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Pulse Width Modulation


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Dear Sir/Madam,
Presently I am doing a project on wheelchair which use a 24V DC motor to drive it. I would like to implement the use of pulse width modulation to control the speed of the motor.
However, at my very best knowledge, all the PWM circuit that I saw is low input voltage which range from 5 to 12V. Hence do you have or know a PWM circuit with an input of 24V? Your suggestions or comments are welcome.

Biomedical Engineering
Ngee Ann Polytechnic

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  • 1 month later...

Do you think a series transistor would work? Set the current you will need when the voltage is 24V. Bias the transistor with the input voltage. Use a second transistor to reverse the junction. Drive that transistor with PWM clock. You can step down the voltage with resistors to run the PWM clock. A PWM clock can be constructed from an inverter oscillator with diodes and resistors to dissipate the voltage from the capacitor. There is an example of this in the theory section called "pulse width modulation".

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That is interesting. But where is the VDS you will need when the transistor is on. The motor will require 24V and you will be left without VDS. I should also note that mine too will have a low VCE at that current. So use a higher input voltage. This is the only way.

I have it. You could operate the transistor at it's saturation current. This is not a linear situation and the performance of the motor will suffer. But since you drive it into cutoff anyway, why not!

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That circuit is good for controlling the speed of small DC motors. However, for a wheelchair, its low 400Hz switching frequency may sound annoying and cause vibration. The circuit's old, slow opamp isn't capable of a higher frequency and its slew-rate is so slow that the MOSFET heats considerably during the switching time.
Modern high-power PWM circuits operate at 20KHz or more. A controller manufacturer's very detailed ramblings are here:

PWM circuits used for motor speed control, lamp dimming or Switched-Mode-Power-Supplies always operate the switching transistor at saturation and cutoff, in order to reduce the heat which is caused when a transistor conducts current with a voltage across it:
a) Transistor is turned-on hard, has low voltage across it and therefore its power dissipation is low. The load receives almost the full supply voltage.
b) Transistor is cutoff, has zero current through it and therefore its power dissipation is also zero. The motor's current continues through the freewheeling diode.
The PWM circuit modulates the average voltage across the load.

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sure, it would take some modification for more current. This is stated in the project. However, this is a step in the right direction from the one in the earlier post.

So, instead of only pointing out what you feel will not work, have you considered posting a link to the forum of a circuit which WILL work from time to time? Or perhaps, what you consider to be a better circuit? I think this would be much more helpful to others.


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You didn't read my link?
This stuff ain't simple.
It has a pretty-good PWM motor controller, but it is limited to only 90 percent of max motor speed, and becomes an expensive smoke generator (that can be fixed) when regererative braking down a steep hill or when being towed. The sub-link to this controller is here:

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Why is PWM so popular in motor control. I know why it is used, but what happened to the AC motor or any type of other variable waveforms that can be applied to motors. What about varying the voltage? How about varying the frequency instead of the pulse width. Can't you realize different motor properties with these other types of waveforms. I don't fully understand the motor but I know that you can have speed, torque and various load situations and that there is a countervoltage that develops. Any tidbits of information would be helpful to my understanding. Thank you.

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To All,

To minimize the temperature in the power stage it is important to have fast rise and fall times =(nice looking square wave). There are two ways to go with the choice of frequency. One is to calculate the optimal frequency for the motor used, second to use a high frequency like 15


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Thanks, Ante,
That CMOS 555 chip looks like it will control PWM much better than an analog triangle generator or simple inverters, due to its well-defined thresholds and speed.
I can use it for the electric model airplane that I won.
With a simple NPN-PNP transistor buffer, like the complicated circuit, it will drive a MOSFET transistor and the wheelchair very well.

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