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Increasing output of fixed V-Reg

Discussion in 'General Electronics Discussion' started by davenn, Jan 29, 2013.

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  1. davenn

    davenn Moderator

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    Sep 5, 2009
    hey Gang

    need to increase the output of a +12V reg (7812) to ~ 13V
    Is there any real hassle just using a resistor in the GND lead to the 0V rail ... ie. lifting that pin above 0V ?
    rather than the more common arrangement of 2 resistors... one from output to GND pin and another from GND pin to 0V rail

    have seen both arrangements in various www sites
    It seems to work well with just the 1 resistor... a 220 Ohm gives me 13.2V output

    Dave
     
  2. duke37

    duke37

    5,364
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    Jan 9, 2011
    The current in the tail of the regulator is not defined so a resistor can not be specified. A voltage divider with a larger current than in the tail can be set better.

    Your example indicates about 5mA tail curent. I have found a similar value with a 7805 regulator.

    A couple of forward biased diodes will give about 1.2V extra.

    The LM317T has a very low tail current and so a resistor will not work.
     
  3. davenn

    davenn Moderator

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    Sep 5, 2009
    hey Duke

    thanks for responding :)

    ummm how / where are you connecting those diodes ?

    Dave
     
  4. duke37

    duke37

    5,364
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    Jan 9, 2011
    The diodes should be connected in the lead which goes to ground and should be forward biassed. Each diode will drop about 0.6V. If you need more voltage than this, you may be able to find a suitable Zener diode.

    I recently wanted more than 5V to drive a transistor radio and used a red led with a 7805. This had the advantage of being pretty.
     
  5. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    Dave, I've been inserting a diode or two in the GND lead of fixed regulators (as Duke described ) for years with no problems.

    Chris
     
  6. davenn

    davenn Moderator

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    Sep 5, 2009
    OK :)

    diode arrow pointing towards GND ( 0V rail) or towards the reg GND pin ?

    Im assuming towards GND 0V rail

    Dave
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Yeah. It uses the forward voltage drop to jack up the output voltage.
     
  8. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    May 8, 2012
    Yes, like that. Cathode connected to circuit GND and the Anode connected to regulator GND pin.

    Chris
     
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