Choosing a mosfet

Discussion in 'The Projects Forum' started by shaggydog, Feb 20, 2014.

  1. shaggydog

    Thread Starter New Member

    Feb 20, 2014
    1
    0
    Hi,
    I am doing a temperature control student project. I am using Labview and a DAQ6008. I want to control the speed of 12vdc fan using a 0-5v o/p from the DAQ. I have a slider on my front panel which I hope to control the speed of the fan. I have used an IRFZ44 N channel mosfet. This will only turn my fan on/off only. The fan is rated at 0.70A. I need a replacement mosfet. Can anyone suggest a suitable replacement.

    Shaggydog
     
    Last edited: Feb 20, 2014
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,553
    2,375
    Slight overkill but the same package, IRL520N Logic gate Mosfet,
    Max.
     
  3. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    You can not use an analog voltage to drive a mosfet.
    You will need a PWM signal to control the mosfet.
    The duty cycle of the PWM signal will change the speed of the fan.

    Bertus
     
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    Sorry Bertus but I will argue that.

    The fan will have a linear range where an analogue voltage controls speed.

    And the FET will also have a linear range where a change in gate voltage (Vgs) causes a change in ON resistance (Rds).

    I'll agree that your suggestion of using PWM is likely to be a much better option, but it's not correct to say it cannot be done linearly.

    Especially since most of these small DC fans now have a tacho feedback output, which could help to eliminate linearity issues with the FET Vgs and temperature issues etc.

    What the OP is probably after is a FET with better linear performance, like some of the audio amplifier linear FETs.
     
  5. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    Ok, you can drive the mosfet in analog mode.
    The effective range will be limited, as the gate threshold voltage of the chosen mosfet is between 2 and 4 volts.
    This means that below the threshold the mosfet will do nothing.
    Also the mosfet must be cooled as the power in the mosfet must be dissipated.
    With PWM the powerloss in the mosfet will be much smaller.

    Bertus
     
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