19v @ 3.42a To 12v @ 4.2a?, DC to DC converter??

Discussion in 'The Projects Forum' started by GRNDPNDR, Apr 24, 2012.

  1. GRNDPNDR

    Thread Starter Member

    Mar 1, 2012
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    I have an external hard drive enclosure that requires 12V @ 4.2A, mostly due to it holding 2 drives, and startup current is high, so running current should be substantially lower. The enclosure is: HERE

    What I would like to do is construct a plug that my laptop power cable will plug into and split it's power between the laptop and the hard drive enclosure. Basically piggybacking the laptop supply.

    my Laptop power supply is 19V @ 3.42A

    If I've figured this out correctly:

    19V*3.42 = 64.98W

    Using that information I then calculated:

    64.98W/12V = 5.42A


    That is plenty of current needed to start the hard drives, after which should settle down to something more reasonable.


    What would be the simplest and/or cheapest way to step down my laptops power supply to 12V so that I can use just one plug to run my laptop and external drives?
     
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    And whats going to power your laptop when you have "stolen" all its power?
     
  3. GRNDPNDR

    Thread Starter Member

    Mar 1, 2012
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    it will only require that high current on startup of the drives. I wish the company would have considered a staggered start for the drives but apparently not.

    Presumeably the laptop is also only in need of high current on startup, and the laptop and drives will not be powered up at the same time.

    I mostly use the laptop at home, and 99% of the time it's plugged in.

    Should the need arise I will have a separate power supply for the hard drives, but in any case they couldn't be used unless plugged into the wall.

    Right now I'm having a hell of a time trying to find a power supply capable of 4.2A @ 12V.

    I did manage to find one, but it's universal and $30 which I don't have at the moment. If the surplus store hasn't sold out when I get some money I'll go pick one up.
     
  4. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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  5. GRNDPNDR

    Thread Starter Member

    Mar 1, 2012
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    No I got it surplus. so SOL for a power supply and I didn't even consider that two drives would consume such a high current on startup.

    The box says 12VDC @ 4.2A so I would like to provide it with it's correct source.
     
  6. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    I was just building a power supply for my single external drive: I measured both the 5V and 12V requirements. At start up, the 5V rail jumped to about 1.8A and then settled down to about 0.8A steady current. The 12V rail was steady at about 0.7A.
     
  7. bountyhunter

    Well-Known Member

    Sep 7, 2009
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    $9 on ebay. free shipping

    http://www.ebay.com/itm/iMAX-Charge...=Laptop_Adapters_Chargers&hash=item483d289f35
     
    Last edited: Apr 25, 2012
  8. t06afre

    AAC Fanatic!

    May 11, 2009
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    I agree with those who say get a separate power supply. Your original plan was kind of dodgy:eek:
     
  9. evilclem

    Member

    Dec 20, 2011
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    You cannot run both at once off the same power supply as there isn't enough current. I don't think your drives will be happy with the voltage collapse (assuming you implement appropriate current limiting to protect the power adaptor) when going into current limit.
     
  10. t06afre

    AAC Fanatic!

    May 11, 2009
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    Also in order to do this. You will have to cut the laptop power cable. And spleising the cable back together would not be an easy task. Unless you are an expert with the solder iron and shrink tube. The cable used for laptop power is not easy to work with.
     
  11. GRNDPNDR

    Thread Starter Member

    Mar 1, 2012
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    First of all, splicing a cable back together is no challange. I've done tons.

    Secondly no way would I cut into that wire. Why wouldn't I just buy a corrisponding female plug, and another male and make a splitter cable?


    I tested this lastnight with a 3.33A supply and it seems to work fine.

    The drives list there requirements as
    0.5A @12V
    0.72A @ 5V

    So adding it all up, both drives combined only pull 2.44A, and obviously nothing is exceeding the power supplies capabilities because if it gets over loaded it just cuts out and this didn't happen.


    HOWEVER, $9 on ebay...... I think I may go that way.
     
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