Depletion mode Mosfet

Discussion in 'General Electronics Chat' started by spike1947, Apr 5, 2016.

  1. spike1947

    Thread Starter Member

    Feb 4, 2016
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    Hi
    Can anyone point me to a simple basic schematic drawing that contains a " n-channel mosfet depletion mode transistor " in the circuit .

    I just really want to know how to turn it off , I am drawing a circuit ( not finished as yet ) for a project , but the info I have about this mosfet is that the Gate voltage has to be around 0.7v min above the source voltage to turn it off !, not sure how to get round that , that is why I am asking if anyone knows of a circuit using a depletion mode mosfet in it , then maybe I can work it out , or if they know how to turn it off in a very simple term .
    .
    sorry if I am not giving all the info that maybe you are wanting , but maybe can if required .

    cheers

    spike
     
  2. dl324

    Distinguished Member

    Mar 30, 2015
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    Vgs needs to be negative to turn off a depletion mode N channel MOSFET. If you apply 0.7V, it will operate in enhancement mode.

    What is the part number?
     
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  3. dannyf

    Well-Known Member

    Sep 13, 2015
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    By putting a sufficiently high drain resistance.

    Most RF MOSFETs are of depletion type. They are not for switching applications so hard to imagine the need to turn them off.

    They are biased with a resistor or led.
     
  4. Marley

    Member

    Apr 4, 2016
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    In an N-Channel depletion mode MOSFET, the gate needs to be negative of the source to switch it off. The device will be fully on if the gate is the same voltage as the source.

    A common type is the J111 series. See the data-sheet here.

    You can see that the gate-source cut-off voltage is about -3.0V to -5.0V depending on type. See the graph "figure 5" in the above data-sheet.

    Have a look at this typical circuit.
     
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  5. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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  6. spike1947

    Thread Starter Member

    Feb 4, 2016
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    Microchip DN2530

    cheers
     
  7. spike1947

    Thread Starter Member

    Feb 4, 2016
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    I am doing a infrared alarm system , were the ir led is on all the time into the ir receiver , then the feed from the ir receiver will feed into the DN2530 to turn it off , when the beam is broken ( ie the path of the ir led is blocked ) there will be no feed to the gate of the DN2530 so then it will operate a relay ! , that is the theory I have ! .

    Thanks all for all your input , i think I am getting to deep here ! .

    cheers

    spike
     
  8. dl324

    Distinguished Member

    Mar 30, 2015
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    The datasheet says you need Vgs of -1 to -3.5V to turn the MOSFET off. Below is the typical IV curve that shows you need a voltage more negative than -1.5V.
    upload_2016-4-5_8-27-55.png

    If you post a schematic, members can help you with your design.
     
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  9. spike1947

    Thread Starter Member

    Feb 4, 2016
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    Thanks for that info ,my schematic is a bit messy at the moment , I need to redraw some of it ,,this is done by hand on graph paper , will have to scan it to put on here .

    cheers

    spike
     
  10. dl324

    Distinguished Member

    Mar 30, 2015
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    If you haven't already purchased the depletion mode MOSFET, you can use a less expensive enhancement mode device.
     
  11. crutschow

    Expert

    Mar 14, 2008
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    You don't need to use a depletion-mode device which requires a reverse-polarity to turn it off (gate polarity opposite the drain polarity. Easier to use a common enhancement-mode device.
    What is the signal voltage from the alarm?
     
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