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  1. #1
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    Make me Understand....

    i have searched a circuit for current sense or current monitoring and i have come across a circuit which i have attached.
    i tried to figure out that how this circuit works and it goes as follows.

    1st consider that load is not present. then Va = Vb and and transistor is in cutt off state where Vc (collector voltage) =

    Va. so at this stage Va=Vb=Vc=V+=V-=Vcc=5v. the transistor will stay off and thus Ve ( emitter voltage) will be 0v.

    Now lets add a little load, thus Va will be slightly greater that Vb. this in turns will make the op-amp go in on state i.e.

    saturation which will then force the transistor to go in saturation. now when transistor will be going in saturation then

    Vc starts decreasing and as it becomes a little less than vb the transistor will be again switched on by the op-amp and

    again when Vc becomes slightly higher then again transistor will go in saturation and this process will keep on going.

    so i thing that this will shows us the voltage in terms of current but with very little oscillations. how little? i m using

    LM358 so it can detect difference of minimum 2mv...so 2mv amplitude of oscillation at the threshold level. which i think

    can be ignored or filtered out by installing a capacitor in parallel.

    in this whole scenario i have supposed that op-amp is acting as comparator. obviously there is no question that its is a negative feed back but there is a possibility that it is some how acting in positive feed back with some finite gain but positive feed back introduces hysteresis doesn't it?

    so plz tell me if im right or wrong and do explain this if i m wrong. ill be thankful. op-amps really confuses me. by the way i have tried this circuit and it works with out any hysteresis.

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  2. #2
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    Re: circuit for current sense or current monitoring

    LM358 not a good choice - you need an op amp that has a common mode range that includes the pos rail, Regards, Orson Cart. The purpose of the circuit is to output a voltage signal corresponding to the current in the 0.2 ohm resistor...



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  3. #3
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    Re: circuit for current sense or current monitoring

    in your schematic the OPAMP is not working as comparator - there is a feedback through the transistor collector!



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  4. #4
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    Re: circuit for current sense or current monitoring

    I have a question regarding this circuitry.

    From the attached diagram, if switching regulator added in between sensing resistor and the load, would it works?

    If not, how to monitor current for the load? (the switching regulator is essential before the load)



  5. #5
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    Re: circuit for current sense or current monitoring

    it should work, this schematic is simply measuring the current. If you galvanically insulate the V+ and V- you can measure everything you want



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    Re: circuit for current sense or current monitoring

    By it's operation principle, the input current of a switching regulator will be different from the load current. I guess, you want to measure the load current. A complete switching regulator would however involve an output capacitor, you would want to place the sense resistor between the capacitor and the load.

    If you mean something different than a true switching regulator, please clarify.



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  7. #7
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    Re: circuit for current sense or current monitoring

    Quote Originally Posted by FvM View Post
    By it's operation principle, the input current of a switching regulator will be different from the load current. I guess, you want to measure the load current. A complete switching regulator would however involve an output capacitor, you would want to place the sense resistor between the capacitor and the load.

    If you mean something different than a true switching regulator, please clarify.
    You've got what I mean, FvM.

    So, you're saying the current sensing circuit will have different power supply to the swithcing regulator input supply?



  8. #8
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    Re: circuit for current sense or current monitoring

    In switch mode regulator, there's a current difference between input and output. In case of a simple buck regulator, it's the diode current. Ignoring losses, I/O currents and voltages are inverse proportional. So the placement of the current sense resistor depends on what you need to measure. The power supply of the sense amplifier is a different thing. It needs to consider the amplifiers common mode range. In case of a buck converter with output capacitor, it usually won't be a problem.


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