Here is a simple adjustable brightness flashlight circuit. This is the main thing I want depletion mosfets for!
Apart from being simple, the circuit has a few practical features.
* Continuous brightness adjustment flashlights are quite rare.
* Flicker-free. Important for video/photography.
* This circuit allows to adjust the brightness in extremely wide dynamic range, from totally off to barely visible in complete darkness to full power the LED can handle.
* The input current required by mosfet gate is negligible, which makes it possible to put very large resistance potentiometer. Dodgy viper contact isn't going to cause any issues.
* There is no need for on/off switch - the adjustment pot does it. The off current can be made as low as desirable by increasing potentiometer resistance, and can possibly be even made negligible compared to self-discharge due to cell protection circuitry.
The main disadvantage of this circuit is that it's relatively inefficient, although I think this is easily compensated for by lowering brightness as needed.
more potential applications:
low or negative voltage drop voltage follower
-> LED brightness control (as just shown)
-> LDO regulators
-> amplifier output stage (rail-to-rail with no bootstrap)
inrush current limiter