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MOSFET PWM Circuit Not Working

Discussion in 'General Electronics Discussion' started by mason5059, Feb 6, 2017.

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  1. mason5059

    mason5059

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    Oct 15, 2014
    Hi,

    I am trying to control the speed of a DC motor using a MOSFET and PWM from an Arduino. I made a small scale proof of concept circuit using the Arduino, a 9V battery, and a very small DC motor. When a button is pressed, a set duty cycle is sent to the gate of the MOSFET. This circuit works perfectly. The motor I plan on using is a larger 12V motor. I set up the circuit exactly the same way as the small scale, except put a flyback diode across the motor and a 220ohm resistor into the gate of the MOSFET. I am using a 12V power supply. This circuit does NOT work. Specifically, the motor will not turn when the button is pressed. I set the duty cycle very low and for a short duration on my first test attempt, and the motor "thought about" turning (audible noise appropriate for the duty cycle and duration I set). The motor has no load. I realize that obviously the biggest difference in these two circuits is the much larger motor and the 12V power supply. To give you an idea of the size, the 12V motor is about 5" in diameter and 5" tall, and came with a 250W power supply (not being used currently), and recommends a 30A fuse. I highly doubt that I will be pulling anywhere near that amount of current with the motor. I found and included a schematic of pretty much the exact circuit I am using right now, only difference being the 220ohm resistor into the gate.

    Why doesn't this circuit work? I have to be doing something wrong. I have checked the wiring multiple times - it all seems correct. Am I missing a key component to get this thing working?

    Thanks for your help.

    MOSFET datasheet: http://www.mouser.com/ds/2/149/FCPF150N65F-965447.pdf

    Power supply datasheet: http://www.mouser.com/ds/2/260/RSP-200-SPEC-806453.pdf


    PWM.jpg
     
  2. duke37

    duke37

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    Jan 9, 2011
    You have less than 5V into the gate, this could give you more than 2.5mA.:)
     
  3. Minder

    Minder

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    Apr 24, 2015
    Often large motors will not operate in the lower PWM range and require 50% and over duty cycle before they start turning.
    I would be inclined to use a lower gate voltage fet, such as logic level gate type.
    You have <5v available at the gate.
    M.
     
  4. mason5059

    mason5059

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    Oct 15, 2014
    I tried 50% and 100% duty cycle, did not work.

    Also, the gate voltage was able to drive the gate in the small scale circuit. Why would this be different in the larger circuit? According to the datasheet for the MOSFET the Vgs(th) (which is what I take to be the amount of voltage needed to switch the FET ON) is 2-4V.
     
  5. duke37

    duke37

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    Jan 9, 2011
    Vgs(th) is the voltage to just start the FET conducting. 10 or 12V is more realistic to turn it on fully.
    As Minder says use a logic level FET.
     
  6. mason5059

    mason5059

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    Oct 15, 2014
    Ok, so I found this logic level FET: http://cdn.sparkfun.com/datasheets/Components/General/FQP30N06L.pdf

    It has a considerably lower Vds (60V) than my current FET (650V). Is the flyback diode (1n4001) good enough that I will not have to worry about the voltage rising to the point of FET failure? I know that once you turn off the circuit there is a large voltage buildup that the diode should take care of. I can't find any logic level MOSFETs that are rated to handle more than 100V (hence the reason I didn't use one initially). Thanks for all your help.
     
  7. Minder

    Minder

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    Apr 24, 2015
    I thought you are driving a 12vdc motor?
    I use the IR range IRL series etc.
    M.
     
    duke37 likes this.
  8. BobK

    BobK

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    Jan 5, 2010
    The reverse voltage on the diode is the same as the voltage to the motor, so 12V is all you need.

    The problem is the current. If the motor came with a 250W supply, why do ypu expect it to draw less than 1A?

    Bob
     
  9. mason5059

    mason5059

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    Oct 15, 2014
    I don't expect it to draw less than 1A. I would be comfortable with a FET that can handle 15-20A, and an IRL that was suggested by another user I found is good for 18A continuous which should be plenty. A 30A fuse is recommended by the manufacturer, which to me is an absolute maximum with a certain amount of "wiggle room" so the motor isn't overloaded. I meant I don't think it will ever draw anywhere close to 30A during use for my specific application.
     
  10. Alec_t

    Alec_t

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    Jul 7, 2015
    Bear in mind that the starting/stall current of the motor could be ~ ten times its normal running current.
     
  11. Bluejets

    Bluejets

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    Oct 5, 2014
    Where is power for the Arduino.?
     
  12. Minder

    Minder

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    Apr 24, 2015
    It is simple to find the maximum current of a motor, supply a small known DC voltage with a locked rotor and measure the current.
    Find the resistance, do not use a ohm meter it is not accurate enough method.
    Also ensure you start with a low PWM signal.
    M.
     
  13. BobK

    BobK

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    Jan 5, 2010
    Yes you are, if you are using a 1N4001. The diode will carry the same current that the motor is drawing.

    Bob
     
    CDRIVE likes this.
  14. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Drawn with a White pen. :D

    Chris
     
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