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7908 voltage regulator

Discussion in 'General Electronics Discussion' started by juh, Apr 25, 2013.

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  1. juh

    juh

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    Feb 21, 2013
    I have newly purchased 7908 voltage regulators. I want to test them first before installing them to the circuit. I supplied -14V dc, 1A. I should expect an output of -8V right? but instead, i keep getting an output of 13V. what could be the problem? thank you!
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Place a small load on the output, say a 1k resistor.
     
  3. juh

    juh

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    Feb 21, 2013
    nothing changed. i still got 13V.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Show is the circuit diagram, and maybe a photo of how you've built it.

    Do you have an input and output capacitor?
     
  5. davenn

    davenn Moderator

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    Sep 5, 2009
    You have probably wired it incorrectly, Did you check the device pinouts with the datasheet ?
    as Steve said we need to see a photo of your construction to confirm that

    Dave
     
  6. juh

    juh

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    Feb 21, 2013
  7. davenn

    davenn Moderator

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    Sep 5, 2009
    you did realise/notice that the pin connections are different for the 78xx compared to the 79xx regulators ??

    Dave
     
  8. juh

    juh

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    Feb 21, 2013
    yes. the middle pin would be the input for the 79xx regulator.
     
  9. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Do you have the capacitors in place as described in the datasheet?
    Without these caps the regulator may show all sorts of strange behavior.
     
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