Getting 18v down to 12v

Discussion in 'The Projects Forum' started by summitlt, Mar 11, 2008.

  1. summitlt

    Thread Starter New Member

    Mar 11, 2008
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    I have a project im trying, i have 3 of those Ryobi 18V batterys, and im looking to use them to run a portable radio, which happens to be a car radio. The fuse on the radio is a 10a, so we'll assume were going to hit 10a max. I highly doubt itll pull that much, but well figure it.

    How can i get the 18v down to 12-14v? 14v being ideal.
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
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    Try a search on the exact phrase "linear regulator" and another one on the exact phrase "dc-dc converter"
     
  3. mik3

    Senior Member

    Feb 4, 2008
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    use this regulator http://www.chipdocs.com/pndecoder/datasheets/MIPAC/42050-128.html

    i think 10amps is too much for a radio but its better to use it as it wont get hot
     
  4. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
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    10 amps is not that much, because 10amps @ 12 volts is 120W, which seems that it could be for example 2x30W amplifier.

    You should use switch-mode regulator because power vasted with linear regulation would be way too much.
     
  5. mik3

    Senior Member

    Feb 4, 2008
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    yes but you dont drive the amplifier at full power all the time and lets say the radio has 4x30 watts , maybe 30watts is the peak power it can deliver and not the continuous power.
     
  6. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Are the batteries NiCd, NiMH, or A123? As noted already, a regulator will work. The linear regulator will effectively throw away the extra power as heat. That is, if the batteries are 2 AH at 18V, the linear regulator will give you something around 2 AH at 12 V. So, you go from great batteries to average batteries in terms of performance per unit weight.

    My approach would be to disassemble the pack and re-assemble it for 12 volts. You have to do that with some care, but none of those chemistries pose the same risk as one can see with Li-polymer and similar lithium batteries. Check on some hobby sites, which give explicit directions. Here is just one such site: http://www.rcgroups.com/forums/showthread.php?t=599316

    John
     
  7. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
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    Hi John,

    What to do with the rest of the cells you took away to make the 18V drop to 12V? Can you parallel them with other cells, and if so, would this present an unbalance?

    You should stay away from linear regulation, due to the tremendous power dissipation you'll need to take care of. I think you may be overshooting the requirements a bit though. If you decide to linear regulate it, only drop the absolute minimum you need to.

    If you go to switching regulation, then be careful of noise. You are feeding a radio and an amplifier, which may not like your selected switching frequency. If it is a modern radio, then you will probably be feeding into a switching regulator anyways. It would be an interesting thing to see anyways... Good luck!

    Steve
     
  8. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    It's for the reasons you state that I would just reduce the cell count. The linear regulator provides no advantage over reducing the number of cells. It just turns the power to heat. The more cells, the greater chance one will go bad. Option #2, can be a bit complicated and produce noise.

    In my view, save the removed cells for another day. Buy two packs and make 3 packs of 12 V each or parallel them and get triple the ampacity. Or, if they are the CR123A cells, send them to me. I'll pay shipping. :D

    John
     
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