Voltage regulator help needed

Discussion in 'General Electronics Chat' started by peck68, May 1, 2010.

  1. peck68

    Thread Starter Member

    Nov 27, 2009
    Hi, today i bought a voltage regulator to have a constant 5v (ish) current to my pics - however all of the fixed ones were out of stock :)mad:) instead I had to opt for the L7815CV (which i have never heard of)...

    Apparently it supports output voltages of 5 to 24V, yet i don't even understand how to use it - I have read the datasheet over and over again and i cant figure out how to do it :(

    I get the multimeter out and it reads 7.7v - which is a 1V drop from my 8.7v PP3


    By the way i am wiring it up like this, sorry it looks a little silly lol
    Code ( (Unknown Language)):
    2.  ______
    3. |_____|
    4. |  |  |
    5. in g  out
    7. g as in gnd

    By the way, at no point have I come across the word "variable" regulator yet - so i am just assuming that it is some special thing
  2. Ghar

    Active Member

    Mar 8, 2010
    The 78xx series has xx as the output voltage.
    7805 is 5 volts, 7815 is 15 volts.

    The 78xx series has output voltages from 5 to 24 V, as in there exist parts 7805 through 7824.

    They're all fixed regulators.
    You need about 2V higher supply than output for these guys to regulate, so this thing won't do anything special for you unless your source is 17 V or higher.
  3. bertus


    Apr 5, 2008

    The 7815 is a 15 volts regulator.
    The 78XX regulators need a voltage drop of minimum 2 Volts.
    Also the capacitors at in and output must be mounted to prevent the regulator from oscillating.
    There are "variable" regulators on the form of an LM317.
    (see attached datasheets).

  4. peck68

    Thread Starter Member

    Nov 27, 2009
    Ahh <snip>, i think that must be why its not working :p

    I will get a stock notice ;) Thanks for the help
    Last edited by a moderator: May 1, 2010
  5. SgtWookie


    Jul 17, 2007
    You can use an LM317 instead, with a couple of fixed resistors on the output.

    Use a 100 Ohm from OUT to ADJ, and 300 Ohms from ADJ to GND for 5v out. I used those values because they're really easy to get, and will provide guaranteed regulation on the output even with no load.

    Use caps on the input and output. 0.1uF is minimum.
  6. Audioguru


    Dec 20, 2007
    You should use a "low dropout' regulator with a little 9V battery that quickly drops to only 6V. An ordinary regulator does not regulate when the battery drops to 7.5V and less.