John said:
I'm building a bicycle light and I've decided to go with the 7.2V
battery pack (for RC) as you suggested. I'm using is 16W xenon bulb. How
can I change the intensity? I assume I have to reduce the voltage?
Yes, but what you don't want to do is just drop the voltage through
something that'll just turn it into heat like a resistor. Instead, what is
often done is to send a square wave with an adjustable duty cycle to a light
bulb. The lower the duty cycle, the lower the RMS value of the voltage the
bulb sees and the less power the bulb will use. This is readily done using
a 555 timer IC and a MOSFET as the switch. I'd suggest running the 555 at
somewhere between 100Hz and some kilohertz, nominally -- lower values can
start to allow you to _see_ the switching action (this is based on the
thermal inertia of the filament -- how quickly it cools down), whereas
higher values will make the switching action slightly less efficient
(although until you get to megahertz, this is probably pretty negligible).
Note that your 16W bulb is going to become a lot less efficient at 5W,
because most of the filament's thermal energy will be emitted as infrared
radiation that isn't going to help you, as a human, one iota. (You might
make the comparison between a single 2 'C' bike light, which often use 2.4W
bulbs, and you setup running at 2.4W -- you'll find that your light will be
much redder and not as good. There isn't much of a way around this problem
with blackbody radiation sources -- if you get ambitious someday, you can
try high intensity discharge bulbs or LEDs which don't exhibit this
behavior.)
What I've described here is on open loop system in that, if the battery
voltage drops a little, the bulb's power consumption will change a little as
well. NiCads have a pretty flat discharge curve so this won't be a problem,
but if you actually want _calibrated_ wattages regardless of battery input
voltage, you'll need a rather fancier design with (most easily) something
like a switching regulator controller IC.
---Joel Kolstad