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I am trying to control the speed of a DC motor speed by varying Vds and keeping Vgs constant at 9volts by potentiometer, but I am not getting appropriate and smooth control of the DC motor this way. The speed increases only at the end point of potentiometer when it starts burning.

Why this is happening?

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    \$\begingroup\$ A MOSFET + DC motor is only usable to fully switch on the motor or switch it completely off. Using it like a variable resistor like you're trying to do puts a lot of stress on the MOSFET and doesn't work properly as the MOSFET isn't designed to do that. A MOSFET is capable of switching on/off very fast and when that is done using a PWM signal, you can control a DC motor. I strongly suggest that you search the internet for projects where a DC motor's speed is controlled and do it the same way. What you're trying to do doesn't work, what makes you believe it would work? \$\endgroup\$ – Bimpelrekkie Dec 3 '20 at 12:16
  • \$\begingroup\$ Show your schematic. \$\endgroup\$ – Andy aka Dec 3 '20 at 12:40
  • \$\begingroup\$ I’m voting to close this question because as explained above the described scheme is fundamentally unworkable regardless of the many missing details. A workable scheme would be entirely different. If after real study of how PWM is done with a low side FET switch there remains a specific question, that distinct, new question could potentially be on topic and productive. But self research on PWM is needed first. \$\endgroup\$ – Chris Stratton Dec 3 '20 at 15:35
  • \$\begingroup\$ (To be clear, even if you wanted to use an FET in a lossly linear manner, you would have to vary Vgs not Vds. Varying Vds would just be using an adjustable power supply or rheostate to control the motor with the FET having no role at all) \$\endgroup\$ – Chris Stratton Dec 3 '20 at 15:38
  • \$\begingroup\$ "I am trying to control the speed of a DC motor speed by varying Vds and keeping Vgs constant at 9volts by potentiometer" - this make no sense. Please show us a schematic of the actual circuit. \$\endgroup\$ – Bruce Abbott Dec 3 '20 at 17:59

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