Apparently I'm not that smart and can't turn an LED on in multisim

Discussion in 'General Electronics Chat' started by durable126, Feb 20, 2016.

  1. durable126

    Thread Starter Member

    Feb 20, 2016
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    1
    Apparently I'm not that smart and can't turn an LED on in multisim

    What am i doing wrong?
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    The led most likely has blown.
    Leds are current controlled devices.
    You will have to use a resistor in series with the led to control the current.
    Also what kind of mosfet do you use?
    Standard mosfets will turn on at 10 Volts on the gate.
    Logic gate mosfets will work as in your schamatic with the 5 Volts at the gate.

    Bertus
     
  3. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,813
    1,105
    Welcome to AAC!
    I don't use Multisim, but it looks to me as though your probes show a 218uA LED current, so it is 'on' (but not very bright :)). What FET are you using? If it's not a logic-level type it may need about 10V to turn it on properly.
     
  4. durable126

    Thread Starter Member

    Feb 20, 2016
    56
    1
    Hey guys thanks for the reply

    This is a generic "ideal" n chanel in multi sim

    Its threshold voltage is set to zero in the parameters therefor it should turn on with anything across the gate to source
    Also the LED don't blow in the simulator

    They are ideal and can take infinite current obviously not in real life but just in the sim

    Any other ideas?

    No one has any ideas on this?
     
    Last edited by a moderator: Feb 20, 2016
  5. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Could you put a probe directly at the drain of the mosfet?
    That way you can see if the mosfet is in conduction.
    If the voltage is 5 Volts at the drain, the mosfet is not conducting.

    Bertus
     
  6. JohnInTX

    Moderator

    Jun 26, 2012
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    I'd try a 2n7000 FET model. That way you know its params (Vgs(th)~=2.1V typ). If all you say is true the LED should be ON but it isn't. Maybe having a Vth == 0V is throwing it off. Try setting it to 2V.

    I'd suspect the FET. I'd also put a 240 ohm resistor in series with the LED.
     
  7. durable126

    Thread Starter Member

    Feb 20, 2016
    56
    1
    Thanks for the ideas

    I have tried all that. Im a reasonably experienced guy so i did try moving the threshold up just to see if it worked. I also have tried it with a resistor and not a resistor in with the LED.

    I have also put a probe on the drain and there is a drop of about 1.5 volts across the LED leaving 3.5 at the drain which is plenty for VDS

    Also i have tried exactly the 2n7000, funny you bring that guy up that is exactly what i tried.

    Still the same issue. No light

    Also just to go way way way back in the debugging i stuck a voltage source on the led to make sure it comes on and of course it does.

    I have to be missing something simple here but can't see it
     
  8. bertus

    Administrator

    Apr 5, 2008
    15,649
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    Hello,

    With 1.5 volts accross the led, the led may still be off.
    What kind of led did you select?
    A white led will need more than 3 Volts.
    A red or green led will need about 2 volts.

    Bertus
     
  9. durable126

    Thread Starter Member

    Feb 20, 2016
    56
    1
    like i said above its an ideal LED

    so it should run on anything
     
  10. absf

    Senior Member

    Dec 29, 2010
    1,493
    373
    I try with proteus and it's woring fine.

    LED IN MULTISIM.PNG

    If I push the button, the current goes up to 407mA and the drain voltage is 1.5V.

    Allen
     
  11. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    I think you need PMOS.


    pmos.jpg
     
  12. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Are you sure about that your diagram is correct ? Do you know why the voltage drop across the PMOS is 0.73V ? Also, have you tried turn off the LED by changing V2 to 0V ?
     
  13. crutschow

    Expert

    Mar 14, 2008
    13,056
    3,245
    No. Not it that circuit.
    A PMOS will be on all the time due the the parasitic substrate diode.
    To use a PMOS, it must be configured as a high-side switch, with the LED going to ground.
     
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