3a 12v to 12v linear regulator

Discussion in 'The Projects Forum' started by yonubear, Sep 9, 2015.

  1. yonubear

    Thread Starter Member

    Jan 10, 2013
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    ok I am hoping someone can help me out with i am trying to regulate a 12vdc battery source that is always on charge when mains is available with an average voltage of 13.6 this is not an automotive application. i will be powering an external hard disk that normally uses a 12v wallwart. I am thinking about using a LM1085 but i was hoping someone might have a better regulator. Since I am sure i will be asked i do want a linear as i am trying to get rid of any additional rfi sources near my HF rig.

    thanks,
    yonu
     
  2. #12

    Expert

    Nov 30, 2010
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    Put two of these in series with the load.
     
  3. crutschow

    Expert

    Mar 14, 2008
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    But that may make the voltage too low when he's operating from the battery (which I assume the op wants to do if the mains quit).

    You might use a regulator that has a lower dropout voltage than the LM1085, which is typically about a volt. Here's some from Linear Technology.
     
    #12 likes this.
  4. yonubear

    Thread Starter Member

    Jan 10, 2013
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    I looked at the page you linked but the 1085 seems to be the only one rated for 3a with 12v output can you suggest one incase I am misunderstanding the chart.

    Thanks,
    Yonu
     
  5. ScottWang

    Moderator

    Aug 23, 2012
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    A rating 3A regulator for a 3A application is not a good idea, it should be needs more current, there is another 5A as LM1084 is better than LM1085.
     
  6. yonubear

    Thread Starter Member

    Jan 10, 2013
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    Ok thanks I hope it really doesn't pull 3a but I haven't had a chance to put a meter on it
     
  7. ScottWang

    Moderator

    Aug 23, 2012
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    Even you just need 2A then it still too hot for a linear regulator, the heat will be hot, hot, hot, you will be need a fan.
     
  8. yonubear

    Thread Starter Member

    Jan 10, 2013
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    Ok I take it a linear with a just a heatsink won't cut it.
     
  9. crutschow

    Expert

    Mar 14, 2008
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    Need to know the current the circuit will actually draw.
    What's the rating of the wallwart?
     
  10. yonubear

    Thread Starter Member

    Jan 10, 2013
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    3a which find hard to believe since my other external of the same model came with a 2a wallwart.
     
  11. tcmtech

    Well-Known Member

    Nov 4, 2013
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    If you are only running at 13.6 volts top end why bother regulating it at all?

    I have a number of old powerpacks for devices such as yours and other computer peripherals that are rated at 12 volts output yet at no load they put out anywhere from 14 - 17+ volts no load and I have yet to see any 12 volt rated devices that run off them blow up and burn down when given the extra few volts.

    My point is being very few transformer based power packs ever put out exactly their rated voltages at light loads and they were the industry standard before the SMPS types came out and they still work with the newer equipment just fine.
     
  12. yonubear

    Thread Starter Member

    Jan 10, 2013
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    I had considered that but I just assumed I should have some type of regulator on it. but if I don't then I will give it a shot with a spare drive.

    Thanks,
    Yonu
     
  13. tcmtech

    Well-Known Member

    Nov 4, 2013
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    If you have ever played around with any types of ATX power supplies you would be familiar with how poorly regulated their 12 supply lines can be.

    I've seen a few of them go as high as 15 volts at light/no loads some go down to 10 volts on high loads. Most good ones are 12 volts +- 1 or so.
     
  14. yonubear

    Thread Starter Member

    Jan 10, 2013
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    Wow I have never noticed one do 15 with no load but then I always check then with at least an hdd hooked up to have a load on the 5v and 12v rail.
     
  15. ronv

    AAC Fanatic!

    Nov 12, 2008
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  16. #12

    Expert

    Nov 30, 2010
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    So...place one diode in series with the battery. It will keep the voltage between 11.8V and 12.8V, but the LM1085 has 1.1 volt drop at 3 amps and will take a 12.6 volt lead acid battery down to 11.5 volts.

    Volts available.....12.6...13.6
    LM1085 provides 11.5...12.0
    1 diode provides...11.8...12.8

    On the low end, the diode solution is closer to 12 volts than the regulator chip.
    On the high end, the diode provided voltage will be less than 7% high.
     
  17. crutschow

    Expert

    Mar 14, 2008
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    Doesn't the HD nameplate list its current draw?
     
  18. yonubear

    Thread Starter Member

    Jan 10, 2013
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    The drive enclosure doesn't actually have a level on it. I did check the label on one of the drives that was out of warranty the level has .75a 5v .75a 12v.

    So give me a honest opinion should I use a regulator or not. Based on my test from the setup I plan to use the highest voltage should be 13.9v.

    Yonu
     
  19. #12

    Expert

    Nov 30, 2010
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    After 5 different people have tried to help you for 3 days, you change both the current and the voltage, then use the word, "honest"? Did you get the impression anybody gave you anything except their honest, best suggestion?
     
  20. yonubear

    Thread Starter Member

    Jan 10, 2013
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    No I think everyone has given an honest opinion. However I have been given 3 different suggestions. As to the current ratings by original 3a figure is based off the wallwart that came with the drive someone asked me what the rating on the drive was. As to voltage I simply stated tat the highest voltage possible should be 13.9. Now I am sorry I have upset you with my last statement but that was no reason to be rude.

    Thanks for everyones help I guess I will go back to the drawing board and not bother anyone.

    Thanks,
    Yonu
     
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