Safety current limit shut-off

Discussion in 'The Projects Forum' started by stephenz, Jul 29, 2014.

  1. stephenz

    Thread Starter New Member

    Jul 29, 2014
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    0
    Hi,
    I've got a brushless DC motor that I am using that I need to automatically trigger off whenever the current draw exceeds 1.8A at the power source (DC 12V).

    The problem that I am running into is that I cannot use a fuse (I tried a few different PTC's models and brands) and they're basically all the same in the sense that you need to chose one with a value that is "far" enough from the nominal power draw.

    In this project, my motor would typically draw 1.5A (nominal) and I need it turned off if the current goes above 1.8A which is pretty close to it.

    I tried using a 1.8, even 2.0A PTC's and while they would do the job at opening the circuit at 1.8A, they would also trigger at 1.5 or 1.6A after hours, sometimes days of operation. Yes, that's another thing in this project, I will leave the motor run for days/weeks, and I don't want it to stop unless the current really goes above 1.8A, not less.

    Using a fuse would have simple and perfect except it won't work in this case...

    Anyway, I was redirected to this forum (new here!) when I found this link: http://forum.allaboutcircuits.com/showthread.php?t=30383

    it doesn't exactly fit my needs but I found the responses very helpful and thought that maybe would be able to help me out with some pointers...

    Any idea?
    thanks
    Steve
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    I recently built a swing gate controller using an Arduino. I put a safety circuit into it such that if the gate hits an immovable object, I detect the increase in motor current and reverse the gate travel automatically. I used a low-side current monitor consisting of a 10mΩ shunt resistor and an LM358 amplifier (gain of 100) also wired as a 2pole Low-Pass filter. The output of the opamp is wired to one of the analog inputs to the Arduino.

    The trip point for my safety circuit happens to be about 2A. AFAIR, 2A makes a voltage near 4V for the Arduino ADC.

    If this would work for you, write back and I will post the circuit.
     
  3. inwo

    Well-Known Member

    Nov 7, 2013
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  4. MrChips

    Moderator

    Oct 2, 2009
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    Find an incandescent lamp that has the right amperage and resistance and use it as a fuse. This is what is used in speaker systems to prevent blowing the loudspeaker when overdriven, especially from acoustic feedback. It behaves as a soft safety switch. The resistance of the lamp increases as the amperage goes up.
     
  5. stephenz

    Thread Starter New Member

    Jul 29, 2014
    4
    0
    thanks for the responses.
    Good suggestions there, however I am looking for something as compact as possible that I could wire in-line with my power wires. I wouldn't mind making a small pcb, if possible without programmable components.

    Steve
     
  6. crutschow

    Expert

    Mar 14, 2008
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    What is the duration and value of the motor startup-current?
     
  7. stephenz

    Thread Starter New Member

    Jul 29, 2014
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    I've only checked a multimeter meter which is going to average it over time.
    How should I measure that transient state with an oscilloscope to get accurate figures?
     
  8. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    On my gate controller (uses a 12Vdc gearmotor), the start-up inrush was much higher than the safety-limit that I wanted to sense. Start up current with full 12V applied to the motor was ~5A. Running current after the motor accelerates the high-inertia load was only ~1/2A. I wanted the over-current trip to be about 1.8A.

    I had two choices. Ignore the high current during acceleration or PWM the motor to accelerate it slowly to keep the startup current low. I chose the latter so that I could sense the gate hitting something even as it starts moving...
     
  9. stephenz

    Thread Starter New Member

    Jul 29, 2014
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    0
    Yes, I am guessing that the latter is also what the controller is doing - the motor has PWM control. But I'd like to make sure of that by measuring the inrush current.
     
  10. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    My Fluke has a 10A current range. I just put that in series with the motor. It seems to have about the right amount of filtering to be able to read the motor current...
     
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