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High frequency noise from DC motor using PWM

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Silver TT

New Member
Hi, I'm driving a 12v 10A DC motor with a micro-controller using a MOSFET. I'm getting a high frequency buzzing noise from the motor that goes away when I just use the 12v straight. Is there anything I can do to solve this?

I want to try using a digital to analog circuit (a resistor-capacitor circuit on the gate). How do I size the capacitor and resistor for this?

Are there any other things? I wanted to increase the PWM frequency but the controller wont allow that.

Thanks!
 

ronsimpson

Well-Known Member
Most Helpful Member
please show your circuit.
What frequency?
What micro?
What MOSFET?
I want to try using a digital to analog circuit (a resistor-capacitor circuit on the gate). How do I size the capacitor and resistor for this?
You can do this but the MOSFET will get very hot.
 

Silver TT

New Member
I'm sorry, I actually don't know the frequency because the manufacturer of the controller hasn't published it. I suppose I could check this with an oscilloscope - if I bought one :)

But this is the MOSFET https://www.sparkfun.com/products/10213.
Motor is on the high side with a snubbing diode across the leads. Pretty simple.

Why will that make the mosfet get hot?
I'm willing to try it, but I don't know where to start with sizing the resistor and capacitor.

Thanks,
Sean
 

ronsimpson

Well-Known Member
Most Helpful Member
Because I do not know much about the motor I will use a 1.2 ohm resistor.
12 volts 10A = 120watts. So the motor will heat with 120 watts.

1)The MOSFET has a resistance of 0.035 ohms. When on hard. So when the motor is on 100% of the time, the MOSFET will have 3.5 watts. (need small heatsink)

2)When the MOSFET is off there is 0 amps, 0 watts, 12V on the part.

When in PWM mode you are ON part of the time and OFF part of the time. At 50% there will be 3.5 watts for 50% and 0 watts for 50% so the MOSFET will heat like 1.25 watts.
-----------------------------------
Using a DAC mode the MOSFET will be turned on so 1/2 the voltage will be across the motor and 1/2 across the MOSFET.
6V on motor and 6V on MOSFET. 5A Motor=30 watts. MOSFET=30 watts. (VERY BIG HEATSINK)
 

dr pepper

Well-Known Member
Most Helpful Member
Another way would be to run the motor on Dc, this could come from a smps operating above audible frequencies, however motors run on Dc are more difficult to control than Pwm as the torque is less due to the dynamo effect in the motor, you can reduce this effect by using back emf as part of a feedback loop in the control system.
This gets complicated and if you dont have a scope then its probably not something you'd want to try.

To find the switching speed of your existing motor, download some software that will generate tones at a fequency you want using the sound card, fire up the motor and adjust the tone so that it matches that of the motor, you'll hear a 'beat' as the tone freq approaches that of the motor which will stop when the 2 are matched, the frequency of your tone will then be the switching speed.
My battery power drills are about 4 khz.
 
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