6V Motor shutoff

Discussion in 'The Projects Forum' started by hoptown, Aug 25, 2013.

  1. hoptown

    Thread Starter New Member

    Aug 25, 2013
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    The project is pretty simple, 6V Motor will be running at a load, designed to run at the max efficiency under load, ~1.4A, the stall current is 7.8A. Since the mechanism components will not handle the torque output of the motor at 7.8A, I need a shutoff when the load increases and the current hits around 2A or 2.5A max. The motor will be running forward/reverse by manual switch.

    I don't have a lot of real estate to do this in, but I was hoping there is some components that I can just hardwire in (without programming) that will shut the motor down at that specific current.

    My research including this forum has pointed me in the direction of current sense resistors as a solution, but not sure what will shut the motor down. Essentially, I need a component to sense the current and then send zero current to motor, send signal to a component to shut the motor down.

    Any help would be appreciated, my background is mechanical, so sorry if this sounds too basic.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    I think I'd use a very low ohms resistor, maybe 0.1Ω or less, and a comparator to sense voltage across that shunt rising above a reference voltage. The output of the comparator could drive a MOSFET switch.
     
  3. tubeguy

    Well-Known Member

    Nov 3, 2012
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    +1
    I think you will also need some time delay which ignores the normal staring in-rush current.
    Adding a capacitor would do.
     
    Last edited: Aug 25, 2013
  4. hoptown

    Thread Starter New Member

    Aug 25, 2013
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    Thanks for the quick feedback, I'll try the resistor/comparator and review the MOSFET switch and add a time delay. Do you think 0.5s would be sufficient time for the starting current to drop down to normal?
     
  5. tubeguy

    Well-Known Member

    Nov 3, 2012
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    I'm thinking some experimentation will be needed, depending on the load.
    How much current can the power supply provide ?

    EDIT: At first I actually typed "How much current can the
    power supply supply." Is that a sentence ??

    Sunday is Fun Day. :cool:
     
  6. hoptown

    Thread Starter New Member

    Aug 25, 2013
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    Tubeguy,
    Actually your first sentence made sense to me!

    I will be running the motor with 4 AA batteries, and while I can provide more current than needed, I do not want to deplete the batteries in short order. I'll be doing some experimentation with the length of time per actuation, and how many actuations will be needed total. The load calcs are theoretical as of now, but always change when you have actual components!
     
  7. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Exactly which AA batteries are you using ?
    How long is each actuation ?
    Do you understand amp-hour capacity of batteries ?
     
    Last edited: Aug 25, 2013
  8. LDC3

    Active Member

    Apr 27, 2013
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    I think there will be a problem with the logic.
    With low current, comparator output is low. When current gets too high, comparator output goes high, shuts off MOSFET, current stops, comparator goes low, MOSFET turns on, repeat.
     
  9. LDC3

    Active Member

    Apr 27, 2013
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    Like a record records sounds. ;)
     
    tubeguy likes this.
  10. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Actually, comparators can be configured for either inverting or non-inverting operation. :)
     
  11. LDC3

    Active Member

    Apr 27, 2013
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    If you're using AA cells, then the current is more like 100 mA unless the cells are expended quickly. Even D cells don't provide 1 A for very long.
     
  12. LDC3

    Active Member

    Apr 27, 2013
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    OK. fix the logic. :D
     
  13. wayneh

    Expert

    Sep 9, 2010
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    In case it wasn't obvious, the shunt and MOSFET need to tolerate the full operating current with room to spare. At 3A, power dissipation in the resistor will be 9R, or about 0.9W using a 0.1Ω resistor. So a 2W rating would be nice. You could get away with a 1W resistor if it's a lower resistance and/or you hold the current below 2.5A.
     
  14. hoptown

    Thread Starter New Member

    Aug 25, 2013
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    This is great feedback, thanks to all involved.

    I didn't think about the MOSFET and having a safety factor(my mechanical verbage) which would take care of and have capability beyond the full operating current.

    I'm currently laying this out in LTSpiceIV, I'll update the thread as I source the components. Learning a lot quickly on this application!
     
  15. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Agreed! It will likely become a big nasty oscillator. :eek:

    Why not just put a current limiting circuit on the motor to limit fault current and startup current to a safe value? It is a popular and well proven system.
     
  16. wayneh

    Expert

    Sep 9, 2010
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    Yeah, my bad. I was thinking latch to off after an over-current event, with a manual reset, but neglected to mention any of that. :(
     
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