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Changing power supply voltage

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    #1

    Changing power supply voltage

    I have an Astec 18V power supply from an HP printer that I want to use on a scanner, but I need to lower the output to 12V.

    It uses a common 3842/3843 IC, which I have the datasheet for, but I'm still not clear on which components need to be changed to alter the output voltage without affecting the 2.5v feedback voltage. I don't have a schematic for the power supply, but I might be able to get some good photos of the top and bottom of the PCB.
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    #2
    Re: Changing power supply voltage

    the voltage spread is too wide to modify and have to remove one or two turn on transmformer and change sense resistors in the feedback circuit to match. Keep looking for another 12V or tap off from computer's 12V with homemade cable adapter (round plug on one end, molex on other end).

    Cheers, Wizard

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      #3
      Re: Changing power supply voltage

      ...or you could add on a 12V linear regulator (7812) to the output. Might get too hot though depending on the current the scanner takes.

      ... or ... string 10 diodes together, each dropping 0.6V to get a total of 6V drop




      ... or not.

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        #4
        Re: Changing power supply voltage

        Wizard is right but you can try if you want do experiment.

        If you change the resistor divider, probably there's one connected to a TL431 driving an optocoupler on the secondary side for voltage feedback, you reduce also the primary supply feeding the controller 3842/43, so the PS will work in hiccup mode unless you remove some turn on the secondary output.

        If you have some experience with electronic, you could built a post regulator using a step down converter.
        If the scanner need a low current, you can try with a common linear regulator , it is simpler but need a big heatsink and it waste more power.

        Ciao
        Gianni
        "In the confrontation between the stream and the rock, the stream always wins...Not through strength, but through persistence."
        H. J. Brown

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          #5
          Re: Changing power supply voltage

          Don't forget you could also use a switching regulator alot less heat. You would need an energy storage inductor however.
          Elements of the past and the future combining to make something not quite as good as either.

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            #6
            Re: Changing power supply voltage

            Well, I guess it's not really worth messing with, since a suitable adapter isn't too expensive.

            I also like the idea of running from the PC power supply. The scanner needs 1.25 amps, but the PC should have plenty of room on the +12V rail for that.

            It's just a visioneer 5800 USB scanner, but I wanted to see if it was better than my ancient HP Scanjet 4C SCSI scanner.

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