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Thread: Operating a LM7812 regulator below minimum input voltage

  1. #1
    Senior Member HID Learner etboost's Avatar
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    Operating a LM7812 regulator below minimum input voltage

    i got question about LM7812
    whats min input voltage? If min voltage below required, it won't regulate output? need clarifications
    thanx...

  2. #2
    Senior Member HID MASTER azdave's Avatar
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    Regulation mins

    I edited the thread title so others could find it easieer in a future search.

    Quote Originally Posted by etboost
    i got question about LM7812
    whats min input voltage? If min voltage below required, it won't regulate output? need clarifications
    thanx...
    It will fully regulate and hold 12.0 volts output when you have about 13.5 volts or greater input.

    When the input is below 13.5 volts the output will be lower by about 1.5 volts. Example put 12.5 volts in and you get 11.0 volts out. 11 volts in 9.5 volts out. This is commonly called "head room".

    With a low-loss regulator (like the Sharp PQ12RD21) you put 12.5 volts in and you get 12.0 volts out. It only needs 0.5 volts headroom to fully regulate.

    Read my first post in this thread.
    http://www.hidplanet.com/forums/viewtopic.php?t=48086

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    Senior Member HID Learner etboost's Avatar
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    ok, thanx Dave...

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