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7356Re: NSLU2 using 7805s and a 12V battery

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  • oabroad
    May 6 9:58 PM
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      I just remembered a design trick for boosting a regulator: one
      circuit I've seen used a PNP power transistor with the base
      connected to the INPUT of the regulator (with a bypass resistor,
      probably 22ohms). By putting the junction on the input side the
      output regulation isn't compromised by Vbe drop.

      --- In nslu2-general@yahoogroups.com, "efilvasers" <efilvasers@...>
      wrote:
      >
      > First off, I'd like to thank you all again for your help.
      > I should have done a bit more research before jumping head first
      into
      > this project.
      > Next time around, I'm definitely going with a switching regulator.
      > My quick solution is this:
      > A 2N3055, bolted to an old CPU heatsink (which still had some
      thermal
      > compound on it), with the base connected to a 7805 (with a small
      heat
      > sink), collector to the battery, and emitter to the NSLU2 (and
      other
      > electronics).
      > Also, the ground pin of the 7805 is connected to the real ground
      > through a 1A diode; when I measured the emitter voltage without
      this,
      > it read about 4.4 volts, went up to about 5.2 after adding it (runs
      > about 4.9 with the NSLU2 on).
      > The 7805 heatsink doesn't even warm up, while the 2N3055 heatsink
      > warms up after a couple of minutes, but went back down to near room
      > temperature when I placed an extra 80mm fan next to it.
      > I'm using the same solution for the 9 volt regulator (both sharing
      the
      > same fan).
      >
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