> Use an ordinary 7805 - it makes a regulated 5 volts, and if you put a
> constant load across that, such as a lamp, then the regulator looks to
> the outside world like a constant current regulator.  Great for battery
> charging!


I used this with a resistive load, to test power packs from Taiwan. Their
spec says so much voltage at so much current, so a constant current load and
a voltmeter made for a great tester.

I wouldn't reccomend a lamp as a load though, their resistance is
non-linear, and highly temperature dependent, and has long term drift as the
lamp ages.. A resistor is utterly predictable.

There are some fun tricks you can do with three terminal regulators that
aren't in any books.  Think about how the reg works. The output V is
constant into a varying load, so the reg has to present a variable impedance
to the supply. Your current waveform is therefore present on the input, but
the input voltage waveform (if any) is not present on the output.  This also
underscores the need for a good input cap, if you have anything else that
feeds from the same unreg source.

Constant current sources are great fun :)