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  • Another Sproggy Schematic Question

    I'm still learning, so please bear with me...

    I was wondering if someone could explain why the design of the Sproggy didn't utilize Maxim's MAX775 for -12V and MAX774 for -5V. Maxim is infinitely easier to get samples for and the sample circuits in their datasheets don't appear to require the flyback transformers that are needed for the Sproggy (and difficult/expensive for most to get).

    The InMotion power supply design looked like it used the MAX775 for -12V. I didn't find any reference to a similar discussion in the search results. Am I missing something obvious?

    Thanks,
    Brian

  • #2
    the -12v comes from the lm2857 which also provides the 12v. Most boards don't use much -12v current, so the extra power and space of the MAX775 is wasted. Since there is so little current used by the -12v, most is still available to the 12v line. unfortunately no one can come up with a high currently 12v line that will accept a wide input voltage. So we are pretty much stuck with the lm2587 for 12v so why would we use 2 circuits when you can get the same amount of power off of 1.

    Comment


    • #3
      The LM2587 supports both of the negative voltages, but the 2 Maxim chips would take up less room than the space needed by the LM2587 and its required flyback transformer. Additionally, the Maxim chips could both be soldered vertically against the heatsink, unlike the flyback transformer.

      Taking that into account, it would seem the Maxim chips would be a better solution. I'm more concerned that there's a fundamental flaw in using those particular chips that I'm missing (i.e. not enough current).

      Brian

      Comment


      • #4
        Either way will work fine, just because there is more tahn one way to skin a cat, it doesn't mean one is right and the other is wrong. Both have there plus and minus points as you and Necare have pointed out.

        The LM7812 chip which the Inmotion design uses for the 12v only supplies 1amp, which may not be enought current for some setups (depending on your setup) however sprooggy get around this problem with the LM2587 which provides both +12v & -12v, and provides a higher current.

        Another chip with will do the job is LM1084, see this thread for more info.

        A quick search of National website www.national.com will provide you with a variety of voltage regulator chips, all of which have there own properties. And also look at this page on Nationals site for info on the LM1084.

        Rob

        PS check out my site for a bit more PSU design info www.mp3mini.tk
        And you say people actually pay money for M$ Windows?
        www.mp3mini.co.uk (Does what it says on the URL) www.openclassic.co.uk (The new car, with zero rust!) www.rob-web.co.uk (My other site)
        Total re-design underway: on the whole progress is very slow as the car is taking up too much time :)

        Comment


        • #5
          Hate to say it Rob, but I think the diagram on your website for "A simplified voltage regulator" is broken. The op-amp has it's - input tied to ground, and the + input goes to a voltage reference. With the high gain of the op-amp, the output will immediately slew to the positive rail.

          I think the negative input should feed into the middle of the feedback resistor network (not ground), and the positive input should be connected to the voltage reference only (not the feedback resistors).


          Regards

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

          Comment


          • #6
            Rob,

            Your 100% correct, that was probably one of the diagrams I drew late at night, fixed it now.

            Cheers

            Rob
            And you say people actually pay money for M$ Windows?
            www.mp3mini.co.uk (Does what it says on the URL) www.openclassic.co.uk (The new car, with zero rust!) www.rob-web.co.uk (My other site)
            Total re-design underway: on the whole progress is very slow as the car is taking up too much time :)

            Comment


            • #7
              Originally posted by Rob
              just because there is more than one way to skin a cat
              you mean a kitten, right?
              -
              My cars
              -

              Comment

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