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    2N7000 strange behaviour

    I want to use a 2N7000 as a switch to protect my circuit for over voltage.
    I have a scenario when i feed 5 volts and everything should be fine.
    But is i feed 24vdc the fet should start conducting and blow the fuse.
    On data sheet of 2N7000 trigger si 0.8 to 3 vdc.
    I attached 2 pictures which should explain better.
    Even if i make GS 1.92 volts, the fet stay open and the fuse does not blow.
    I need help as it is first time when i work with Mosfets.
    Click image for larger version. 

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  2. #2
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    Re: 2N7000 strange behaviour

    One minute ago on another forum I asked if the Mosfet will blow before the fuse blows. I think the low current Mosfet will blow much faster than the fuse will blow.
    Also I explained that some 2N7000 Mosfets are more sensitive than others and the sensitivity changes when the temperature changes so a Mosfet cannot be the voltage reference and a comparator IC that you need.



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    Re: 2N7000 strange behaviour

    Quote Originally Posted by Audioguru View Post
    One minute ago on another forum I asked if the Mosfet will blow before the fuse blows. I think the low current Mosfet will blow much faster than the fuse will blow.
    Also I explained that some 2N7000 Mosfets are more sensitive than others and the sensitivity changes when the temperature changes so a Mosfet cannot be the voltage reference and a comparator IC that you need.
    Thank you but let me explain something more to be clear.
    The power supply is short circuit protected at 2A.
    Now, looking better on data sheet i see that the max DS current is 115 mA.
    Ok i was wrong.
    What i want to do is to blow the fuse if the input voltage is 24VDC, (make the fet to act like a close circuit and drain all current until the power supply go in protection mode)
    When the input voltage is 5 vdc, the fet should stay open.
    Can you suggest a fet fo 40 volts/4-6A ?
    For testing i prefer through hole, so i can use a breadboard.
    Thank you



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    Re: 2N7000 strange behaviour

    Where is a voltage reference and comparator IC to detect that the supply is more than 5V? Then the comparator turns on the power Mosfet.

    There are thousands of power Mosfets available. Select a "Logic level" one that conducts well when its gate-source voltage is 5V.


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    Re: 2N7000 strange behaviour

    Thank you.
    I am sorry but i looked for something else.
    All i wanted was to make it simple. Voltage divider.
    I will try to find an answer by myself doing trial and error.
    Thanks for suggestions.



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    Re: 2N7000 strange behaviour

    Hi,

    Datasheet says 0.8 to 3V VGSth, for 1mA drain current, 1.92V is too low. Maybe paralleling a 20V Zener with a small resistor, only to protect the gate at 24V, might work. The NMOS needs to be fully on or fully off if used as a switch; as is, it looks like it doesn't fully turn on.

    I agree with previous explanation, the circuit would benefit from a comparator to detect the 24V.

    I might be wrong, but maybe the MOSFET also needs a source load resistor in order to sink sufficient current to blow the fuse when it turns on.

    - - - Updated - - -

    Correction(s): the Zener should be 15V or 18V, and is in series with the resistor to the gate, if a resistor is used.



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    Re: 2N7000 strange behaviour

    To blow a fuse, you should consider a crowbar SCR. It must be rated to stand the fuse Iēt integral.

    2N7000, presumed you manage to turn it fully on, will hardly stand the fuse current, you most likely blow the MOSFET together with the fuse.



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    Re: 2N7000 strange behaviour

    Do not think about "the threshold gate-source voltage", because that is when a Mosfet is almost turned off and barely turned on only a tiny amount. Instead look at when the very low on-resistance is stated with a gate-source voltage of 4.5V or 10V.



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    Re: 2N7000 strange behaviour

    Use a SCR instead.
    The trigger voltage is more predictable.



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