N-MOSFET Help Please

Discussion in 'General Electronics Chat' started by Paradoxz, Aug 23, 2011.

  1. Paradoxz

    Thread Starter New Member

    Aug 2, 2011
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    0
    Yesterday was my first time working with a N-MOSFET, or any Transistor as I am still a newb =) I found a great video on youtube:
    http://youtu.be/Te5YYVZiOKs

    Why is it so easy for the guy in the video but I can't achieve the same results?

    Even with nothing hooked to the gate (example image below), the LED lights. According to the video you have to apply voltage to the gate to transfer the ground from the drain to the source.

    I have tried 10000x attempts now, resistors in every possible order, multiple mosfet's, everything. What am I missing?

    [​IMG]

    [​IMG]
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You need a 10k resistor from the gate to the source to keep the MOSFET turned off by default.
    Your supply negative lead should be connected to the MOSFET source terminal.
    Your LED cathode should be connected to your MOSFET drain terminal.
    Your LED anode should have a current limiting resistor connected between it and the battery positive terminal.

    To turn the MOSFET on, you need to put a positive voltage on the gate. If it is a standard-level MOSFET like an IRF510, you will need 10v from the source to the gate in order to turn the MOSFET on fully. Logic level MOSFETs can generally work when Vgs is 5v.
     
  3. Paradoxz

    Thread Starter New Member

    Aug 2, 2011
    8
    0
    That is the second time I have seeked help, and the 2nd time you have came to my rescue. I really appreciate it. I was doing two things wrong:

    1) I had the drain and source backwards.
    2).... Yes, it's a real bad newb mistake.... I had the pin from the microcontroller connected to the gate... but there was no ground from the microcontroller (yea, I know.. what good is the output pin only).
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Just so you know, there is a "body diode" in the MOSFET between the source and drain terminals. If the drain is more positive than the source, the body diode won't conduct; but if you connect them backwards, the diode will conduct, and it will more-or-less look like the MOSFET is shorted.

    Glad that helped you figure out the difficulties you were facing. :)
     
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