MOSFET Switch help

Discussion in 'General Electronics Chat' started by spinnaker, Dec 18, 2012.

  1. spinnaker

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    I have the switch on the left working just fine.

    [​IMG]

    What I would like to do is for my light to share a common ground.

    Will the configuration on the right work just as well for me? Any issues I need to be aware?

    The LED is going to be one of those 12V LED bulbs. The Mosfet is an FQU13N06 N channel logic level mosfet.
     
  2. Audioguru

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    The circuit on the left needs a "logic level" Mosfet.
    The circuit on the right WILL NOT WORK because when the gate of the Mosfet goes to +5V then the source pin that drives the LED barely gets any voltage.
     
  3. #12

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    Yeah. Big problem. The mosfet is going to act like an emitter follower so the output voltage will never be 12 volts. In addition, the gate of the mosfet must be a few volts higher than 12 volts to get most of the 12 volts to the LED.
     
  4. spinnaker

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    Thanks

    Is there any way I can have a logic level mosfet switch and share a common ground with the light?

    Would a P channel configuration work?
     
  5. #12

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    Use a mosfet for the transistor on the left.
    I recommend this circuit so often that I keep a copy on my desktop.
     
  6. spinnaker

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    Sorry I am confused where is the load? And that would be a P channel mosfet?

    Why would I need a transistor and a mosfet if I have logic level mosfets?
     
  7. Ron H

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    FQU13N06 is not a logic level device.
     
  8. spinnaker

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    I left out the L From the datasheet FQD13N06L / FQU13N06L 60V LOGIC N-Channel MOSFET
     
  9. Ron H

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    What is your PIC supply voltage? What is the Fwd voltage of your LED? The current?
     
  10. spinnaker

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    The pic pin is 5V. The voltage to be switched is 12V. In the actual project it will be an 2 12V LED bulbs in parallel containing it's own current limiting device. Each bulb has 60 leds and is rated at 3W.
     
  11. spinnaker

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    I would also like to know how to search for logic level mosfets on mouser. I asked this question before and was told search for "logic level" . Well that does not work. For example NDP6020P is a logic level P Channel FET yet does not come up on mouser when "logic Level" is searched.


    Edit:

    A log of searching I found this:

    http://forum.allaboutcircuits.com/showthread.php?t=27262

    from Ron.

    So I need a mosfet with a vgs of 4.5V and a low rds(on) is there anything else I need to know other than the parameters on the voltage and current I plan to switch?

    Edit again:

    Rats It looks like mouser does not have a search for vgs on mosfets. How on earth are you supposed to find logic level mosfets?
     
    Last edited: Dec 18, 2012
  12. spinnaker

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  13. #12

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    OK. I changed the symbols to mosfets and labeled the load terminals.
     
  14. spinnaker

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    What are the values of the resistors? Again why do I need two mosfets?
     
  15. Ron H

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    You can use an NMOS and a PMOS, an NPN and a PMOS, an NMOS and a PNP, or an NPN and a PNP. The first option is the most efficient, and you only need the two transistors and one resistor.

    The attachment shows how I would do it.

    This all assumes that you aren't going to be doing any high-speed switching.
     
  16. #12

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    It's ok if you don't want to use the circuit I posted.
     
  17. spinnaker

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    Gee thanks. I guess it is too much trouble to answer my question.
     
  18. spinnaker

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    I might want to do PWM at some point. How would that change things?

    Also can you please tell me why two mosfets are needed? And can you recommend 2? I would assume the one driven by the mcu would be a general purpose mosfet?

    I think I have seen p channel switches with one mosfet for mcus. Or am I wrong?


    Edit: Oops I see you made a recommendation for the first mosfet. Thanks


    Edit: I do have some SPP18P06Ps on hand but I don't have any GP MMOS but I do have some GP NPNs . Can the SPP18P06P be used for that second PMOS?
     
    Last edited: Dec 18, 2012
  19. Ron H

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    What would your max PWM frequency be? PWM requires faster rise and fall times than quasi-static operation. You would probably need a more complex driver, or at least waste more current in the first transistor.

    The reason you need two transistors is because you want a high side driver that can switch 12V, and you have a 5V control signal. You can't use the circuit you posted because, with a MOSFET, Vgs has to be several volts before the device begins to turn on. When your gate is at 5V, the source will be around 2-3V. If it tries to go higher, it turns off. You can't turn on a 12V LED lamp with 2 or 3 volts.
    You can't use a single PMOS because the source has to be at +12V, and Vgs has to be zero volts to turn it off. This means your gate voltage has to go to +12V. Your PIC only goes to +5V.
     
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  20. spinnaker

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    Not sure right now what my PWM would be. I'd need to check my source code and see what freq I am currently running to see what pwm freq is possible.

    Thanks for the explanation. But I am still a little confused with my NMOS, I am currently switching 12V off and on with 5V on the gate. But I didn't measure the voltage. Are you saying I would not have 12V on my source (in my right most figure)?

    Sorry for being so dense. :)
     
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