Constructing a DC motor driver

In summary: The non-inverting terminal is the negative terminal and the inverting terminal is the positive terminal. They are named this way because when the op amp is in the "off" state, the voltage on the inverting terminal is higher than the voltage on the non-inverting terminal. When the op amp is in the "on" state, the voltage on the inverting terminal is lower than the voltage on the non-inverting terminal.
  • #1
Xyius
508
4
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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  • #2
Xyius said:
Why would changing the resistance of the input change the polarity of the signal input to the OpAmp?

Thanks!

Think voltage divider
 
  • #3
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.
 
  • #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
 
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  • #5
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:
 
  • #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.
 
  • #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
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. :(
 

Related to Constructing a DC motor driver

1. What is a DC motor driver and how does it work?

A DC motor driver is an electronic circuit that controls the speed and direction of a DC motor. It works by receiving input signals from a microcontroller or other control device and converting them into the proper voltage and current levels to drive the motor.

2. What components are needed to construct a DC motor driver?

The main components needed to construct a DC motor driver include a power source, transistors or MOSFETs, diodes, and resistors. Other components such as capacitors and ICs may also be needed depending on the specific design.

3. What are the key considerations when designing a DC motor driver?

There are several key considerations when designing a DC motor driver, including the power requirements of the motor, the desired speed and torque, the available space for the driver, and the control signals needed. It is also important to consider protection features, such as overcurrent and overtemperature protection, to ensure the safety and longevity of the motor and driver.

4. Can a DC motor driver be used for different types of DC motors?

Yes, a DC motor driver can be used for various types of DC motors, such as brushed and brushless motors. However, the specific design and components used may vary depending on the type and specifications of the motor.

5. Are there any safety precautions to keep in mind when constructing a DC motor driver?

Yes, it is important to follow proper safety precautions when constructing a DC motor driver, as it involves working with high voltages and currents. This includes wearing appropriate protective gear, such as gloves and safety glasses, and ensuring all components are properly connected and insulated. It is also recommended to test the driver with a smaller motor before using it with a larger and more powerful motor.

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