A discussion was originally started in this thread: http://www.explorerforum.com/forums/showthread.php?t=216990 about various methods of controlling hydrogen generators. Most run on plain DC while some use PWM (pulse width modulation) to run a step up transformer. This would have an AC output unless a voltage rectifier is used. Some other methods include inverters, but keep in mind that current is very important. Current drops when voltage is increased.

This website has some schematic diagrams, and block diagrams for various gadgets: WWW.Better-Mileage.Com.