7812 gets hot. how to reduce the current.

Discussion in 'General Electronics Chat' started by imraneesa, Jun 8, 2015.

  1. imraneesa

    Thread Starter Member

    Dec 18, 2014
    184
    4
    i tried to connect 7812 to laptop charger 19V 3.5A. and the wire start to smoke. 3.5A is too much for the regulator?
    how to limit it? it is not possible to put resistor in series becoz we need so much wattage.
    what alternate things i can do.
    i have checked the pinouts in datasheet. so i connected them correct.
    do so much current passes through regulator even though no load is connected?
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,093
    3,030
    It's hard to understand your results. It sounds like the regulator got hot with no load attached?

    Did you use input and output capacitors as recommended in the data sheets? The ripple in the supply voltage may have been causing problems for the regulator. Filtering it may help.

    The normal 7812 cannot handle 3.5A, even with a large heat sink. The application circuits in the dat sheet will usually describe how to build a higher current circuit.
     
  3. dl324

    Distinguished Member

    Mar 30, 2015
    3,235
    619
    Are you saying that the charger wire started to smoke? Was there a load on the 7812 at the time?
    Yes. Maximum current tolerated depends on package and heatsink.
    Determine what is drawing so much current and go from there.
    Quiescent current for a 7812 should be around 10mA.
     
  4. Roderick Young

    Member

    Feb 22, 2015
    408
    168
    What causes the LM7812 to get very hot is when there is a large voltage across it at that current. You have 19 volts in, and 12 volts out, so that means there are (19 - 12 = 7) volts across the regulator. At 3.5 amps, that's (7 * 3.5) = 24.5 watts, the same as a small soldering iron.

    If you need 12 volts at 3.5 amps, your best bet is to buy a ready-made adapter with that output on eBay or somewhere. It won't be very expensive. Second choice, if I was doing it, would be to buy a switching regulator on eBay, perhaps based on the LM2596, and put a big heat sink on the actual regulator part. Last time I got one of those modules, it was less than $3 from eBay.
     
  5. imraneesa

    Thread Starter Member

    Dec 18, 2014
    184
    4
    Thank you for all the comments. i went to recheck and found how stupid i was. i connected hot wire to ground and ground to input.

    i spoiled the regulator finally. but i learned something. thank you so much everybody.
     
  6. imraneesa

    Thread Starter Member

    Dec 18, 2014
    184
    4
    what is quiescent current mean. can you please explain it in easy words? thank you.
     
  7. OBW0549

    Well-Known Member

    Mar 2, 2015
    1,301
    880
    Quiescent current is simply the idle current: the current the internal regulator circuits take for their own operation, regardless of load current.
     
  8. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    There is no better experience than letting the magic smoke escape. You won´t do that same mistake anytime soon.
     
  9. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
    782
    The 78xx appnotes describe how to add an external bypass transistor to get more current - its been discussed several time just recently on this forum.

    If you're burning up wires, you have a short somewhere.
     
  10. dl324

    Distinguished Member

    Mar 30, 2015
    3,235
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    I used the terminology used by the manufacturer. Quiescent == inactive or idle. Quiescent current changes depending on the regulator's load. That is important to know if you're inserting a resistance or diode in the ground connection to change the output voltage. See specs attached below.
     
  11. ian field

    Distinguished Member

    Oct 27, 2012
    4,413
    782
    I couldn't find the bit in the manufacturer's literature about the wires started to smoke.
     
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