9v regulator 7809 off 12v

Discussion in 'General Electronics Chat' started by stoopkid, Sep 27, 2011.

  1. stoopkid

    Thread Starter Member

    Mar 3, 2011
    I'm working on a project that will run off arduino and also needs a 9v line(guitar pedal circuit). I have a 7805 for the atmega328 so I figured I can just use a 7809 for the pedal line. Additionally it seems most logical to run them both off a 12v wall wart. But all the 7809's I'm finding says primary input voltage of 15v. Is that a problem or can I still use 12v?

    Also whats the difference between Low Drop Out and Linear if they both claim a drop out of 2v?
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    A LM7809 datasheet shows a min input Vi of 11.5V with a dropout of 2V
    A low dropout version like the KA78R09C shows a dropout of .5V
    Not sure where you are getting your information from.
  3. debjit625

    Well-Known Member

    Apr 17, 2010
    Most 7809 will work with 12V input as they typically drop 2V.I dont know which 7809 you are talking about ,can you provide us the datasheet?
  4. SgtWookie


    Jul 17, 2007
    The dropout on the 78xx series regulators is ~2v at low current. It basically depends on your load as to how much that dropout will increase.

    If you are using an unregulated "wall-wart" type plug supply, you will likely find that it has a low value for the filter cap; you'll need to add more capacitance than the mandatory 0.33uF input caps to the regulators.

    Also, unregulated "wall wart" supplies output within roughly ~10% of the voltage at the rated current; at low current the output voltage will be a fair bit higher.