MOSFET control

Forums dropController Forum General Questions MOSFET control

This topic contains 4 replies, has 2 voices, and was last updated by  Ed 1 week ago.

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  • #3315 Reply

    Ed Lomax

    I like the design of the version 3 but I have one question
    Given that the MOSFET control pins will always be HIGH or LOW then the gate of th MOSFET can never float. This seems to make the 10k pull down resistor unnecessary. The only time the gate can float is if the Arduino fails.
    Am I missing something

    #3316 Reply

    Martyn
    Keymaster

    Once the Arduino is powered the gate on the mosfet should follow the Arduino pin and the pull down resistor is not required. However, while the Arduino is starting, or it is being reset, Arduino pins default to inputs until the sketch starts and the mosfet gate will be floating.

    #3317 Reply

    Ed

    Thanks Martyn. I guess I need the resistor. I do not have 10k but i presume a 3k will be adequate

    #3319 Reply

    Martyn
    Keymaster

    The larger the resistor the better but 3K will be OK if using a IRL540N or similar (give it a try and see what happens)

    The way the resistors are connected forms a voltage divider. So the smaller the second resistor the less voltage gets to the mosfet.

    With a 10k resistor the amount of voltage lost/diverted is negligible
    5v in, R1 = 220, R2 = 10,000. V out is around 4.9v.

    With a 3K resistor
    5v in, R1 = 220, R2 = 3,000. V out will be around 4.6V
    The IRL540N is OK for 4.0v so 4.6v will be ok.

    IRL540N
    RDSon @ 5.0V = 0.053 ohm
    RDSon @ 4.0V = 0.063 ohm

    You could reduce the 220 ohm resistor to 100 ohm. This is work OK and increase v out slightly.
    V in 5, R1 = 100, R2 = 3000, V out will be around 4.8v

    #3320 Reply

    Ed

    Thanks Martyn
    I am using IRLZ44n which is similar
    I will try the 100ohm/3K combination

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