Acceptable voltage drop?

Discussion in 'General Electronics Chat' started by tracecom, Feb 23, 2011.

  1. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
    I put a LM7812CT regulator circuit on the output of a wall wart that was labeled 12 vdc at 1000 ma. (Without the regulator, the actual output measured over 15 vdc under a 184 ma load; I never loaded it any heavier.) With the regulator, I get the following readings:

    no load: 12.03 vdc
    250 ma: 11.96 vdc
    333 ma: 11.95 vdc
    500 ma: 11.90 vdc
    1000 ma: 10.82 vdc

    Obviously, it's not capable of a full amp, but is it ok to use continuously at 500 ma or does the 1/10 vdc sag indicate something's overloaded?

  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    Changes of a few hundreths of a volt are acceptable. Using the reg. @ 500mA is very doable.

    If no heatsink is supplied, long duration use may see the voltage sag even at only 500mA loading.
  3. SgtWookie


    Jul 17, 2007
    Check the voltage across the regulator (in vs out) when you have a 500mA load. You'll probably see somewhere around 2v-2.2v.

    The 500mA is certainly do-able. Calculate your power consumed in your load vs the regulator. Make sure your heat sink will dissipate that power.

  4. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    The dropout voltage for an LM7812 is 2V. More than 14V, across the load range, is required from the wall wart for the 7812 to maintain regulation. Check the output voltage of the wall wart at 500mA (or 1A).