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Need a little advice for a LM317T voltage regulator circuit

Discussion in 'Electronic Basics' started by Neil Brown, Jul 16, 2004.

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  1. Neil Brown

    Neil Brown Guest

    I'm looking to build a voltage regulator which will allow me to
    power/charge my palm from a battery pack or a car cigarette lighter.
    I'm thinking about using the "LM317T Variable Voltage Regulator"
    circuit from Bill Bowden's webpage:

    http://ourworld.compuserve.com/homepages/Bill_Bowden/page12.htm

    ( first one on the page )

    My device requires 5v 1100mA, and for size reasons I'd like to keep my
    battery pack option to 4 AA cells if possible. Bill recommends on his
    page that the input voltage be 3V above the output voltage though, so
    my question is this: Can I just throw a voltage doubler on the input
    side or is there a better way to tackle this?

    adv( thanks )ance

    -o-
    Neil
     
  2. Rok Sitar

    Rok Sitar Guest

    Putting voltage doubler on input is a great idea but you have to be careful
    that it can handle current. Your device has to have 1A at 5V. Tell me why
    don't you use the lm7805 regulator that can provide what you need. It can
    handle 2.2 A peak current.
     
  3. Byron A Jeff

    Byron A Jeff Guest

    -I'm looking to build a voltage regulator which will allow me to
    -power/charge my palm from a battery pack or a car cigarette lighter.
    -I'm thinking about using the "LM317T Variable Voltage Regulator"
    -circuit from Bill Bowden's webpage:
    -
    -http://ourworld.compuserve.com/homepages/Bill_Bowden/page12.htm

    Standard issue. Not real good for this application.


    -
    -( first one on the page )
    -
    -My device requires 5v 1100mA, and for size reasons I'd like to keep my
    -battery pack option to 4 AA cells if possible.

    It won't last long. AA batteries have between 1000 and 3000 mAH worth of
    juice, and that's at a 20 hour discharge rate. You be lucky to get 20 minutes
    discharging at that rate with a top range battery.

    - Bill recommends on his
    -page that the input voltage be 3V above the output voltage though, so
    -my question is this:

    As I said not real good for this application. The voltage doubler will both
    consume power and will cause the LM317 to burn even more power that it was
    before, draining your batteries even faster than the 10 minute estimate that
    I gave above.

    -Can I just throw a voltage doubler on the input
    -side or is there a better way to tackle this?

    A couple of ideas. But be aware that it'll be quite a bit more money and more
    effort.

    For the battery consider upgrading to lithium polymer. Light, compact, and
    allows you to get most of the charge out even at extremely high current draws
    (which BTW 1.1A is minimal. 12A typical). For example consider this
    LifeFlightRC pack here:

    http://www.lightflightrc.com/HTML/products/2400-2S2P-E.htm

    1.3x2.4x1 in in size, and 102 g in weight is similar to 4 AA's. But the higher
    7.4V voltage, 2400mAH capacity, and the ability to draw nearly all of the power
    from the pack makes it ideal for this application. According to their charts
    the run time is pretty much linear based on current draw with 24 minutes of
    run time at a 6A discharge. So to extrapolate that to a 1A current draw would
    estimate 144 minutes of run time, over two hours. A whole heap better than
    the 20 minutes or so with standard AAs. BTW since the battery voltage is higher
    the current draw will even be less than an amp. An off the cuff estimate of
    full power draw at 5V would have an input current draw of about 800mA @ 7V
    on the battery pack.

    Now for the regulator. Linear regulators will gobble up much of your power.
    You really want a high efficiency switcher that delivers nearly all of the
    power to the load. A simple switcher such as a NatSemi LM2596, along with a
    couple of components, gives you a compact, cool, efficient regulator for
    your project.

    You'll end up spending about $60 for the job. But it's a job that will actually
    get done with better components.

    Hope this helps,

    BAJ
     
  4. Byron A Jeff

    Byron A Jeff Guest

    -Putting voltage doubler on input is a great idea but you have to be careful
    -that it can handle current. Your device has to have 1A at 5V. Tell me why
    -don't you use the lm7805 regulator that can provide what you need. It can
    -handle 2.2 A peak current.

    Because with 2.5V of headroom, you're pretty much guaranteed to burn up
    about 4 percent of your power budget on the regulator?

    BAJ
    -
    --> I'm looking to build a voltage regulator which will allow me to
    -> power/charge my palm from a battery pack or a car cigarette lighter.
    -> I'm thinking about using the "LM317T Variable Voltage Regulator"
    -> circuit from Bill Bowden's webpage:
    ->
    -> http://ourworld.compuserve.com/homepages/Bill_Bowden/page12.htm
    ->
    -> ( first one on the page )
    ->
    -> My device requires 5v 1100mA, and for size reasons I'd like to keep my
    -> battery pack option to 4 AA cells if possible. Bill recommends on his
    -> page that the input voltage be 3V above the output voltage though, so
    -> my question is this: Can I just throw a voltage doubler on the input
    -> side or is there a better way to tackle this?
    ->
    -> adv( thanks )ance
    ->
    -> -o-
    -> Neil
    -
    -
     
  5. Neil Brown

    Neil Brown Guest

    Thanks a bunch for the response!
    20 minutes for a single AA I'm guessing? Hmm... The minimum 1100mA
    draw is an upper limit when recharging the internal battery of the
    unit and using the unit at the same time. With 4 AAs I guess I
    wouldn't be able to do both - maybe just give it a charge.
    My primary goal here is to be able to use the unit for longer than the
    internal battery allows. I want to stick with AAs since they're small
    enough to not be bulky, and I can go to WalMart and buy some more if
    I'm in a bind. Even just one extra charge would put my usable time up
    to 5-6 hours, which is probably longer than I want to spend staring at
    that tiny screen anyways ;) How about going to 8 AA cells and the
    LM2596? That would put my input voltage at 12V and double the
    discharge time.

    I had a look at the spec sheet and the example circuit they have looks
    pretty easy:

    http://cache.national.com/ds/LM/LM2596.pdf

    Any idea what the LM2596 does when the input voltage begins to drop
    below output?

    -o-
    Neil
     
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