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Thread: Sproggy PS problem

  1. #1
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    Dec 2001
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    69

    Post Sproggy PS problem

    I have gotten the +12V and +5V lines woring perfectly. I cannot get the -12V line to work very well. I am using the exact same type of inductor as used in the 5V stage, I have tried the MBR745 and another generic schottky rectifier with the same results. I have rewired te circuit three times with the same results. What happens is that the output voltage changes with differint size capacitors. If I remove the large (470uF) cap the voltage just to -100 or greater and current draw becomes very large. I have been able to get it close to -12V but if i even touch the tab on the MBR745, the voltage goes haywire. I am guessing possibly the diodes? but why on earth would the voltages change with the size of capacitor?

    Very confused
    -Alex Channell

  2. #2
    Newbie
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    Jul 2001
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    Post

    well, one possibility is that the cap is in backwards, and is shorting out the supply, and different values put different loads on the circuit..

  3. #3
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    a backward cap won't short out.

    Id check to see if the tab on the MBR745 needed to be electricaly isolated or maybe grounded.
    As i understand things the large cap on the -12 line is thier to store power. The smaller cap is the one used to determine the voltage.
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  4. #4
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    The cap is not in backwards, and the diode is electrically isolated. It was suggested to me by Sproggy that it could be because I am using to large of an inductor (5.5 Amp continuous). Does this sound resonable? Any electrical engineers out there?

  5. #5
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    You probably have ringing on your diode. Use a big fat 10 gauge wire to connect the SMPS IC to the diode. This is the most critical connection, the others aren't as critical. If you have ringing it fudges the loop response, so who knows what you get. You could even make it oscillate and that's real bad. If you have a scope, poke around some. It doesn't sound like too large of an inductor. Inductor current isn't really a factor as long as the inductor holds it's expected inductance at the rated max current.

    Presslab

  6. #6
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    Dec 2001
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    I have tried directly soldering the leads to the chip to no avail. I don't believe this is the problem, although I don't have a scope to test it. If it means anything, the output is DC, no fluctuations(at least none that would make a frequency counter chow anything but 0). I plugged in the -5V chip to compare to the -12V chip and both output right at -4V. (there are slight differinces in output between differint chips of the same rated output voltage)

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