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P channel MOSFET Current Flow

Discussion in 'General Electronics Discussion' started by Nishitha M C, Oct 19, 2019.

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  1. Nishitha M C

    Nishitha M C

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    Oct 19, 2019
    Dear Sir,
    I am using FDS4935 P channel MOSFET for my application. I have a doubt, when the gate voltage is 0v and source will be 5v, then our P channel MOSFET will be ON. I learned it When Vgs < Vth MOSFET will be ON then what will be the drain current? when i connect the LED at the drain, whether full 5A will flow as per datasheet.
     
  2. Nishitha M C

    Nishitha M C

    3
    0
    Oct 19, 2019
    And also what abt the Vds and Vd?
     
  3. WHONOES

    WHONOES

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    May 20, 2017
    The LED will only draw as much current as it needs. If it is rated at 20mA then that is what it will demand regardless of what the FET is rated at. Bear in mind that the LED will need a series resistor to limit the current into it. As an example, if your supply is 5V and the LED forward voltage is 2V and its current demand is 20mA, then the resistor needs to be 3 / 0.02 = 150Ω.
    Drain source voltage needs to be enough to accommodate the power supply voltage with a bit in hand. eg, for a 20V supply, a 30V Fet would be adequate though there is nothing to stop you using one with a higher Vds.
     
  4. Nishitha M C

    Nishitha M C

    3
    0
    Oct 19, 2019
    Ok sir.... Another question is I am using source voltage is 5v and gate voltage is 0v means what about the voltage will appear in the drain because i am connecting the LED at the drain pin. LED required some forward voltage to be active. If my source is 5v means drain voltage is also 5v?
     
  5. WHONOES

    WHONOES

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    May 20, 2017
    If you power supply voltage is 5V then the voltage that will appear on you Fet drain minus the forward voltage of the LED. If you include a current limiting resistor in the LED as you should if it does not have one then, the drain voltage will be almost zero.
     
  6. BobK

    BobK

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    1,688
    Jan 5, 2010
    Unless you are approaching the current limit of the MOSFET, the voltage at the drain can be considered to be 5V when the MOSFET is fully turned on. You cannot just connect an LED between 5V and ground, it will likely be destroyed, so, as already stated, you must have a resistor in series with the LED.

    Bob
     
  7. WHONOES

    WHONOES

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    May 20, 2017
    Surely the drain voltage will be less than the supply voltage when the Fet is fully enhanced?
     
  8. BobK

    BobK

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    Jan 5, 2010
    Yes. The datasheet for that part indicates a 28 mOhm resistance at -4.5V Vgs. So, with a 20mA LED powered through the drain, the voltage at the drain will be:

    5 - 0.028 * 0.02 = 4.9994V

    And at 1A it will be 4.972V

    That is why I said it can be considered to be 5V.

    Bob
     
  9. WHONOES

    WHONOES

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    May 20, 2017
    Must confess, I was thinking in terms of an N channel device. That's what you get for only scanning a post. You are correct for a P device the voltage would be within 7mV of the 5V rail. The datasheet I found quoted Rds as 35mV for 4.5V Vgs.
     
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