LM78L05 vs. LM7805 Output Transient Suppression

Discussion in 'General Electronics Chat' started by tracecom, Sep 6, 2011.

  1. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
    The National Semiconductor datasheet shows a .01 μF cap on the output of the LM78L05 regulator but shows a .1 μF cap on the output of the LM7805. The datasheets show them both as somewhat optional. I have a circuit that uses a LM7805 which I plan to replace with a LM78L05. Do you think that National really intended to use two different value caps for these two devices or is the value (.1 vs .01) not really that important?
  2. tom66

    Senior Member

    May 9, 2009
    The 78L05 can supply less current than the 7805 so might not require such a large transient cap.
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    Concider the manufacturer of any device you intend to use to have the name "Simon." Therefor, if Simon Sez, you do it.

    I'm not sure where you got your info on the NS LM7805, as they have discontinued that part and I get an LM7812 data sheet searching their page.

    The LM78L05 does indeed have a .01 cap on the output and the following comment: Note 4: Recommended minimum load capacitance of 0.01μF to limit high frequency noise.

    Note "minimum." More is OK. These little regulators can actually oscillator under some conditions, all of them have little to none cap there. An exact value of cap is unimportant, a min value is important.
  4. SgtWookie


    Jul 17, 2007
    A few years ago, I did a very informal test on a random selection of 7805 regulators that I'd accumulated over the years. I found a couple of them that oscillated at frequencies in the MHz range. Adding the recommended 0.1uF cap eliminated the oscillations, and the regulators performed as specified.

    Always have a cap on the output. It may not oscillate without one, but it's not worth the trouble trying to diagnose it if it does start oscillating.
  5. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    Always. Especially if you work for a large company where the purchasing people think it clever to find "substitutes" for the manufacturer and part numbers you spent days selecting and years knowing what they are.

    They save a few cents per unit. You spend a weekend troubleshooting boards due to "off brand generic 7805" regulators that oscillate no matter how much capacitance you put on the output.

    Not my experience, but something I've read ("Tales from the Cube," perhaps.)