MOSFET's - Just curious

Discussion in 'General Electronics Chat' started by Ford Prefect, Nov 26, 2017.

  1. Ford Prefect

    Thread Starter Senior Member

    Jun 14, 2010
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    16
    Hi guys,

    I have been using MOSFET's for years and all the schematics that I see have the MOSFET's connected as shown in schematic 1 below and I have followed this method/rule/convention of connecting the MOSFET with the load between the Vcc and the MOSFET Drain.
    But I am just curious and have not tried it, but can a circuit be connected as in schematic 2 with the load between the GND and the MOSFET source?
    If it is possible to connect it as in schematic 2, why is it that all schematics are shown as in schematic 1 - or is it just a convention that people use?
    MOSFET.jpg
     
    Last edited: Nov 26, 2017
  2. bertus

    Administrator

    Apr 5, 2008
    18,374
    3,523
    Hello,

    For using a N-fet as a high side switch, you will need a high side driver.
    The N-fet will only work when the gate voltage is higher than the source.

    Bertus
     
  3. Ford Prefect

    Thread Starter Senior Member

    Jun 14, 2010
    135
    16
    Hello Bertus, thanks for your answer. But a MOSFET is just and acts like a toggle switch, if used as in schematic 2 it is just providing the Vcc to the other side of the load which is connected to GND. - Or am I missing the point?
     
    Last edited: Nov 26, 2017
  4. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    It will work but, BUT,
    If you have a standard MOSFET that needs the gate to be 10v above the source, the gate, in drawing #2 will need 22v if V+ for the relay is 12V
     
    Last edited: Nov 26, 2017
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  5. joewales44

    Member

    Oct 8, 2017
    143
    4
    finally someone with the same question i had.
    i had a hard time grasping this too.
    just remember the gate must see a 'difference' in voltage from the source/drain, not just any signal.
    most need 10 volt difference to fully turn on.
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    hobbyist likes this.
  7. dl324

    AAC Fanatic!

    Mar 30, 2015
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    You should just try it. When you do, you'll find out why it has issues and a P MOSFET would normally be used to drive a relay high side.
     
  8. Ford Prefect

    Thread Starter Senior Member

    Jun 14, 2010
    135
    16
    Aah. got it. many thanks for the explanations. :)
     
    GopherT likes this.
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