For general purpose experiments I use deep cycle marine battery
chargers. They deliver about 1.5 volts up to about 11 volts at 11
amps. They have a 6 volt and a 12 volt switch. I don't know what is
going on but I can't get 12 volts. I use one of those heavy duty
rheostats to vary the voltage. These things ARE student proof. A dead
short will kick the reset after about 5 seconds. Check them out at
K-Mart.
If you are still using fuses in your power supply you need to
get-with-the-times and install an in-line breaker.