-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