop carries back issues for all months (with some exceptions!) from 1997 to date. Some popular issues are sold out, and some months are getting quite low. But if you want a particular issue, you can order it for just $12.00 INCLUDING P&P* – while stocks last! 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