Need help with a hovercraft!

Discussion in 'The Projects Forum' started by laxman0324, Apr 9, 2009.

  1. laxman0324

    Thread Starter New Member

    Apr 9, 2009
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    As stated in the title, I am building a hovercraft for a class project. It is powered by a 14.8V battery rated at 6.5A max output. I have one lift fan rated at 1A draw and 2 thrust fans and 2 directional fans (these 4 are the same, and they draw a whopping 5A at 9V). The battery is Li-ion, so it doesn't give any power with that kind of draw attached to it. I figured out that the fans need to be put down to 4V to attain a 1.5A draw, which is much better since only two of the fans will be constantly running, the other two will be pulsed as needed.

    So in order to achieve this voltage I need an adjustable voltage regulator. It needs to be able to handle 14.8V input, give me at least 3-4V output, and most importantly have an amp output of 6A (at least). I have done a lot of searching and it has been exhausting and yielded me no good leads. Any information on where I can get this type of regulator would be much appreciated.

    Also, do you think that I will need a heat sink for my given setup?

    Thanks ahead,
    Mike
     
  2. PRS

    Well-Known Member

    Aug 24, 2008
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    Great project. Hope you succeed. :)

    Looking through my Jameco catalog I find one switching regulator that will output 8 amps. It operates at 4 MHz. It cost 13 bucks. I'll give you the number if you want it. Though I may be wrong, I don't think you'll find a simple analog type of regulator to handle this high current.

    The switching regulator sounds like a pain in the neck. Personally, I would make my own regulator of a zener diode, a resistor and a big BJT in a TO3 case -- such as the 2N3055 -- operating in the follower mode. Use the zener and a resistor to bias the base.

    Your supply will cause 14.8 - 4 volts = 10.8 volts Vce. If you want to supply 6 amps to the load then you need more current than that going through the transistor in order to keep it going. Let's say 7 amps. With Vce at 11 volts and Ic at 7 amps, you're looking at 77 watts. Of course you will need a heat sink -- a good one. And the resistors have to be big. Also the zener. I'm sure you can figure out their values and their power requirements, but if you can't, or you're pressed for time, let me know.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    I suggest that Paul's switching regulator is a good idea. If you attempt to use a linear regulator, you will be dissipating (wasting) a great deal of power just in the regulator. Additionally, you will need a rather large heatsink per regulator.

    While you may be able to incorporate a clever heatsink design into your basic framework (for example surrounding the tips of the fan blades with copper/aluminum shroud) you will be far better off if you don't need the heatsink to begin with.

    Were your power requirements less, you could have used something like this:
    http://www.dimensionengineering.com/DE-SW050.htm
    basically, a switching drop-in replacement for a 7805 regulator.

    You could possibly use this:
    http://www.dimensionengineering.com/DE-SWADJ3.htm
    another switching regulator, but adjustable, and capable of 3A.

    However, your needs for the lift fans might be met by simply using PWM with an adjustable duty cycle with a power MOSFET to switch current, and a "flywheel" diode across the motor to provide a current path when the MOSFET is off.

    For your directional control fans, you might need an H-bridge. An L297/L298 pair per fan may do the job for you (look up their datasheets; ST Microelectronics and others make them). Fan blades designed to produce airflow in a particular rotation are extremely inefficient when rotated in the opposite direction, but still may produce a modicum of useful thrust.

    My time here is quite limited lately; I'm just throwing some ideas your way.
     
  4. laxman0324

    Thread Starter New Member

    Apr 9, 2009
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    Well this is a school project, so we are on a tight budget while doing this. And to be honest, my electrical knowledge is very basic, so all of these complex structures are way beyond my reach for an engineering project while taking other classes. This will run for maybe a 15 mins at a time, and i plan on putting on a heat sink. I am really hoping there is some regulator that I can use instead of trying to build custom circtuitry. I really appreciate all of the fast responses!

    Mike
     
  5. laxman0324

    Thread Starter New Member

    Apr 9, 2009
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    BTW: the only fans seeing the regulator are the thrust and directional fans, NOT the lift fan. It will be ran in parallel with the regulator.
     
  6. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    I did not see a V rating for lift fan motor. Do directional fans need to be reversed? Can you use two batteries of lower Vs? PWM not costly, more in postage.
     
  7. PRS

    Well-Known Member

    Aug 24, 2008
    989
    35
    After reconsideration, I think you'd be best off picking motors that all have the same voltage rating. I hope you know about using an H-bridge for speed control by way of PWM, pulse-width-modulation, as Sgt Wookie said. This is almost a must for your project. When I graduated I was as yet ignorant about it. Maybe someone could point you to a good free source on line. I don't know of one. The best source I found on the subject was a great big paperback called Robot Bonanza.

    By the way, is this an RC project or are you using a teather with control and power wiring inside a cable? This could make a big difference. If the latter is the case, then by all means use a second battery so that you don't have to waste power just to reduce the voltage!
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Here's a pretty simple PWM circuit based on a 555 timer that Bill Marsden posted in his blog:
    [​IMG]

    Basically, the output of the 555 could be connected to the gate of an N-ch power MOSFET via a resistor in the range of 39 to 62 Ohms, and the MOSFET used to sink current from the motor.

    The PWM'ed MOSFET in conjunction with the motor's coils would in effect create a buck-type switching regulator, and would be quite inexpensive to build. You will also need a "flywheel" diode across the motor to provide a path for current when the MOSFET is turned off.
     
  9. laxman0324

    Thread Starter New Member

    Apr 9, 2009
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    I really appreciate the replies you guys have given me. That stuff is way past the level of this class (and my experience). I ended up getting one of these http://www.micrel.com/_PDF/mic29710.pdf

    It is working great except it shuts itself off after 5 seconds or so because I am dissipating about 70 watts of power through the little tab attached to it. Does anyone know of a good heat sink I can use? The best one I have found so far has a thermal resistance of 2.6C/W, but 70W of power puts it at 182C, which is well over the 125C. Can I attach multiple heat sinks? Any suggestions are more than appreciated.

    Thanks,
    Mike
     
  10. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    Try positioning the heatsink & regulator in one of the airstreams. It won't be a complete fix, but it will help.

    Also, check Digikey or Newark or their competitors for a better heatsink!
     
  11. laxman0324

    Thread Starter New Member

    Apr 9, 2009
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  12. SgtWookie

    Expert

    Jul 17, 2007
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    Sorry, but I just don't think you're going to get where you need to be without a PWM solution. You'll dissipate far too much heat in a linear regulator.

    Why don't you start off with a single 555 and build the circuit Bill Marsden drew up that I posted a few back, and get it working? Compute the RC values for the frequency range to be around 200Hz or so. Start with C being a commonly available value like 10uF or so, and compute R from there.
     
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