(click to enlarge)
I'm not going to count this in the April Maker challenge. I don't know why.
I hacked together a really hokey power supply for my tabletop foam cutter and as-yet-unbuilt acrylic bender (although strictly speaking, the portion of the foam cutter heating element that runs underneath the acrylic "table" has done a nice job of melting it). JTBarclay is using it for driving a 24V nigh-5W fan under a stir-plate, and Pat is building one for HIS foam cutter.
It does a fairly good job, although it has no check on the amount of current it draws from the unregulated DC supply, which is a MAJOR short coming. The IC is an 8-pin PIC that I've programmed with a PWM controller that reads a pot to determine the duty cycle (at least, in this configuration it does). Because of the way the design works, you can put a limit on the duty cycle by inserting a resistor between the potentiometer and the ground rail. Using a smaller potentiometer is wise if you were going to do that- you want to keep the overall resistance down. The larger the resistor, the lower the maximum duty cycle will be. If the resistor is equal to the value of the potentiometer, you'll be limited to a 50% duty cycle; if it is twice the pot's value, you'll be limited to 33%. The equation is Rpot / (Rpot + Rbias), where Rbias is the value of the added resistor.