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  1. #21
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    Re: Voltage controlled current source circuit needed for high currents

    crutschow and Klaus here is the reason I didnt use a non inverting opamp:

    Simulations were not how I wanted.

    If I use LMC6482 as in following way:
    Click image for larger version. 

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    Here are plots for input to the non inverting input(blue), output of the op amp and the power dissipated:
    Click image for larger version. 

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    As you see above output voltage is not pulled to zero also note that the pulses get distorted. And so much power is dissipated.

    And if I use the following one:

    Click image for larger version. 

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    Now the power dissipation is less but there is like 8mV offset again in the output when PWM is OFF. In INAMP it goes to zero.
    I also tried with LM358 same issue.

    Maybe better to draw a sketch of the topology what you mean if my circuit is wrong. Thanks.

    - - - Updated - - -

    Quote Originally Posted by CataM View Post
    What Klauss is trying to say here:

    is that your IRL3803 is already a logic level N channel mosfet (a very good one for this application by the way..), which means, you can drive it directly from the MCU_PWM pin with a properly selected resistor at the gate.

    -----

    That is necessary only if you implement the active low pass filter suggested by Klauss. If you stick with the Instrumentation OP amp, you are good...
    I didnt get the last part. If I use the INAMP above the MOSFET as in my circuit, I cannot use an active filter? Better to draw than words I guess,
    Last edited by doncarlosalbatros; 24th April 2017 at 16:05.



    •   Alt24th April 2017, 16:10

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  2. #22
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    Re: Voltage controlled current source circuit needed for high currents

    Quote Originally Posted by doncarlosalbatros View Post
    I didnt get the last part. If I use the INAMP above the MOSFET as in my circuit, I cannot use an active filter? Better to draw than words I guess,
    Solution 1): you can use an active filter with high order (e.g. 3rd order suggested by crutshow) but with no gain. You can use a simple passive RC LPF.
    Solution 2): doing it like Klauss suggested, current sensing must be on the soruce path.


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    •   Alt24th April 2017, 16:31

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  3. #23
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    Re: Voltage controlled current source circuit needed for high currents

    How can I design an active 2nd order butterworth with gain for about 20Hz LPF?
    Is there a practical tool for that when you plug in 20Hz cut off and gain it outputs the component values on a topology?

    If I can have that I wouldn't need the -5V voltage converter and an INAMP.



    •   Alt24th April 2017, 18:14

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  4. #24
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    Re: Voltage controlled current source circuit needed for high currents

    Quote Originally Posted by doncarlosalbatros View Post
    How can I design an active 2nd order butterworth with gain for about 20Hz LPF?
    Is there a practical tool for that when you plug in 20Hz cut off and gain it outputs the component values on a topology?
    http://www.analog.com/designtools/en/filterwizard/



  5. #25
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    Re: Voltage controlled current source circuit needed for high currents

    This should help.

    But I would go with a Bessel rather than a Butterworth response to minimize overshoot.
    Zapper
    Curmudgeon Elektroniker



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