Discussion in 'The Projects Forum' started by vini_i, Apr 7, 2011.

  1. vini_i

    Thread Starter New Member

    Apr 19, 2010
    My coworker built his own submersible propulsion system. It uses at car battery (deep cycle), an electric boat motor (40 amp) and a solid state relay to control the whole thing. The contraption is housed in a water tight metal cylinder that is neutrally buoyant. On the inside he mounted a magnetic alarm switch from a door and he uses a magnet in his scuba glove to trigger the switch to turn the relay on and off. What he doesn't like is that it has one speed, Fast. It's not even the speed so much as the acceleration. It nearly rips your arms off because the motor goes full speed as soon as it's turned on.

    I was thinking that if the speed of the motor can be ramped up slowly to full speed that the vehicle would be much more manageable, and that a transistor like a mosfet could be used.

    My question is what could i use to build a circuit that slowly ramps up a duty cycle to a maximum predetermined amount and give me two adjustments. One would be that maximum amount and two, how long it takes to reach it. also so the when turned on the circuit will ramp up to full and stay there until turned off.

    Any ideas would be greatly appreciated.

  2. russpatterson

    Senior Member

    Feb 1, 2010
    You can use an N-Channel MOSFET and switch the negative side of the motor. Take a look at this schematic that runs a pump.

    In the upper right you'll see the section I'm talking about. You'll need all that stuff, including the diode, rated at 1/2 the current your motor will draw.

    I would use an Arduino (if you're new to micro controllers). Use the PWM on the Arduino to drive the MOSFET gate (with the series resistor).

    You can then write some simple code to ramp up the duty cycle. Use an Arduino input pin to get the user commands via the magnet to relay input.
  3. wayneh


    Sep 9, 2010
    Could there be a simple mechanical solution? I mean, if the pump blades are inside a venturi chamber or such, maybe there could be a way to throttle the water input or output. No water, no propulsion. The motor would need to be able to withstand the higher RPM, just like when you choke off a vacuum cleaner.
  4. russpatterson

    Senior Member

    Feb 1, 2010
    I guess simple is all relative to what you're comfortable with. I think you could also devise 555 timer based system to slowly ramp up the duty cycle using capacitors and resistors for the timing.

    One issue I've run into that you should be aware of is that you need to overcome the initial resistance of the motor so it starts spinning. For my pumps I run them 100% duty cycle for 1/2 second to make sure they start spinning before I scale the duty cycle back to 50% (or whatever). If you start at a low duty cycle you may not overcome that resistance, motor doesn't turn, and you're in danger of burning through windings on the motor as all the current passes through the same spot and it heats up. It's all those edge cases that make the software based solution attractive for me. Check out the Arduino. You can be putting out variable duty cycles in a couple of hours. They've made it really simple.

    They even make some really small ones.

    Here's a buying guide

    I use the PIC microcontrollers and make my own boards but Arduino is an easy way to get started.