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How to keep 7805 from overheating?

Discussion in 'General Electronics Discussion' started by Mat17, Oct 21, 2015.

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  1. Mat17

    Mat17

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    Oct 13, 2015
    I have a solar panel maintaining a 12V battery 24/7 and its powering an arduino (it's not built yet). If you're wondering why it's 12V, it's because I have to power a solenoid on the same source. I was wondering if I could put an LED before the 5V regulator or something that could use up some power before big turned into heat.

    Any suggestions? I imagine the 7805 will get pretty hot.
     
  2. rickselectricalprojects

    rickselectricalprojects

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    Feb 1, 2015
    it will get very hot, either use a massive heatsink or a switching regulator.
     
  3. Kiwi

    Kiwi

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    Jan 28, 2013
    How much current will the 7805 be carrying?
     
  4. Mat17

    Mat17

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    Oct 13, 2015
    The arduino only uses about 40mA I believe. I'm only using a photresistor as an input, then outputting a small signal. The input would only be for about 3 seconds. So it's really not that intensive.
     
  5. AnalogKid

    AnalogKid

    2,352
    659
    Jun 10, 2015
    The power dissipated in a series voltage regulator such as the 7805 equals the differential voltage across the device times the current through it (plus a small amount of power used by the regulator itself). With 12 V in and 5 V out, the differential voltage is 7 V. At 40 mA out, that's a peak device power of 0.28 W. If the 40 mA is constant, then the average power also is 0.28 W. Go through the datasheets and add up the currents of everything that is powered all the time separate from the things that are powered intermittently. To the "all the time" list add in the regulator circuit power. As I recall, a 7805 has about 10 mA of current in the ground pin. This is coming directly from the input, so it adds 0.12 W to your list. With these numbers you can calculate the peak and average power dissipations.

    OK, so what. In very round numbers, at 1/4 W in free air a TO-220 package will get warm but not hot. At 1 W it will be painfully hot and at 2 W -ish it will go into thermal shutdown or fail. This should give you a starting point for determining how to protect your circuit.

    ak
     
  6. BobK

    BobK

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    1,685
    Jan 5, 2010
    Don't Arduinos have a built in regulator?

    Bob
     
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