amp sensor switch

Discussion in 'The Projects Forum' started by ozioho, Jan 28, 2014.

  1. ozioho

    Thread Starter New Member

    Jan 20, 2014
    15
    0
    I did a search and found sensing modules, but is there such a module that can read amps and turn off at a low set point. Current sensing switch?
    I have a small water pump that runs off 12v DC. At load (water in the line pumping) the motor draws .36 - .40 amps. When you cut off the water supply, it drops to .15 amps instantly. The purpose is to turn off motor if it gets air an pocket or runs dry and keeping pump from burning up. I have found big switches for large pumps, but nothing for a micro motor.

    Thanks for the help
     
  2. GopherT

    AAC Fanatic!

    Nov 23, 2012
    5,998
    3,756
    Why would drawing fewer amps cause the motor to burn up (unless it us not rated for continuous service).
     
  3. crutschow

    Expert

    Mar 14, 2008
    12,995
    3,229
    You could use a comparator, such as an LM339, to detect the voltage across a small shunt resistor in series with the motor. If the voltage drops below the set point it can shut off the motor using a relay. You will need a momentary bypass delay in the circuit to allow the motor to start from zero current.

    Is that something you would be interested in building or do you want an off-the-shelf solution?
     
  4. ozioho

    Thread Starter New Member

    Jan 20, 2014
    15
    0
    I am in the middle of a project that needs this type of a switch in an area of 1"x2"x1". I would love to off the shelf solution, but I don't mind trying putting one together if one this size is not available. Can such a module be built in the measurements given? I tinker in my garage, this might be over my head

    Thanks
     
  5. crutschow

    Expert

    Mar 14, 2008
    12,995
    3,229
    Below is a simulation of a circuit that should do what you want and fit within your allocated volume on a small perfboard. The comparator monitors the voltage drop across shunt resistor R2 and cuts off the motor if the current drops below about 200mA.

    The trip point is determined by R3 and R4. If the 12V varies significantly then you may need to add a reference zener diode to stabilize the trip point.

    C1 momentarily turns the MOSFET on at startup otherwise the motor wouldn't start. Once the motor is tripped off you need to reset the voltage to restart the motor.

    The MOSFET can be just about any N-MOSFET that has a 20V and 5A or more rating.

    Motor Cutoff.gif
     
  6. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    Last edited: Jan 29, 2014
    ozioho likes this.
  7. ozioho

    Thread Starter New Member

    Jan 20, 2014
    15
    0
    Thank you,

    Unfortunately, this is over my head, but it is very important that I get this built. With the posted drawing and recent posts, can the right person build it?
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,515
    2,369
  9. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    A reed relay may be a good choice. Fitting your space and budget.

    All I had to try was one of these from a similar project.

    http://www.digikey.com/product-detail/en/59070-010/59070-010-ND/43968

    But it works pretty well.

    If you can tolerate 1/2 ohm.
    Tested@:
    100T #27 pulls in at .4A and drops out at .18A.

    Seems to be room for larger gauge and same ampere-turns if 1/2 ohm is to much.
    An authentic reed switch should have less differential.
     
    Last edited: Jan 31, 2014
  10. crutschow

    Expert

    Mar 14, 2008
    12,995
    3,229
    Someone with a little experience in building circuits should be able to readily fab it.

    But note that some of the part values may need to be tweaked during circuit testing from those shown, since the real circuit may operate somewhat differently than the simulation due to various parameter tolerances of the parts. If there's a problem with the built circuit, someone here should be able to help you resolve the issue.
     
    ozioho likes this.
  11. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    Have you changed your mind?:cool:
     
    ozioho likes this.
  12. TANDBERGEREN

    Member

    Jan 20, 2014
    74
    4
    Small and relatively inexpensive is the ACS712 cirquit. 8-pin smd unit wich can neassure the current to the motor. The output can be fed directly to a comparator where You set the trip-point to be at the desired low current value.

    I could draw it up for You but the datasheet for the ACS712 is quite self-explaining.
    Also with examples for exactly Your application.
     
  13. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    And a picture.................................................................
     
    • ct.png
      ct.png
      File size:
      12.6 KB
      Views:
      24
    ozioho likes this.
  14. TANDBERGEREN

    Member

    Jan 20, 2014
    74
    4
    Not tested, not evaluated in any way, just a basic idea.

    [​IMG]
     
  15. ozioho

    Thread Starter New Member

    Jan 20, 2014
    15
    0
    I am meeting with a person today about the project. Will be taking all this great info to see if he can build this for me. I appreciate all input and ideas from everyone that has helped me on this topic. I will keep checking for new ideas, so keep them coming. I will give updates on this project as it happens.
     
  16. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    If you are in the states, I could put the current relay in an envelope for you.

    It's not perfect, but a one component solution.
    Not counting the cap and possible series resistor, providing extra starting current, to latch relay on start-up.
     
  17. ozioho

    Thread Starter New Member

    Jan 20, 2014
    15
    0
    o

    That would be great! This would take a lot of pressure of me. Can't find a PM button on the forum, but if I got some info, "invoice" I would be moooore than happy to compensate you for the parts and time.
     
  18. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    Last edited: Jan 31, 2014
  19. ozioho

    Thread Starter New Member

    Jan 20, 2014
    15
    0
    Maybe the moderator will give you my email. My email is my social type email anyway. Just a hotmail account, so I already have to delete 100 emails per day out of it. Or I can open a new email just
     
  20. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,515
    2,369
    Why not use 'Ask a question' request using inwo's ebay link?
    Max.
     
Loading...