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I am working on a project; driving a DC MOTOR using an NMOS MOSFET amplifier and Arduino PWM signal.

My problem is my transistor amplfiler is always on when I vary the PWM input value from 0 to 255. Also, the motor runs faster in smaller PWM value and runs faster in larger PWM value. But, my intention is to run the motor faster in larger PWM value..

The below is my current circuit diagram. My transistor is "RFP30N06LE" NMOS MOSFET, my motor is Buehler 4-wire Precision DC motor That can run from 6V to 24V.

enter image description here

Could someone educate me on why my transistor is not getting turned off, and the motor runs faster with smaller PWM value to the amplifier please ?!

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  • \$\begingroup\$ Please provide a link to the datasheets for the power supply, the motor, and the MOSFET. You have connected the motor in parallel with the MOSFET which is clearly wrong...they should be in series, with the MOSFET closer to ground...but without more information it's hard to give constructive advice. Where are the other two wires for your motor connected? \$\endgroup\$ – Elliot Alderson Dec 30 '18 at 3:05
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In the wiring diagram that you're showing, you won't see any PWM regulation as you're applying directly 24.5V to the motor when the transistor is off. You're basically shorting the power supply if you turn the FET on. Below you can see the typical low side switching configuration.

enter image description here

Image source: https://www.gammon.com.au/motors

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  • \$\begingroup\$ Thank you. I modified my circuit as the one you shared. It functions normal. \$\endgroup\$ – Steve Chung Dec 30 '18 at 2:15
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Could someone educate me on why my transistor is not getting turned off,

from your schematic drawing, we can see that your motor always have the voltage your battery can supply because you connect it parallel with the battery. Turned on MOSFET sucks more current. Usually, this causes the VDS at your circuit drops. But as long as your battery can supply enough power (current) at constant voltage, your motor always gets the required voltage it needs.

and the motor runs faster with smaller PWM value to the amplifier please ?!

in your circuit, smaller PWM causes less current flows on the MOSFET than when the PWM is higher. In other words, in low PWM, your motor could receive more current. Again, it is under assumption that your battery is an ideal voltage source.

to make it acts like what you wanted, I suggest you to check what is called double pulsed circuit like in the link below. Double Pulse test

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  • \$\begingroup\$ Thank you for knowledge sharing. I modified my circuit as Xavier shared. My motor is in serial between a 12V power supply and the drain of my amplifier, and the motor only runs after a certain PWM value from 0. \$\endgroup\$ – Steve Chung Dec 30 '18 at 2:20

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