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Question about 78xx AND 79xx regulators in TO-220 cases

Discussion in 'Electronic Basics' started by PatrickM, Nov 20, 2004.

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  1. PatrickM

    PatrickM Guest

    I'm curious about a little something. Why do positive voltage regulators
    (like 7815) have their case connected to GND and negative ones (7915) have
    it connected to Input voltage (Vin)? Seems to me it would be more logical
    and practical if both had it grounded. Can someone enlighten me on the
    reason for this? TIA
  2. John Larkin

    John Larkin Guest

    The silicon chip is soldered to the can for best heat transfer, and in
    most linear IC processes the silicon substrate has to be the most
    negative potential to allow using back-biased junctions for isolation.

    There are a few exceptions, like LM317s I think.

  3. Dan Dunphy

    Dan Dunphy Guest

    Typically the substrate of the IC is eutectically, bonded to the case
    or heat sink, in case of a TO-220. The substrate, in a typical NPN
    process must the be the most negative point on the device. In the 78
    series, gnd is the most negative, in the normal configuration, and
    V-in in the 79 series.
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