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Constructing a DC motor driver

  1. Feb 14, 2012 #1
    So in my SKIL course, (SKIL is the name of the lab courses (1 through 6) physics majors take. This course is about non-linear circuits) we need to develop some type of project that incorporates what we learn in the labs. The course deals with..

    -Operation Amplifiers (Op Amp's)
    -BJT Transistors. (Or BJT's.. I guess saying BJT transistors is a little redundant.)

    So I was looking up project ideas that incorporate all of these things and I found something I am very much interested in.

    http://www.electronics-project-design.com/DCMotorDriver.html [Broken]

    Since my knowledge of transistors and op amps are limited, I was wondering if anyone could answer a few questions I have about the workings of this circuit.

    My understanding of OpAmps is that they look at the voltage difference between the + and - sides, and if the input (+ side) is negative with respect to the - side, the op amp will put out a positive current to balance the potential between the two points. Likewise, if the input is more positive with respect to the - side, it would put out a negative current. The amount of amplification depends on the ratio of the resistors the op amp is connected to.

    Is this an accurate description? I feel like it is not completely correct.

    So I can pretty much see how this circuit works I think. The main source of confusion is the way the potentiometer works. Why would changing the resistance of the input change the polarity of the signal input to the OpAmp?

    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Feb 14, 2012 #2
    Think voltage divider
  4. Feb 14, 2012 #3
    Hmm.. would this be correct?

    The negative input in the OpAmp is at zero potential at first. On the positive input, the initial position of the resistor is 0 resistance so the +9 and -9 voltage cancel out and the + input on the OpAmp has an input of 0 volts. When the resistor is moved downward, it puts resistance on the negative side, thus reducing the voltage. This means that the net voltage is positive. And the same can be said about the other direction.

    Also, I still do not understand why the output of the OpAmp has to go back into the negative side.
  5. Feb 14, 2012 #4
    Look at your voltages. One is +, the other is -. the ground for both is were? The voltage divider will drop from + to _ and opposite. What happens to the input voltage on the op amp, as the potentiometer is changed? Which transistor is on when? Remember your ground is both + and _ .

    What is confusing you is they show the polarity and not both batteries as whole in the schematic.
    Edited to add uses a dual power supply not a single as in two 9v batteries. Remove a bit of confusion.. Sorry
    Last edited: Feb 14, 2012
  6. Feb 14, 2012 #5
    Looks like the ground is on the other side of the motor.

    Was what I said before incorrect completely? :(? What was wrong about it? I don't understand. :confused: The top transistor is on when the output current from the op amp is positive, and the bottom is on when the output current is negative.

    Man every time I think I got it, I actually don't :cry:
  7. Feb 14, 2012 #6
    You mostly have it. One transistor is NPN and the other is PNP,
    I can be confusing..

    When the input voltage of the op amp is changed , from + to _: or opposite:
    The output of the op amp changed polarity also, it switches on and off the transistors as it does so.
    Higher + or _ voltage at the input will increase the flow to the motor.
    This device works as a speed control and a direction switching control in one for the motor. Hence the dual powersupply or two batteries.
  8. Feb 14, 2012 #7
    Ah I think I see now! Sorry, this stuff is still new to me, I am learning :)

    But as a general question about OpAmps, why does the output current need to be fed back into the negative side? I see it set up this way a lot.
  9. Feb 14, 2012 #8
  10. Feb 15, 2012 #9
    I still do not fully understand. I read it a couple times and went through the math, but the conceptual understanding isn't there. Why is one terminal called the "non-inverting" terminal and "inverting" terminal? Is there a reason behind these names? I still do not understand the concept of negative feed back. :(
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