12V to 5V USB charger

Discussion in 'The Projects Forum' started by imperatormk, Jul 31, 2013.

  1. imperatormk

    Thread Starter Member

    Mar 1, 2011
    96
    2
    Hey.

    I want to get 5V off 12V car battery. At the moment I am using LM7805 together with several 1N4007 diodes to lower the input voltage.

    The problem is the IC and the diodes heat up to a point that they are too hot to touch. I am using a small heatsink for the regulator. The input current is ~0,6A and the input voltage of the IC is ~7.6V thanks to the diodes.

    I need to know whether it's OK for the components to heat that much knowing that the current flowing through them is less than the max specified in the datasheet.

    Thanks!
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,446
    3,361
    Yes, the diodes and regulator will get hot because 60% of your power must be wasted as heat.

    A more efficient solution would be a switching regulator.
     
  3. Shagas

    Active Member

    May 13, 2013
    802
    74
    The LM7805 has an operating temperature range of up to 125 Celcius so I guess you can spray some water on it and if it doesn't sizzle then it's less than 100 and it should be fine .
    Also if you are worried about high temperatures then get a bigger heatsink

    Anyone else think that that is a good/bad idea?
     
  4. Dr.killjoy

    Well-Known Member

    Apr 28, 2013
    1,190
    156
    I would use a car chargerand use that or mod it from there
     
  5. wayneh

    Expert

    Sep 9, 2010
    12,126
    3,048
    I mounted my 7805 inside an Altoids tin, and screwed it right to the metal wall. It stays mildly warm most of the time, except when I plug in a discharged phone. Then it gets hot enough to discolor the paint around the mounting screw, and obviously wicked hot to the touch. I believe in that mode the current is restricted by internal protection of the 7805, and some devices don't like that. This thing has lasted for years. Dropping the input voltage with diodes would help spread out the heat.
     
  6. #12

    Expert

    Nov 30, 2010
    16,298
    6,810
    I have seen 780L5 chips intentionally used for their current limiting behavior. They work.
    They get hot as a match head, but they work.
    Bolt ANY chunk of scrap aluminum on it to spread the heat.
     
  7. blueroomelectronics

    AAC Fanatic!

    Jul 22, 2007
    1,758
    98
    It's one of those things where you could buy a cheap USB car adapter (they're switching regulators) for about $2. I've seen them at the local dollar stores for that. Some can also supply much more current than the 7805.
     
  8. wayneh

    Expert

    Sep 9, 2010
    12,126
    3,048
    I agree. Time has marched on.
     
  9. imperatormk

    Thread Starter Member

    Mar 1, 2011
    96
    2
    Hey. I have bought a USB car adapter, and now experience a little problem. After ~10 minutes of charging, the cellphone's battery gets fairly warm, which never happens when I charge it off the power grid. I have also tried a different model of USB adapter, to no avail.

    The both chargers I tried can supply 1000mA, while the 220V adapter can supply 850mA, so it shouldn't be the problem here.

    Any suggestions appreciated.
     
  10. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,770
    970
    Battery heating during car charging is "normal". Every phone I've ever had would always seem warmer when charged in the car vs home
     
  11. imperatormk

    Thread Starter Member

    Mar 1, 2011
    96
    2
    It makes sense. Thanks!
     
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