I'm using a $35 (plus tax and the obligatory 'do you want to purchase that on your frys card?') Velleman: PSSMV7V-5/25VDC, Switching Power Supply I bought from Frys, a $2 100k audio taper pot and two pigtails. A more advanced version would use a 2 pin wirewrap socket or similar, combined with a panel mount display. The voltage resistance relationship is roughly 55kΩ/(v - 5V) as can be seen by the following graph:
Most likely the powersupply feedback loop looks something like this (the reason for the choice of 2.5V will become clear):
(Though I'm not exactly sure, it might be that the feedback loop includes the 2.5V zener?) I'm using the log taper pot as a poor mans approximation of 1/x taper (and because I had one). It might be worth opening the box up and replacing both sides of the feedback with a linear potentiometer, though perhaps log scale makes sense for voltage dials too?
ACSpike suggested measuring the voltage across the pot to confirm this circuit. We see a steady 2.5V rather than the 5V seen on the output at infinite resistance. This suggests that in fact the circuit is more likely to be this:
The power capacity is a respectable 4A @ 5V (20W) up to 1.5A at 24V (36W) and appears to track smoothly. The highest voltage I've measured is 48V, I'm not sure whether this is out of the design range of the power supply - output capacitors might have a working voltage of only 30V, the best way to find out would be to look at the circuits. Next I think I'll take it apart and see how to add current limiting. Really these devices can be considered a low cost 35W high efficiency op amp directly driven from the mains. I wonder what other interesting circuits we might make (power amp? servo control?). Perhaps a down to 0V supply could be constructed using a 5V off-the-shelf supply.