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Is there a MOSFET that turns OFF when voltage is applied to gate?

Discussion in 'General Electronics Discussion' started by supak111, Jan 9, 2015.

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  1. supak111

    supak111 ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★

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    Is there a MOSFET that turn OFF when voltage is applied to gate? And is ON when voltage is removed? Or any other transistor that would do the job? Switching on 12V, 1 to 7 amps.

    I need a mosfet that resembles a PNP bjt so basically I need something that turns ON when there is 0V in the base/gate..

    I would use a BJT but then I need a big BJT and to add extra resistors and stuff, MOSFET would be better..
     
    Last edited: Jan 9, 2015
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    There are JFETs that conduct by default unless reverse bias is applied to the gate, but 7A could be a problem for them. And there are P-channel MOSFETs that conduct when the gate is brought negative relative to the source, which is comparable to a PNP transistor.

    Why don't you post the circuit you have so far and tell us what you want to do. Then we can suggest the best option for you.
     
  3. supak111

    supak111 ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★

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    It really isn't much of a circuit, I'm basically just trying to turn ON my cars fog lights anytime the headlights are OFF. Wanna do something different and not use a mechanical relay. Sooo no voltage in the headlights turns ON the mosfet and the FOG light come ON. I will look into the P mosfet.
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

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    [​IMG]
    This is a basic schematic how the circuit could look like based on Kris' PMOS suggestion: Rheadlight and Rfog are stand-ins for the respective lamps.
    M1 has to be selected for the load presented by Rfog.
    Rprotect is uncritical, it serves to limit current through Dprotect.
    Dprotect can be any zener diode >= 16V.

    A PNP instead of the PMOS could also be used, however Rprotect must be reduced to provide enough base current.
     

    Attached Files:

  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Check out this one: http://www.digikey.com/product-detail/en/SUP75P03-07-E3/SUP75P03-07-E3-ND/2623113

    Very good specifications and nice and cheap.

    MOSFETs are not as robust as relays and need to be protected from various nasties that occur in the automotive environment. You can protect the gate using a zener from gate to source, and you should also protect the device against overvoltage at switch-off due to the inductance in the wiring. This is what I suggest.

    272158.001.GIF

    Make sure D2 and C1 are connected directly between the MOSFET's terminals using thick, short wires.

    Edit: Harald's circuit is pretty similar but the diode should be directly across the gate-source to protect the device properly.
     
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  6. supak111

    supak111 ★ƃuᴉɯǝɥɔs sʎɐʍlɐ★

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    Thank you both very much. Really wanted to do this and not have to run wiring into the cabin of the car, install an extra switch in my dash, and a relay in my car. This way everything stays under the hood.
     
    Last edited: Jan 9, 2015
    KrisBlueNZ likes this.
  7. Harald Kapp

    Harald Kapp Moderator Moderator

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    Kris: you're absolutely right about the diode.
     
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  8. duke37

    duke37

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    Do you want both head and fog lights to be off together? The circuit as shown always energises one or the other.
     
  9. Arouse1973

    Arouse1973 Adam

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  10. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Interesting! Depletion-mode MOSFETs rated for 500/1000V and up to 20A.

    Not relevant here, I don't think, but I'm sure they have some benefits for certain applications. Thanks for the info Adam :)
     
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