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current limiter

Discussion in 'Power Electronics' started by Emesine, May 25, 2021.

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  1. Emesine


    Jun 19, 2017

    I am filling a 12V, 5 farad capacitor array. The design of my power source requires that I limit charging current to approximately 80 mA.

    I am looking for a simple solution for a current limiter. I'm considering the attached design based on the lm334.

    I'm pretty sure this circuit will work, but I think it will oscillate. I'm interested in suggestions as to where to place a capacitor or resistor to limit oscillation. Thanks!


    Attached Files:

  2. crutschow


    May 7, 2021
    Below is the LTspice simulation of a simple, 2-transistor circuit that should not oscillate and should do what you want.
    It uses the base-emitter voltage of Q1 as a reference for the current limit.
    The limit is about 0.65V / R3.

    The load current increases until the voltage drop across R3 starts to turn on Q1.
    Q1's collector current then raises the voltage across R1 until M1 starts to turn off and limit the output current to maintain the drop across R3 equal to about 0.65V.

    As the simulation shows, the source current (green trace) stays constant as the capacitor charges (yellow trace).

    The PNP and P-MOSFET types are not critical and can be just about any devices you have.

    Note that it takes over 13 minutes to charge the cap with an 80mA charging current.

    Last edited: May 25, 2021
    Emesine and Harald Kapp like this.
  3. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    The LM334 is a programmable current source and is imho not the right component for this purpose.
  4. Emesine


    Jun 19, 2017
    Thanks Crutschow. I like your schematic and will incorporate it in my design.

    My son is building a base station for launching model rockets. It needs to produce enough power to fire a 12V model rocket igniter using the 5V usb power supply from a laptop computer. I'm using a 1 watt boost converter to fill supercapacitors at 12V. Works beautifully, but will try smaller caps next time. At my first trial the capacitors lost ~15% of their voltage during a 1 second firing, and the igniter was fully aflame after ~140 ms. I'm also thinking about using a larger boost converter, though I am limited by the 500 mA available from my computer USB power supply.

    Again, thanks for the schematic. Really helps.

    Harald, lm334 officially scrapped!

  5. Emesine


    Jun 19, 2017
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