N channel MOSFET not working properly

Discussion in 'Power Electronics' started by zazas321, Jun 14, 2019.

  1. zazas321

    Thread Starter Member

    Nov 29, 2015
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  2. ericgibbs

    Moderator

    Jan 29, 2010
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    hi Z,
    If I am reading your circuits correctly, you are shorting the 12V to 0V.!!!!
    E
     
    sagor likes this.
  3. zazas321

    Thread Starter Member

    Nov 29, 2015
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    I was under impression that everything should be connected to a common ground? Where should I connect the negative side of +12V supply?
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    When you want to switch the lamp X1, then it should be in series with the mosfet.
    Place the lamp X1 between de Drain of the mosfet and the +12 Volts.

    Bertus
     
  5. zazas321

    Thread Starter Member

    Nov 29, 2015
    412
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    Please note another odd thing I have come across.
    upload_2019-6-14_11-9-41.png

    upload_2019-6-14_11-9-56.png

    Note the voltages after the switch. How can I have 2.5V after the switch if the switch is open? Shouldnt it be 0V?
     
  6. zazas321

    Thread Starter Member

    Nov 29, 2015
    412
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    By placing the light bulb in series with the mosfet as bertus suggested, the bulb is permamently off despite the switch status.
    upload_2019-6-14_11-14-29.png
    upload_2019-6-14_11-14-37.png
     
  7. ericgibbs

    Moderator

    Jan 29, 2010
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    hi,
    How have you determined the N MOSFET specification.?

    Try an actual MOSFET model?.
    E
     
  8. zazas321

    Thread Starter Member

    Nov 29, 2015
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    What is the difference between using a single MOSFET to drive a higher voltage and the following circuit? :

    upload_2019-6-14_11-30-1.png

    I think this simulation software is a bit shit
    I am adding a picture of MOSFET specs - it doesnt help me much.
    upload_2019-6-14_11-31-48.png
     
  9. ericgibbs

    Moderator

    Jan 29, 2010
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    hi z,.
    Are these circuits part of a college assignment.?
    E
    BTW: the last two images are too small to read.
     
  10. Kjeldgaard

    Member

    Apr 7, 2016
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  11. zazas321

    Thread Starter Member

    Nov 29, 2015
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    That is not a part of any assignment. I am trying to build a DALI controller for lightning and I require to control 18V with 3.3V logic. My apologies, let me upload the pictures again:
    upload_2019-6-14_11-40-10.png

    upload_2019-6-14_11-41-23.png
    With this configuration, the lightbulb is also constantly ON and doesnt change if I press the switch
     
  12. ericgibbs

    Moderator

    Jan 29, 2010
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    hi,
    Check the orientation of Q2.!
    E
     
  13. zazas321

    Thread Starter Member

    Nov 29, 2015
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    Yes I have flipped the Q2 mosfet the other way arround as shown in the example picture - now the lightbulb is permamently OFF .
    upload_2019-6-14_11-51-3.png

    upload_2019-6-14_11-51-14.png
     
  14. Kjeldgaard

    Member

    Apr 7, 2016
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    What does the simulator measure of voltage on the Q2 gate?
     
  15. zazas321

    Thread Starter Member

    Nov 29, 2015
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    upload_2019-6-14_12-1-4.png

    upload_2019-6-14_12-1-45.png

    I am measuring 12V when switch open and 11.75 when closed
     
  16. ericgibbs

    Moderator

    Jan 29, 2010
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    hi,
    How have you determined the N MOSFET specification.?

    Try an actual real MOSFET model?.
    E
     
  17. zazas321

    Thread Starter Member

    Nov 29, 2015
    412
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    upload_2019-6-14_12-11-15.png
    Please see the specifications on the left if you can see it. Also, this is the only N type mosfet available on this application.
     
  18. ScottWang

    Moderator

    Aug 23, 2012
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    The IRF530 is not a 4.5V or 5V logic level N MOSFET, you can use a npn bjt to replace the IRF430, to add a Rb as 2.2K ~4.7K, and a Rbe as 10K.
     
    shortbus likes this.
  19. zazas321

    Thread Starter Member

    Nov 29, 2015
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    Im not actually using IRF530 , im just trying to simulate any basic MOSFET switch.
    upload_2019-6-14_13-0-1.png

    upload_2019-6-14_13-0-16.png

    I have swapped the first MOSFET with the BJT transistor and im trying to switch P type MOSFET. I have put loads on both sides as I was not sure which side do I need to use but it does not show any signs of life
     
  20. ScottWang

    Moderator

    Aug 23, 2012
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    For a P MOSFET that you should in series the load with D pin, and R1 can be higher as 4.7K, the Ic of npn bjt only needs less <10mA at here, Ib = Ic/10, so the Ib can be <=1mA.

    Edit : You missed the Rbe, if there is no Rbe, when the S1 switch is OFF then the input will be as a floating status, the noise will be generates as a AC signal.
     
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