Motor over current sensor

Discussion in 'The Projects Forum' started by Matt D., Jan 6, 2016.

  1. Matt D.

    Thread Starter Member

    Nov 10, 2013
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    Hello,
    I found this circuit and wondered if this can also do the opposite. Instead of sensing over current, can it sense under current?

    Would this work with batteries or would there have to be a voltage regulator with batteries to keep things consistent when the battery voltage drops over time?

    Thanks,
    Matt
    http://www.romanblack.com/current.htm
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,553
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    Just use the info in reverse, set so that it switches on at normal current level, not over current.
    The other way is to monitor in a analogue fashion.
    Max.
     
  3. BR-549

    Well-Known Member

    Sep 22, 2013
    2,004
    394
    I can't see anything on your link.
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,024
    3,236
    Works fine for me.
     
  5. BR-549

    Well-Known Member

    Sep 22, 2013
    2,004
    394
    All I get is text with grey background.
     
  6. Matt D.

    Thread Starter Member

    Nov 10, 2013
    65
    0
    Here it is..if I uploaded correctly.

    tmp_31298-current611494361.gif
     
    Last edited by a moderator: Jan 6, 2016
  7. Matt D.

    Thread Starter Member

    Nov 10, 2013
    65
    0
    I want it to stop the motor when the current drops. So the motor will be under a load and then when the load is removed but the motor will still be turning, but at a reduced current. Then cutoff the motor.
     
  8. BR-549

    Well-Known Member

    Sep 22, 2013
    2,004
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    I can't see those gifs either.
     
  9. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    Can you see it now?
    What's your computer system and which browser?
     
  10. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,553
    2,375
    I would detect the voltage across the sense resistor in an analogue fashion and input into AtoD input in the Pic.
    Max.
     
  11. BR-549

    Well-Known Member

    Sep 22, 2013
    2,004
    394
    Thank you, I can see the gif here and on your link now.

    Win10, FF.
     
  12. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    That is a really crappy current sensor. Look at what it does as a function of ambient temperature. Roman should be ashamed...

    If you are worried about over-discharging a battery, that is usually done by detecting a drop-off of battery voltage; not load current.
     
  13. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I have the Pic Motor Development board and have used their similar method, Check out the schematic and see how it is done. (r26) 505HR100 current sense resistor.
    Max.
     
    Last edited: Jan 7, 2016
  14. Matt D.

    Thread Starter Member

    Nov 10, 2013
    65
    0
    Thanks. I was not planning on using the board, but rather use a latching relay to turn off the motor?
    I don't understand too much of this stuff, but could I set up a sense resistor with the motor and go to the input side of a Schmitt trigger?
     
  15. MaxHeadRoom

    Expert

    Jul 18, 2013
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    That was going to be another suggestion, if you don't want to go with the analogue approach, a comparitor circuit also could be used with the current sense resistor.
    You can also get mosfets with extra pins with current sensor internal.
    The development board is just for design purposes and testing.
    Max.
     
  16. Matt D.

    Thread Starter Member

    Nov 10, 2013
    65
    0
    Thanks. I'll probably need a bit of hand holding to get through this. And to be able to pick out a sense resistor.
    I'm planning on using TI Schmitt SN74AC14 with 4 AA alkaline so about 6v.
    I don't quite understand at which voltage it switches. On the top of the 3rd page of the datasheet it has positive going threshold voltage and negative going. For 5.5V and positive going it lists min 1.6V and max 3.9V. For 5.5V and negative going it lists min 1.1V and max 2.3V. I don't understand why they overlap. Is this kind of a floating imprecise range?
    The hysteresis is min 0.5V and max 1.6V.
    The switch could be anywhere in the min through the max voltage ranges for either positive or negative going?
     
  17. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Schmidt trigger is mainly for squaring up a pulse, a comparitor, LM311 may be a better choice to set up the trip point.
    Max.
     
  18. BR-549

    Well-Known Member

    Sep 22, 2013
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    Hey Max, can you give me a pointer to those mosfets with extra pins with current sensor internal. I would like to learn about those.
     
  19. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Search Int. Rectifier site for HexSense.
    AN959.
    http://www.irf.com/technical-info/appnotes/an-961.pdf
    IRCZ24/34/44 etc.
    Max.
     
    BR-549 likes this.
  20. Matt D.

    Thread Starter Member

    Nov 10, 2013
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    Can anyone help me understand what I was asking about the datasheet?
     
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