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DC motor very slow discharge, mosfet to small ? 5v circuit.

Former Member

Been playing around with a DC motor #learningByDoing

 

And found for some reason, my DC motor wont discharge at a normal rate/instant,

It's as if it keeps power, but it's slowing down, slower.. slower.. slower..  until it finally stops.

 

image

The motor is connected to the black/red wires to the left, and have a diode as fly back.

 

It's grounded through the P-fet in the top, via the red wire, and green to ground.

Yellow is the signal from the arduino on port A0,

 

See the video --

 

 

So what am i doing wrong here, since the charge is not instant?

 

any ideas ? image

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    You say your using a PFET

     

    Is this a P Channel MOSFET or something else, can you please provide the part number

     

    The way you have everything connected as best I can see, the motor is on the high side of the supply and the FET is taking the black wire of the motor to ground based on the output of the Arduino, please confirm, provide a hand drawn schematic if needed to help

     

    If my assumption is correct then you have the wrong device

     

    PChannel fets need their gate taken to a negative value relative to the source and as this is connected through the motor to plus V, it should turn on but then once on it will sit at a floating voltage as now a current is passing through it, the source is no longet at +V, even when the gate goes back to +V, the back emf even with the diode still may be enough to sustain a small current (Heavy capacitive coupling between the pins of a fet), if you have a scope, you may see it oscillating

     

    If the PFET is connected to the high side of the power then you would need to pull the gate atleast 4V below the source to get it to turn on and even then only partly, this is the prefered configuration for a pchannel fet used as a switch, for low side, use an NChannel fet

     

    Hope this helps, if not, provide the schematic and a part number of the fet in use

Reply
  • 0

    You say your using a PFET

     

    Is this a P Channel MOSFET or something else, can you please provide the part number

     

    The way you have everything connected as best I can see, the motor is on the high side of the supply and the FET is taking the black wire of the motor to ground based on the output of the Arduino, please confirm, provide a hand drawn schematic if needed to help

     

    If my assumption is correct then you have the wrong device

     

    PChannel fets need their gate taken to a negative value relative to the source and as this is connected through the motor to plus V, it should turn on but then once on it will sit at a floating voltage as now a current is passing through it, the source is no longet at +V, even when the gate goes back to +V, the back emf even with the diode still may be enough to sustain a small current (Heavy capacitive coupling between the pins of a fet), if you have a scope, you may see it oscillating

     

    If the PFET is connected to the high side of the power then you would need to pull the gate atleast 4V below the source to get it to turn on and even then only partly, this is the prefered configuration for a pchannel fet used as a switch, for low side, use an NChannel fet

     

    Hope this helps, if not, provide the schematic and a part number of the fet in use

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