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    overcurrent and reverse polarity protection

    Can someone explain the attached circuit. It is overcurrent and reverse polarity protection. How to choose the values and selection of mosfet.

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    Re: overcurrent and reverse polarity protection

    The left MOSFET is a low voltage drop polarity protection. It will be off if the input voltage is reversed.
    When the current gets too high, the bipolar transistor will turn on and remove the gate voltage from the right MOSFET.
    The MOSFETs are P-channel.


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    Re: overcurrent and reverse polarity protection

    Hi,

    Give each part in your schematic a name (R1 .., Q1...), so we can talk about it.

    It would be good if you have some other specifications, too. Voltages (min, max...), current (expected max normal current, max allowed overcurrent...)

    Klaus
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    Re: overcurrent and reverse polarity protection

    This is a great opportunity to learn a simulation tool.

    Download LTspice.


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    Re: overcurrent and reverse polarity protection

    Designations are noted.

    Input could be 9V battery. Nominal output load will be 5mA.



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    Re: overcurrent and reverse polarity protection

    Hi,

    Sadly you didn't give more informatiin about current limit...

    --> Just say you want it to be limit to 10mA nominal.
    R1 .... is the "shunt" for current measurement.
    Q1 .... should become conductive during current limit.

    --> Q1 needs about 0.6V V_BE to become conductive. Now with 0.6V and 10mA you are able to calculate R1.

    --> R2 is no needfed, as long as V_GS_max of Q3 is higher than the battery voltage.

    --> R3 and R4 are not critical. They just need to drive the tiny gate current of Q3/Q2 (and the leakage current of Q1). Choose 100k down to 10k

    --> Q1, Q2, Q3 need to widthstand battery voltage and max limiting current.

    Klaus
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    Re: overcurrent and reverse polarity protection

    For my application, I want to limit max allowable current to 50mA. Upon that, the circuit should be protected.

    From this, I have calculated R1 as 0.6V x 50mA => 30mOhms
    R2 is no needed means. Then should I short the connections of Q1 collector and emitter? Kindly clarify.
    R3 = 100K; R4 = 10K;
    Q2 and Q3 => BSS84-7-F, MOSFET P-CH 50V 130MA SOT23-3.

    Are these values are ok.



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    Re: overcurrent and reverse polarity protection

    Quote Originally Posted by hemnath View Post
    For my application, I want to limit max allowable current to 50mA. Upon that, the circuit should be protected.

    From this, I have calculated R1 as 0.6V x 50mA => 30mOhms
    R2 is no needed means. Then should I short the connections of Q1 collector and emitter? Kindly clarify.
    R3 = 100K; R4 = 10K;
    Q2 and Q3 => BSS84-7-F, MOSFET P-CH 50V 130MA SOT23-3.

    Are these values are ok.
    Ohms law is U = R * I
    Can be rewritten as R = U / I
    In your case:
    R = 0.6V / 0.05A = 12 Ohms



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    Re: overcurrent and reverse polarity protection

    Quote Originally Posted by hemnath View Post
    ...........
    R2 is no needed means. Then should I short the connections of Q1 collector and emitter? Kindly clarify.
    R3 = 100K; R4 = 10K;
    Q2 and Q3 => BSS84-7-F, MOSFET P-CH 50V 130MA SOT23-3.

    Are these values are ok.
    R2 is only needed if the input voltage is higher than the Vgs(max) rating of the MOSFET.

    You leave the collector-emitter connection open.
    If you short that connection how could the transistor work?

    You could make both R3 and R4 100kΩ.
    Zapper
    Curmudgeon Elektroniker



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