Water switched mosfet switching power mosfet

Discussion in 'The Projects Forum' started by kiwifisher, Sep 13, 2016.

  1. kiwifisher

    Thread Starter New Member

    Sep 13, 2016
    6
    0
    I want to use two mosfets to switch on the motor on a model boat.
    The first will power up a timer whose output will switch a second mosfet which will connect motor circuit.
    The first mosfet to have two external connectors which will connect thru the water when lowered into it
    I have very llimited experience in this feild but understand the basic circuitry.
    The motor is 12V and draws up to 55A .
    Any assistance on mosfet and resistor selection would be most appreciated.
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,155
    3,061
    Since the first MOSFET is switching only the timer, how much current does the timer draw? What is the output of the timer?

    Why the timer - are you just delaying the start?
     
  3. kiwifisher

    Thread Starter New Member

    Sep 13, 2016
    6
    0
    no setting the time off.
     
  4. #12

    Expert

    Nov 30, 2010
    16,347
    6,835
    So much missing information! Do you want it cheap and dirty or reliable in the long run? What kind of timer? Do you already have the timer? How much time? Is it salt water? Is the water rather pure like a mountain spring? Is the water at least an inch deep? Is the water in the bilge (captive) or is it free flowing?

    I'm miserable at mind reading.:(
     
  5. kiwifisher

    Thread Starter New Member

    Sep 13, 2016
    6
    0
    hi expert,
    In the long run reliable.
    The switching medium is salt water.
    The timer is a digital one. but I do not yet have it. Time is up to 30 minutes currently - battery limited pretty much.
    Longterm this will be part of a much bigger control system but I am testing each cicuit seperately, to ease debugging.
    ;)
     
  6. #12

    Expert

    Nov 30, 2010
    16,347
    6,835
    Bi-polar power supply to make a square wave which is sent to a standard sized probe made of two carbon motor brushes encapsulated in plastic with a slot cut between them to let the water in. Salt water on Earth is rather predictable as to its conductivity range. That will give you the resistor size to compare the signal to. After your circuit has made the detection of salt water, you use that to fire up a CD4020 timer chip and that chip fires the power mosfet.

    This is not a $2 circuit. It's how to make something that lasts for years and can be put on a production line.
     
  7. kiwifisher

    Thread Starter New Member

    Sep 13, 2016
    6
    0
    I was looking at using two "probes"(stainless buttonhead bolts) moulded into the plastic hull, and attaching wires to these. Hadnt thought of carbon brushes due to the difficulty of preventing them being totally overmoulded during manufacture. the bolts I can push silocone tubes over.
     
  8. #12

    Expert

    Nov 30, 2010
    16,347
    6,835
    The carbon brushes are supposed to be over molded. That's why you cut a slot in the potted assembly. Two rather inert plates with a known width of gap and a known depth of cut makes a known area of salt water. Cheaper than silicone, very repeatable, less number of parts, no worries about leakage in the sleeves, and less labor cost to make.
     
  9. kiwifisher

    Thread Starter New Member

    Sep 13, 2016
    6
    0
    I will not have access to make fixings on the brushes post moulding.
    worst case scenario with bolts is unscrew and replace using a proprierty sealant.
     
  10. #12

    Expert

    Nov 30, 2010
    16,347
    6,835
    I think the big difference here is that you started out:
    and now you're at
    Dropping a hull into the water is very different from dropping a probe into the water.

    ps, I'm called, "Number Twelve".
    That, "expert" thing is a label created by the website software.
     
  11. kiwifisher

    Thread Starter New Member

    Sep 13, 2016
    6
    0
    ok #12
    No it is immersing two conductors into seawater, no difference to me.
     
  12. Kjeldgaard

    Member

    Apr 7, 2016
    73
    17
    What about using a capacitive sensor, glued on the inside of the hull?

    The principle is used (in reverse mounting) to control the water level in some jacuzzi.
     
    wayneh likes this.
Loading...