Logic Level Mosfet Switch - fail

Discussion in 'The Projects Forum' started by mndless, Aug 15, 2012.

  1. mndless

    Thread Starter New Member

    Jun 1, 2011
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    There is something I am failing to understand properly.

    The objective is to use a 3.3V microcontroller to switch a gang of LED's that needs 9V...

    I first used a 2N7000TA mosfet and could not get more than about 2.5V out of the Source... so... did some more research and decided that maybe the gate threshold on that mosfet was too high..

    Determined that I needed to be using something more in line with a Logic Level Mosfet... so, I plugged in a PSMN022 Logic Level Mosfet... as shown in the attached picture....


    [​IMG]

    the ciruit is this... just replace the NPN graphic with a mosfet...

    [​IMG]

    No real change in the output voltage... I know I must be missing something simple and obvious...

    Any and all input and direction would be greatly appreciated....
     
  2. tracecom

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    It's impossible to tell from the schematic, but it sounds like you have the source and the drain on the MOSFET reversed.
     
    Last edited: Aug 15, 2012
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  3. Papabravo

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    Feb 24, 2006
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    What would you expect from a guy who can't even find the correct symbol for his schematic. There was a time when we could draw any symbol we could imagine.
     
  4. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Take a peek at Figure 1. On-Region Characteristics for the 2N7000 and show me where you are operating, keeping in mind that curve is typical and not guaranteed. (Translation: logic level FETs are usually for 5V logic, not 3.3V).

    Now the IRLZ44N tracecom looks a little better down at that level: It's curve indicates it should handle 10 amps (hopefully an overkill) when driven by just (again, typically) 3V.
     
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  5. bretm

    Member

    Feb 6, 2012
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    But since it's an N-MOSFET it should be used as a low-side switch. You have 3.3V at the gate, but 2.5V at the source, so there's only 0.8V Vgs.

    Connect the source to GND, and put the LEDs between Vcc and the drain. Then Vgs will be the full 3.3V.
     
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  6. mndless

    Thread Starter New Member

    Jun 1, 2011
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    Papabravo, here you go my friend... this should help...

    To the rest, thanks for taking the time to have a look... looks like just keeping the mosfet on the low side should be simple...

    Thanks again, great help.
     
  7. #12

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    Nov 30, 2010
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    I just clicked a "Thanks" on the post that said what I would have said.
     
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  8. Papabravo

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    Feb 24, 2006
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    CAD tools don't always allow us to rapidly express our intent. People in a hurry tend to take shortcuts. Learning the discipline of Engineering teaches us that taking shortcuts often leads to disasters of monumental proportions. Using an NPN symbol for a FET creates the negative impression. Nothing I say or don't say can change that.

    I am quite well satisfied with the state of my mental health, so no thanks for the impertinent suggestion. I think I'll take a pass on that one.
     
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  9. mndless

    Thread Starter New Member

    Jun 1, 2011
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    I knew a thoughtful response was in there somewhere...

    For the record, I'm a 50 year old Structural Engineer, .. I'm not an electrical guy. That is why I asked a question on this forum, looking for some input from the crew much more versed in things elecrtical than I....

    as stated in my initial post, "Please ignore the graphic for the NPN..." guess you didn't take the time to read carefully or as I suspect, when given the choise of posting a thoughtful and helpful reply or one that is snarky and disrespectful, you opt for the later... hense my link ... I think you should spend some time there doing some reading...


    Thanks again for taking the time to type out a reply!!
     
  10. tracecom

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    Apr 16, 2010
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    So, do you have the circuit working?
     
  11. mndless

    Thread Starter New Member

    Jun 1, 2011
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    Going to breadboard up the changes this evening... haven't done that yet, but, should be headed down the right path at this point....

    Thanks!
     
  12. tracecom

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    Apr 16, 2010
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    Since my last post in this thread, I remembered the attached project. It shows four N-channel MOSFETs being driven by a microcontroller. I actually built this circuit and it worked well. Good luck.
     
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  13. Papabravo

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    Feb 24, 2006
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    You have me all wrong. I much prefer snarky and disrespectful. BTW imprecision in structural engineering has profoundly negative consequences as I'm sure you are aware. I would expect you to know better, and I'm surprised at your apparent lack of seriousness.
     
  14. mndless

    Thread Starter New Member

    Jun 1, 2011
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    Thanks Papabravo, I hope you are as successful in your warm welcome to future new participants to this forum...
     
  15. Ron H

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    2N7000 is not guaranteed to have low ON resistance at Vgs=3.3V. It is only specified down to Vgs=4.5V. The worst case threshold voltage @ Ids=1mA is 3.0V.
     
  16. tracecom

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    The OP has changed to a PSMN022-30 MOSFET. See post #1.
     
  17. Ron H

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    Doooohhhh...
     
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