Constructing a DC motor driver

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    Dc Dc motor Motor
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Discussion Overview

The discussion revolves around the construction and understanding of a DC motor driver circuit, incorporating operational amplifiers (OpAmps), diodes, and BJT transistors. Participants explore the theoretical and practical aspects of the circuit, including voltage division, transistor operation, and feedback mechanisms.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant describes their understanding of OpAmps, noting that they amplify the voltage difference between their inputs, but expresses uncertainty about the accuracy of this description.
  • Another participant suggests considering the voltage divider concept to understand how changing resistance affects the OpAmp input.
  • A participant proposes a scenario where the negative input of the OpAmp starts at zero potential, and changing the potentiometer alters the voltage, impacting the OpAmp's output.
  • There is a discussion about the role of the ground in the circuit and how it relates to the voltage levels at the OpAmp inputs.
  • One participant mentions the need for feedback from the OpAmp output to the negative input, questioning its purpose and commonality in circuit designs.
  • Another participant clarifies that the circuit uses both NPN and PNP transistors, explaining how the OpAmp's output polarity affects transistor operation and motor control.
  • A participant expresses confusion about the terminology of "inverting" and "non-inverting" terminals of the OpAmp, seeking clarification on these concepts and negative feedback.

Areas of Agreement / Disagreement

Participants express varying levels of understanding regarding the OpAmp's function and the circuit's operation. There is no consensus on certain conceptual aspects, such as the necessity of feedback and the terminology used in OpAmp configurations.

Contextual Notes

Some participants note confusion regarding the schematic representation of the circuit, particularly the ground reference and the dual power supply configuration. There are also unresolved questions about the conceptual understanding of feedback mechanisms in OpAmps.

Who May Find This Useful

This discussion may be useful for students and hobbyists interested in electronics, particularly those learning about operational amplifiers, transistor circuits, and motor control applications.

Xyius
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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)
-Diodes
-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

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?

Thanks!
 
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Xyius said:
Why would changing the resistance of the input change the polarity of the signal input to the OpAmp?

Thanks!

Think voltage divider
 
lostcauses10x said:
Think voltage divider

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.
 
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:
lostcauses10x said:
Look at your voltages. One is +, the other is -. the ground for both is were?
Looks like the ground is on the other side of the motor.

lostcauses10x said:
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?

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:
 
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.
 
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.
 
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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