I'm doing an op-amp project with a textbook ±17V supply and was wondering how you guys would recommend BEST, most-efficiently (in terms of cost, component count and energy wasted) doing an additional 5V supply for logic without adding another transformer (I've already got a toroidal guy with two 18V secondaries lying around that I'm going to use). I could obviously grab the post-bridge-rectified, pre-regulated positive rail and just dump it into a 7805, but I hate wasting that much voltage and generating that much heat. Is it stupid to do a resistive divider to step it down and then regulate? Or a zener to bump it down to 9-ish volts and then regulate? Or how 'bout this absurd one: grab one leg of one of the 18V secondaries, half-wave rectify it with a beefy power diode and huge reservoir cap then regulate? That's probably a really dumb idea.