Drive mosfeet n-channel with transistor (?)

Discussion in 'General Electronics Chat' started by alan1975, Oct 22, 2012.

  1. alan1975

    Thread Starter Member

    Jan 1, 2012
    42
    0
    Hello,

    I have about 3.x volt signal.
    I need feed it to mosfeet n-chanell, but problem is it won't like to open at 3.xv.

    I have 12V voltage rail, so i can feed it to mosfeet, but i need some kind of transistor acting like switch.

    I used before NPN transistors, and i connect it between my load and collector.
    But here, that trick won't work... i try resistor ( nchanell gate) and colelctor, but it work in reverse, when transistor is off n-channell got voltage, when transistor is on, n-channel is off. That is easy to understand, because we get all current into transistor. I check voltage on emitter, and it's almost same as base voltage.


    Do You have any circuit for using transistor as switch for n-channel mosfeet ?

    Regards
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,022
    3,236
    Post a diagram of what you are trying to do.
     
  3. alan1975

    Thread Starter Member

    Jan 1, 2012
    42
    0
    Here You go!

    At 5V input to NE555, i can't drive that n-mosfeet.

    I know i should use logic level, but i don't have any right now.
    That's the reason i use non logic level.

    Regards
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    With a supply voltage of only +5V then the output high from an ordinary 555 is +3.7V.
    Even many "logic-level" mosfets will barely turn on.

    So why not power the 555 from your +12V so its output high is +10.7V?
    Then ANY mosfet will turn on properly.
     
  5. alan1975

    Thread Starter Member

    Jan 1, 2012
    42
    0
    You have right, i just start from idea using LMxx5 to allow circuit work from many voltages 12-24v, but now i realize, i just need use 9V stabilizer and should work fine... i don't know why i stick to that 5V, just stupid...

    But, anyway i found a solution for my problem!.
    I tested it on breadboard and work well !

    I put a optocoupler, and zener diode ( i need this to run safe on 24V ), to prevent sending this voltage to gate ( i name rail 12V but it can be 12V - 24V ) .
     
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  6. Audioguru

    New Member

    Dec 20, 2007
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    The photo-transistor in the opto-coupler is an emitter-follower fed from the 8.6V zener diode so its maximum output voltage is only 7.9V which might not be high enough to fully turn on the Mosfet.
     
  7. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    That's a common misconception. The phototransistor has no connection to its base. The electrons generated by photons impinging on the base-collector junction can drive the transistor into saturation in common emitter or common collector mode. See the attachment.
    Yes, I realize this is a simulation. This app note supports my viewpoint.
    I encourage others to check this with hardware.
     
    absf likes this.
  8. crutschow

    Expert

    Mar 14, 2008
    13,022
    3,236
    You have it right Ron H. A phototransistor can go into full saturation in either a common emitter or common collector configuration, makes no difference.
     
    Ron H likes this.
  9. Audioguru

    New Member

    Dec 20, 2007
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    It is amazing to learn that a photo-transistor emitter-follower can saturate. Thanks for letting me know about it.
     
  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    657
    I know. For years, I thought Vce would be about 0.7V.
     
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