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7343Re: NSLU2 using 7805s and a 12V battery

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  • bullfrog528
    May 3, 2008
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      Hi

      Must admit I'm somewhat rusty on my electronics theory. Regulators
      are rated with suitable heatsinks fitted.They are very derated
      without heatsinks. Various problems here - the regulator has to
      disapate a 7V drop as well as the actual current drawn. There is a
      way to use a 78xx regulator with an external transistor to boost
      current.

      See page 22 of the datasheet below for a 7805 which shows a typical
      cct.

      http://www.datasheetcatalog.org/datasheets/228/390068_DS.pdf

      A large Watt rated resistor between the 12V and the input to the
      regulator may also help, but regulation might become an issue as the
      volt drop across the resistor would increase as current increased.

      Jeremy (Bullfrog528)




      --- In nslu2-general@yahoogroups.com, efil vasers <efilvasers@...>
      wrote:
      >
      >
      > I'm trying to run my NSLU2 (with a 1GB flash drive, connected to
      an old wifi
      > router) from a 12 volt SLA rechargeable battery (5Ah) using 7805
      voltage
      > regulators.
      > So far, I've tried a few different regulators (rated 1 or 1.5
      amps), and
      > when I turn the NSLU2 on, after about 30 seconds, the regulators
      are too hot
      > to touch and soon shut themselves down.
      > I'm not sure what the problem is.
      > Just to try, I connected it with a single 7805 (1 amp), which
      immediately
      > shut itself off.
      >
      > I connected the regulators in parallel with each other to provide
      up to 3
      > amps (more than the NSLU2 power supply), so I figured that I
      wouldn't need
      > heatsinks for them. Am I wrong, or is my NSLU2 just behaving
      strangely?
      > How many people out there have tried this?
      >
      > (For those wondering, this is for a robotics project for school,
      and the
      > NSLU2 is running Debian)
      > --
      > View this message in context: http://www.nabble.com/NSLU2-using-
      7805s-and-a-12V-battery-tp17030363p17030363.html
      > Sent from the Nslu2 - General mailing list archive at Nabble.com.
      >
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