Reduce voltage from battery to power rc vehicle

Discussion in 'Power Electronics' started by wesleyg, Jul 5, 2016.

  1. wesleyg

    Thread Starter New Member

    Jul 5, 2016
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    Hello. What I would like to do is reduce the voltage output from a 3s lipo battery (11.1v +/-) down to something around 9-9.5v. The catch being the rc boat i'm trying to power draws up to around 30amp peak. It normally runs of a 2s lipo (7.4v) but I am trying to sneakily one up a friend of mine with the same boat. I've read about people running them straight off a 3s but they dont seem to last too long. I would love to get just an extra 1.5-2v so as to get a few more rpm out of the motor but I cant figure out a simple way to do this with the amp draw required. Any help would be great.
    Thanks
     
  2. #12

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    Nov 30, 2010
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  3. wesleyg

    Thread Starter New Member

    Jul 5, 2016
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    Thank you very much. Just one more question. Do you know roughly how much each one will drop the voltage? Just need to know how many to buy.
     
  4. #12

    Expert

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    Look at page 2 of the datasheet I provided and see that the answer is about 1.2 volts at room temperature and 1 volt each at 100 degrees C. One volt at 30 amps is 30 watts. Rtheta JC is 1 degreeC/ watt at the case. Then you add a heat sink to keep them from burning up. Their melting temperature is 175C. Ambient is 25C. 150 degrees/30watts is 5C/W max and 1C/watt is already used up by the case, and some will be used up by the mounting interface, so you will need something like what's on this page:
    http://www.mouser.com/Thermal-Manag...?Rl=5gg0ZerfnZ1yhgqirZ1yyms4lSGT&Ns=Pricing|0


    That's some pretty massive metal, so you better plan on bolting these puppies to some kind of steel or aluminum bulkhead. Forced air flow seems like a very good idea in this case.

    Did you say, "boat"? Excellent option for "water cooled". You're going to spend more time doing the cooling design than the electrical design.
     
  5. #12

    Expert

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    Maybe this one in the 3C/W size:

    The point is, something as massive, aluminum or copper is preferable to steel.
    46 grams is about 1.5 ounces, in a very special shape. 3 or 6 ounces in a much cruder shape would work. You will need to use an insulator kit to keep the metal backs from shorting together unless the heat sinks are completely separate from each other.
     
  6. wesleyg

    Thread Starter New Member

    Jul 5, 2016
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    Thanks a lot for the info, I really appreciate it. I'm gonna try and use the water cooling the boat already has set up. Not a hole pile of room in there for heat sinks. Thanks again
     
  7. ian field

    Distinguished Member

    Oct 27, 2012
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    I'm thinking aluminium hull and plenty of heatsink compound to stop the mounting bolt leaking......................
     
  8. #12

    Expert

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    I think heatsink grease is fairly poor as a water seal. Some RTV in the bolt hole, but heatsink compound between the diode case and the metal.
    It is important that the heatsink metal be flat on its mating surface for good heat transfer.
     
  9. ian field

    Distinguished Member

    Oct 27, 2012
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    Its usually an oxide in a silicone grease base so its pretty water repellent.

    RTV would do but not so good for thermal.

    Once or twice I've burned the silicone grease off compound and stirred the remaining oxide powder into 2 part epoxy - you can probably buy that off the shelf if you know where to look.
     
  10. WBahn

    Moderator

    Mar 31, 2012
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    Depending on the size of the craft -- or, more specifically, the amount room available to put things, there are lots of switch-mode power supplies (usually called "battery eliminators" in the RC world, from my little bit of exposure a few years ago) that can easily work at those currents and are pretty cheap. I have a few that display both the output voltage and current that were very handy for powering my airborne equipment from a bunch of LiPo batteries.
     
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  11. #12

    Expert

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    A 30 amp switch mode supply is good about minimizing heat problems, but it won't be smaller or simpler than a couple of TO-247 diodes.
    The TS will have to make a judgement call on this one: larger, complicated, and cool...or smaller, simple, and hot?
    The idea of deception seems to suggest something small and inconspicuous, not something with a digital display.
     
  12. WBahn

    Moderator

    Mar 31, 2012
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    I just mentioned that the ones I used happen to have a digital display -- this made them very convenient for setting the output voltage. Most of the ones available do not have such a display and are most quite a bit smaller. But whether it has a display or not isn't the issue when it comes to being inconspicuous -- if it can be placed out of sight, then what does the presence of a display matter? But, yes, the smallest and easiest to hide ones are almost certainly not going to have a display.
     
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