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7349Re: NSLU2 using 7805s and a 12V battery

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  • oabroad
    May 4, 2008
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      One option worth considering is a switching regulator. These have the
      huge advantage that they are efficient so don't create all that waste
      heat and could halve the current drain from the battery.

      If you can get hold of one you could do worse than get the Texas
      instruments TPS5430EVM-173 evaluation module, I've seen it priced at
      £7.99 and is supplied configured as a 5v 3A regulator. Failing that
      I've got a design for a 5V 1A switch reg that can be built on
      stripboard for a BOM cost of about £2.50 last time I costed it.


      --- In nslu2-general@yahoogroups.com, "efilvasers" <efilvasers@...>
      wrote:
      >
      > Thanks for all your responses.
      >
      > I'm using a 12V battery because it's what I happened to have on
      hand.
      > The NSLU2, three Arduinos, and a few various ICs and LEDs need the 5
      > volts, while the wifi router needs 12, and all the motors use 9 (the
      > actual robot itself is made with LEGO).
      >
      > As for solving this problem, I googled around a bit and found a
      > solution for connecting 78xx's in parallel; it involves diodes on
      the
      > outputs to isolate them, and one between their ground pins and the
      > real ground to cancel out the voltage drop.
      >
      > The other solution I am considering is from the datasheet that
      > bullfrog528 linked me to; on page 23, adding a PNP power transistor
      > and 3 ohm resistor.
      >
      > I'll try both of these later tonight once I get myself to
      RadioShack.
      > I neglected to mention before that this project is already about one
      > day late and I don't have the time to wait for orders to ship.
      >
      > --- In nslu2-general@yahoogroups.com, "bullfrog528" <yahoo@> wrote:
      > >
      > > Is there any particular reason your trying to run this off a
      12Volt
      > > battery?
      > >
      > > If you need 12V to run the rest of the project.You could have two
      > > 6Volt batteries in series use the the 12V to run whatever it is
      and
      > > tap 6V out from the link between the two and regulate that down
      to
      > > 5Volts. The lower battery will discharge quicker mind.
      > >
      > >
      > > |------ 12v ---> to rest of project
      > > []6volt batt
      > > |------6V ----> to 5volt reg and onto nslu2
      > > []6volt batt
      > > |______ov
      > >
      > >
      > > As they say there is always more than one way to string a cat.
      > >
      > > Bullfrog
      > >
      > > --- In nslu2-general@yahoogroups.com, "bullfrog528" <yahoo@>
      > > wrote:
      > > >
      > > > 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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