Question About 78xx Series Voltage Regulators

Discussion in 'General Electronics Chat' started by K-Young, Dec 8, 2010.

  1. K-Young

    Thread Starter Member

    Aug 22, 2009
    27
    1
    Alright I've been playing around with a LM7812 and built the following circuit:

    http://img202.imageshack.us/img202/6627/circuitg.jpg

    Now everything seems to work, but I have no idea how to use the formula provided in the datasheet. I'm looking at the following datasheet:

    http://www.fairchildsemi.com/ds/LM/LM7805.pdf


    On page 24 there is an exact circuit, minus the 1K resistor that seems to have been forgotten or not included. Either way, there is very little difference in output voltage when I remove the 1K resistor. Now on page 23 of the data sheet the formula is explained. I don't know what VXX
    is supposed to mean & is R2 the potentiometer?

    When the pot is turned to zero I'm getting about 13.9V, which makes sense according to the datasheet. At 5K I'm getting about 16.7V and at 10K I'm getting 15.1V.

    I'm trying to figure out what's going on, but I'm so lost...:confused:
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,635
    2,342
    Hello,

    The difference between the schematic on the site and in the datasheet is the 1 K resistor as you say.
    There is a current in the ground-pin of the LM78XX (IQ).
    In the schematic in the datasheet all the current must be absorbed by the opamp.
    In the schematic from the site part of the current goes into the 1 K resistor and the rest goes into the opamp.

    The VXX in the schematic is the basic voltage of the 78XX.
    An 7805 has a VXX od 5 Volts.
    An 7812 has a VXX of 12 Volts.

    Bertus
     
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