My Brother-in-law runs an excavator, and expressed a desire to be able to plug his cell phone charger into the excavator's electrical system. Unfortunately the charger is 12V, while the electrical system on the excavator is 24V.
No problem I figured, it shouldn't take too long to cobble together a simple linear regulator, and slap it into a case with a cord on each end.
As it turned out, this task was much more difficult than I expected. The problem, which non-electrical types don't tend to appreciate, is that vehicle electrical systems are very dirty things. According to MIL-STD-1275, one must regularly expect to deal with 300V spikes in a 24V system!
No pre-packaged regulators were available (that I could find, in any case) that could handle anything approaching this voltage, so I had to build the circuit up from discrete components. Then things got complicated from the heat point of view. The system needed a heat-sink, strain relief on the cables, and an armoured case.
Skipping a large number of dead ends, here's the finished circuit.
A couple of neat things about this circuit:
All in all, I'm pleased with it. Most of the credit for this circuit design must go to my little bro Jurg, who does analog electronics for a living, and came up with the core of the circuit in 20 minutes (after I'd beat my head against the problem for a couple of days).
For a case I used a piece of 3/4" copper pipe, with caps soldered onto each end. Holes drilled through the caps acted as the strain relief on the cables (with a couple of knots). A small flattenned piece of copper plate soldered across the bottom of the pipe provided the mount for the FET. I'll try to put up a picture or drawing at some point.
Now all there remains to do is to carve some ratings and my name into the side of the case.