L7805 capacitors

Discussion in 'General Electronics Chat' started by msr, May 2, 2010.

  1. msr

    Thread Starter Active Member

    Jul 8, 2008
    62
    1
    Hello,

    In the L7895 datasheet (from ST) they use a 0.33uF input capacitor and a 0.1uF output capacitor. Why that values? Should it be changed? Will a 220nF input capacitor has the same effect of a 0.33uF?

    Thanks for your patience!
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    You can probably get away with 220nF (0.22uF). It's best to stick with the manufacturer's recommendations. If you don't, your mileage may vary.

    If your filter capacitor is within a couple of inches of the regulator, you might get away without it at all. However, you will be better off to include it rather than leave it out, have strange problems, and have to add it in later.

    I did an informal bench test on a number of different 78xx series regulators that I had sitting around. A couple of them that oscillated at 3MHz to 10MHz unless the caps were included. When they were oscillating, instead of 5v out I was reading about 1.7v, with a 1.5v p-p poorly formed sine wave riding on it.

    The thing here is that I was specifically looking for such a malfunction. Very small capacitances on the output can cause the output to oscillate. Using the 0.1uF cap swamps the effect.
     
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