12vdc to 5vdc using L7805

Discussion in 'General Electronics Chat' started by ATM, Oct 14, 2009.

  1. ATM

    Thread Starter Member

    Oct 8, 2009
    31
    0
    Hello guys.

    I plan on dropping 7v from a 12v car battery down to 5v to operate a sensor. The sensor requires a current of 6mA. So if my calculations are correct the 7805 will need to dissipate 42mW. I have a couple of questions:

    1. If I use the typical circuit below:

    [​IMG]

    Will I need the caps? Also, I can disregard a heat sink if only 42mW needs to be dissipated right?

    Thanks.
     
  2. russ_hensel

    Well-Known Member

    Jan 11, 2009
    818
    47
    one: Yes
    two: Yes
     
  3. ATM

    Thread Starter Member

    Oct 8, 2009
    31
    0
    In this instance what are the caps for if you don't mind me asking? I presume the 0.1 is to help smooth the output?
     
  4. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    The capacitor are there to prevent the regulator from oscillating.
    It is a stability question.

    Greetings,
    Bertus
     
  5. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    More precise 0.33u is required if the regulator is located more than 3" from the power supply filter. Will prevent regulator from oscillating. But I have never experienced a oscillating 7805 even in my most "quick and dirty" prototype boards. The 0.1u is the recommended minimum load capacitance to limit high frequency noise. I like to combine this with a 10U tantal cap
     
  6. ATM

    Thread Starter Member

    Oct 8, 2009
    31
    0
    Nice, thanks for the replies guys.
     
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