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I am using a mosfet (IRFZ44N) to control a dc motor, and a separate one to control a led strip. Both are using the same circuit running to different pwm pins on the arduino.

The light strip is working fine (can be turned on an off as expected), but the motor runs at a low level when the pin sends a LOW state, and revs up when it is sent a HIGH state.

They are both receiving power from the VIN pin of the arduino which in turn in plugged into a 12v 6a dc adaptor.

Any ideas what could cause this behavior?

(Apologies if this is a messy schematic, I am a beginner)

schematic

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  • Please include a schematic of ypur circuit. Generally, this transistor is a bad choice due to its high gate-source-saturation voltage of 2-4V. For switching applications you want to choose one with a Vgs,sat several times lower than your logic level. There are dedicated "logic-level mosfets"
    – Sim Son
    Oct 22, 2019 at 15:51
  • Just added a schematic, hope it represents what I have here. I'm finding that the mosfet gets very hot, maybe it can't handle the current going through it.
    – ltrell
    Oct 22, 2019 at 16:05

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There are some mistakes in the provided schematic:

  • the nmos has to be connected to motor-

  • the freewheeling diode has to be the other way around

  • the resistor doesn't make sense. If it's meant to be a pulldown is has to connect the gate with ground and have a much higher value (~10kOhm). If it's meant to be an inrush limiter it has to be in series with the gate

Actually I don't trust you schematic at all, because I wonder why the motor runs at all without damaging your arduino. Have a look at the low-side transistor configuration here to see how it is done.

As I mentioned in a comment, this mosfet is a bad choice because it has a too high gate-source threshold voltage Vgs,th (that's the gate voltage at which the transistor starts to conduct. The voltage at which it has the specified on-resistance Rds,on is several times higher than this. Choose a device with a Vgs,th below 1V or go for a logic-level mosfet.

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  • Thanks, I will try to replicate the circuit in the article
    – ltrell
    Oct 22, 2019 at 16:44

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