# Debugging my bipolar stepper motor design

I'm trying to connect a NEMA 17 Bipolar Stepper Motor to an Arduino Uno using an L298N Stepper Motor Controller Module and control its speed using a 10K potentiometer.

I haven't found any examples online on how to do this explicitly (using the L298N controller module), so I've resorted to trying to design the circuit myself. This is, literally, my very first attempt at designing a circuit myself. I have no background in electronics or electrical engineering. I've gathered as much info as I can from the arduino website and the manufacturers of the motor itself. This much I'm certain of:

1. The motor should be powered independently from the arduino, using 2.55 volts and 1.7 amps.

2. The motor and its power source must be connected to an H-bridge before connecting to the arduino inputs.

3. Pairs of wires on the stepper motor are connected and can be determined easily by hooking an LED light to a pair of wires and rotating the motor manually. Connected wires will cause the LED to light up.

4. All grounds should be connected together.

In addition to this, I'm also making a few (hopefully true) assumptions:

1. The L298N Controller module should provide the necessary H-bridge.

2. The H-bridge inputs should be connected to a contiguously indexed array of digital pins on the arduino.

3. Despite the specs indicated the need for 2.55 volts, a 2.5 volt source should be sufficient for this motor.

4. Since a 2.5 volt source is not readily available, I can create one with a voltage divider.

5. Reversing the polarity of the stepper wires to the Controller Module (h-bridge) does not matter. That is, if a pair of connected stepper motor wires Wire1 and Wire2 are wired to Out1 and Out2 on the L298N Controller Module (respectively), then hooking up Wire1 to Out2 and Wire2 to Out1 would produce the same effect as the original wiring.

Based upon the above facts/assumptions, I created a schematic of how I think everything should be hooked up:

Note that the two instances labeled 2.5V are connected to each other. Additionally, all grounds are also connected together.

I would like some feedback from the community on my proposed stepper motor design. In particular, I would like to know if my assumptions above are correct and whether I can obtain the necessary current for my stepper motor using my voltage divider as displayed in the picture above. Any suggestions to improve / ensure functionality would be greatly appreciated!

If anything is unclear about my schematic or assumptions, please let me know and I will clarify as best I can.

• Voltage dividers are not regulators. – Ignacio Vazquez-Abrams Dec 21 '13 at 23:30
• using 2.55 volts and 1.7 amps These low impedance motors are not meant to be driven with a constant voltage but with a constant current which can deliver a higher torque as the step speed increases – alexan_e Dec 21 '13 at 23:38
• The L298N has a minimum driver voltage rating of (Vih + 2.5V) which after looking at Vih min, the driver voltage needs to be no lower than 4.8V. In addition, the logic supply is only rated for 5V logic (4.5V min) . You need to choose a different h-bridge or drive it using a chopper drive with a higher voltage. – Tut Dec 22 '13 at 15:41

Okay, you have a couple of issues:

1. Your voltage divider will present ~2.55V at it's centre, but only unloaded - due to Ohm's law as soon as you present a low resistance load the voltage will drop severely. For instance, if your stepper coil has a resistance of around 3 Ohms, then the voltage will drop to ~9mV!. See the various wiki pages on maximum power transfer and voltage dividers.

2. Rather than making sure you have exactly the right voltage, you would be better off with a stepper motor driver that will ensure no more power than the motor can handle will be used. They do this by "chopping" the voltage into pulses, and there are many to choose from. The L298 is pretty old now, uses bipolar transistors and after the source/sink saturation voltages you will have almost nothing left to drive the motor. I would read this wiki page on Stepper Driving (and related pages) plus ask people who have already used these setups - the reprap folks are probably a good source of info.

On dropping small amounts of voltage, a series diode can come in handy to drop between 0.3 and 0.8V or so per diode. But for efficient power regulation, depending on the difference between input and output, linear or switching regulators are the norm, (almost) never voltage dividers for supplying power.

• If I were to acquire a regulator, what properties should it have in order to provide the necessary power? Where should I insert it into my circuit? – Paul Dec 23 '13 at 1:32

Voltage dividers are not the right option for driving motors. Following are the reasons

1) The voltage provided from a voltage divider varies as per the current consumed. So when the motor draws more current, the voltage at the voltage divider wont be 2.5V. For constant supply voltage, constant voltage regulators are required. These voltage regulators keep their output always constant whatever is the current drawn by the motor.

2) If you use a voltage divider, the resistors used in the voltage divider should be able to handle higher current rates as required by your motor. To solve this, you need to use high wattage resistors.

So use a voltage regulator which can provide 2.55 volts(there are variable voltage regulators which can provide this voltage.) Also look for proper current ratings since your stepper needs 1.7A.

Using L298N for driving stepper motor is not that easy. L298N is suited for driving DC motors. Stepper motor drivers are the best for driving stepper motors (see this http://www.pololu.com/product/1182 There are even cheaper driver modules available with the same IC)

There are certain advantages for using a stepper motor driver

1) You can feed the driver with a 5V supply. These stepper drivers have current limiting options(which you can set using a variable resistor) which will save your stepper motor when supplying voltages than the rated.

2) You can reduce the software overhead of creating the step pulses.

But still if you want to use the L298N, this link might be helpful http://arduino.cc/en/Tutorial/MotorKnob

• So use a voltage regulator which can provide 2.55 volts It's pointless because the L298 with introduce a Vce voltage drop of about 2-2.5v for the high side switch and another 2-2.5v for the low side switch so even starting with 5v would result to almost no voltage on the motor coil. – alexan_e Dec 22 '13 at 9:41
• @robomon: If I were to invest in a driver for this particular stepper motor, what sort of qualities should this driver have? That is, how will I know if will be able to drive my motor? – Paul Dec 28 '13 at 15:40