In my last post, I talked about my newest project; a 24V DC brushed motor controller shield for the Arduino. I mentioned my setbacks in my post and that in my new post, I will give an update for the project. Unfournately, the only thing I could do last week was replacing my old Arduino Duemilanove with the Arduino Uno. Until I can test the circuit proper, I decided to spend this post discussing how I intend to control this 24V motor.
If you’re a regular visitor of this blog, an electrical engineer, or just an electronics hobbyist, then you’re probably familiar with an Hbridge. An Hbridge is an electronic circuit that changes the direction of current flowing to a motor in order to change the rotation its shaft. To change the flow of current, four switches are used. Using the figure 1 as a reference, the motor’s shaft rotates clockwise when S1/S4 turns on, and when S2/S3 turns on.
Today, these Hbridge switches are replaced with transistors. Lower voltage Hbridges can be made using Bipolar Junction Transistors (BJT) since they require little current to properly control lower voltage motors. However, if you’re control 12V and more motors, then BJTs would be horrible as they will require more current to control these motors. So instead, metal oxide semiconductor field effect transistors (MOSFET) are used since they “ideally” consume no power to turn them on. For motor control, Power MOSFETs are used. There are two types of MOSFETs: Pchannels and Nchannels. I will not discuss the physical difference between them, but I will mention that Nchannels turn on when you apply a positive voltage to its gate, and Pchannels turn off when you apply positive voltage to its gate.
Alas, MOSFETs also have problems to consider. One problem is that if you try to pass 12V or more through the Nchannel MOSFET, then you need at least 12V to turn them on. Also, to turn off a pchannel, you to apply the same voltage you’re sinking through the MOSFET to its gate. I seen many forum posts in which people blown out their MOSFET forgetting this important fact. Of course I’m no different since I made the same mistake senior year of college…and in my last post.
Now that I talked about the theory of this motor control, let me talk about my proposed circuit. In my Hbridge circuit, I have my upper switches comprised of pchannnel MOSFETs and my bottom switches made up of nchannel MOSFETs. To turn on just one pchannel MOSFET, the Arduino will activate a low current NPN transistor, which will turn on the pchannel MOSFET because of the voltage divider at its gate. However, when the NPN transistor is off, then the voltage at the pchannel’s gate will be equal to the voltage being applied to it. By having the voltage applied to equal to the voltage at its gate, the Pchannel turns off. This method is repeated for the other pchannel MOSFET.
Finally, to control whether an nchannel is off or on, 12V-20V must be applied to the gate. Unlike a pchannel MOSFET, the voltage applied to the gate can be a separate voltage from the vvoltage applied to it. To accomplish this, I used low side gate drivers (IR4427) to provide the right voltage to turn the bottom Nchannels on or off.
Now that I explained the theory the best way I could, let me discuss what’s next for this project. Although I did not test my Arduino to my motor control circuit yet, there’s a glaring problem with the circuit. There is no isolation between the Arduino and the circuits responsible for driving the MOSFETs. In order to achieve this, not only will have to look into optoisolators, but also isolated DC to DC DIP modules. But that’s another post. Another item that I will have to look into is setting up an undervoltage lockout circuit for the circuits driving the MOSFETs gates. By adding an undervoltage lockout circuit, the motor will not turn on until the right amount of voltage is applied to the MOSFETs.
Anyway, that’s it for me today. If you have a question, comment, or concern, feel free to leave a comment. I will see you guys, next week.