7805 votlage regulator overheating

Discussion in 'The Projects Forum' started by lordofentropy, Jul 26, 2012.

  1. lordofentropy

    Thread Starter Active Member

    Apr 10, 2010
    52
    0
    I have a circuit which needs to operate on 5 VDC and measured current is 208.5 mA. The power supply generates +12V (max 2 amps). I wired the regulator as you normally would (per datasheet), .33uF cap on input voltage line to ground and a .1uF cap on output voltage to ground. However, with a heat mount on the regulator, and a high watt resistor (to bring the input voltage down some) the regulator still is over heating. I have never ran across this issue before using a 7805 (1amp max out). Any ideas, suggestions?

    Thanks
     
  2. paulktreg

    Distinguished Member

    Jun 2, 2008
    611
    120
    So how many volts have you got on the input to the 7805?

    It only needs one or two watts and 7805 will feel hot but there's hot and damaging hot if you know what I mean.
     
  3. Papabravo

    Expert

    Feb 24, 2006
    10,144
    1,790
    Here is the calculation:
    Code ( (Unknown Language)):
    1.  
    2. PD = (12V - 5V)* 0.208A ≈ 1.46 Watts
    3.  
    If the thermal resistance is 45°C/W and the ambient temperature 22°C then the part will be at
    Code ( (Unknown Language)):
    1.  
    2. 22°C + 1.46Watts*45°C/W = 87.7°C
    3.  
    Don't know 'bout you I'm not going to put my lips on that!
     
  4. lordofentropy

    Thread Starter Active Member

    Apr 10, 2010
    52
    0
    I've tried from 11V-7. something volts and results are all the same, at the lower voltages as an input going into the 7805 it just takes longer...and i am talking Temps above 100 degrees F. I've had 8-9 volts going into it and w/ a thermal couple saw temps reach well over 125 degrees, trying to push it and see if it ever would reach a plateau I stopped at 178 degrees and it was still rising.
     
  5. bretm

    Member

    Feb 6, 2012
    152
    24
    Papabravo did the math for you. 88 degrees C would be normal for the voltage and current you listed, and that's 190 Fahrenheit. Why do you consider that overheating? What is the part rated for? (125 deg C would be a common number)
     
  6. lordofentropy

    Thread Starter Active Member

    Apr 10, 2010
    52
    0
    Auh...totally missed the Celsius..my bad; interesting; I never had one ever get that hot on me before
     
  7. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    Which package device is the 7805?

    You need to use a TO-220.
     
  8. Papabravo

    Expert

    Feb 24, 2006
    10,144
    1,790
    I just looked at the 7805 datasheet. R-theta-J-A for the Fairchild Part in a TO-220 package is actually worse than I quoted above. It is 65°C/W. So plugging that into the calculation above makes it worse. This one most definitely needs a substantial heatsink.
     
  9. takao21203

    Distinguished Member

    Apr 28, 2012
    3,577
    463
    You need a LM2576 or MC34063.
     
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