I am using PWM from a micro-controller to drive a DC motor. I know how to design amplifiers: common emitter, common collector, common base. My problem is, I don't know how the motor will act as a load when I attach it to the final stage. I don't know much about DC motors. I am using http://www.pololu.com/product/2271 and I have a TIP142. My thought process is that, I should make a common emitter to amplify the PWM to 6V and feed it to a common collector to get the current needed to drive the motor. I'm not sure how to make the common collector since I don't think I can treat the motor as a simple resistor.

 

A reversible DC motor is normally driven with a H-bridge. Current goes through the motor in one direction for cw rotation and in the reverse direction for ccw rotation. So the PWM will control the average current and thus the torque. You do know that the faster the shaft rotates the less torque it produces -- right?

http://en.wikipedia.org/wiki/H_bridge

 

Thanks Papabravo. I don't think I need to use an H-bridge. I am using PWM from a microcontroller to control the speed of the motor. I do not need direction control. The problem I am having right now is that I cannot power the motor directly from the microcontroller and I am trying to make an amplifier using a transistor to drive the motor.

 

OK so what you need is called a half bridge. Something like this:

http://www.irf.com/product-info/datasheets/data/ir2183.pdf
http://www.powerguru.org/the-half-bridge-circuit-revealed/

 

Or like this with the TIP. You will need a good sized heat sink. You could use a FET and get better performance.

 

thanks Papabravo, I will try the half bridge circuit with some time; I don't have the components now. Thanks Ronv, I got the circuit to work with those schematics.

 

OK so what you need is called a half bridge. Something like this:

http://www.irf.com/product-info/datasheets/data/ir2183.pdf
http://www.powerguru.org/the-half-bridge-circuit-revealed/

I don't understand this statement. Why would he need a half bridge? Wouldn't just a low side mosfet switch do it?

 

The half bridge will offer braking if needed.
Max.

 

The half bridge provides active control of motor current. With a low side switch you rely on a passive component to reduce the current to zero, which may take longer. This may be important if you wish to use PWM.

 

Thanks guys. Guess I've (in my ignorance) been lucky in just using a low side with PWM to control motor speed.

 

The half bridge will offer braking if needed.
Max.

How is that different from a diode across the motor?

 

A freewheeling motor generates same polarity as supply.
Max.