>
>Actually, what I'd like to see would be some smaller wall warts.  Although
>there have been a few that use HF transformers with optocoupled feedback
>(an oscillator on the "hot" side generates a signal which is fed through the
>transformer; the oscillator adjusts itself according to feedback about the
>regulated voltage) they've tended to be expensive.  What I'd like to see,
>though, would be a wall wart that skipped the feedback step and simply gave
>you pretty much unfiltered DC which was 18-24v peak.  The user application
>could then regulate the output in whatever manner was desired (7805 plus a
>resistor and a couple caps being a nice choice).  This should be cheaper
>than an optocoupled feedback circuit, but would allow the wall wart to fit
>in a much smaller plug than normal.
>
I reverse engineered the power supply that Igloo uses to power their
thermoelectric cooled ice chests and it is semi-regulated without
opto-couplers, etc. It is a self-oscillating switch-mode converter, similar
to that used in low voltage florescent lamps but is powered off the mains.
It has three secondary windings. One to provide about 13.5 volt, 5 amp
output another returns surplus power back to the filter capacitor accross
the rectified mains input, and another to provide the oscillation feedback.
The system has fairly good load regulation, but its output is affected by
the mains voltage, just like a wall wart. A small version of this would be
just what you need. It uses 2 transistors, a MOSFET for the
oscillator/driver and I think the small bipolar one is used to limit the
duty cycle by cutting off the feedback/drive signal when the current
exceeds a preset limit. This keeps it running in the continuous mode (core
not allowed to saturate) for better efficiency. The circuit is extremely
simple and could easily be made into a small, cheap wall "pimple". I figure
Igloo would not appreciate me publicizing their design on the piclist.

Regards,   Andy Morris