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Thread: Would this schematic work?

  1. #1
    Newbie dsoares's Avatar
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    Would this schematic work?

    Would this schematic work?



    Post your comments, please....


    Thanks,
    Diego Soares

  2. #2
    Low Bitrate
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    I remember reading somewhere the following:


    I think it will not offer any current limiting/ protection. Also the heat dissipation by the transistor would be great.

    A similar idea was to use just a 7805 instead of a switcher. The current through it will be limited to around 1A and the rest would be supplied by the bypass transistor.

    You could add a crowbar for protection.

  3. #3
    Newbie dsoares's Avatar
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    Originally posted by abbas
    I remember reading somewhere the following:


    I think it will not offer any current limiting/ protection. Also the heat dissipation by the transistor would be great.

    A similar idea was to use just a 7805 instead of a switcher. The current through it will be limited to around 1A and the rest would be supplied by the bypass transistor.

    You could add a crowbar for protection.
    Well... you're right.
    MAX787 can supply 5A and I'll be using only 1A.

    OK!

    But, what do you say about this schematic:



    I know the heat dissipation in the transistor would be great too, but, in this way, we could have a more powerfull +12V PSU.

    I don't know what will happen when cranking because the collector voltage will be minor than the base voltage.
    Diego Soares

  4. #4
    FLAC
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    Originally posted by dsoares
    Well... you're right.
    MAX787 can supply 5A and I'll be using only 1A.

    OK!

    This won't work at all.
    MPEGBOX - Plexiglass Computer
    www.mpegbox.com

  5. #5
    Newbie dsoares's Avatar
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    Originally posted by zootjeff
    This won't work at all.
    Why?
    Diego Soares

  6. #6
    Raw Wave Rob Withey's Avatar
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    Originally posted by dsoares
    Why?
    I'm not quite sure what you are trying to achieve, but you appear to have bolted a linear pass transistor onto the backend of a switcher. There is a reason that people don't use linear regulators for the 12v section of their power supply. That transistor requires 0.6v headroom. It introduces a series resistance (the Vbe sense resistor) into the only section of the power supply that is capable of supplying 12v when the input voltage drops.


    Rob
    Old Systems retired due to new car
    New system at design/prototype stage on BeagleBoard.

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