Power panels do not convert the 12 volts to 1.2 volts. It works by pulse width modulation. It put a 12 volt pulse onto the plug for approximately 1 millisecond, turns it off for 9 milliseconds, back on for 1 millisecond etc. etc. for an average voltage of 1.2 volts. When you turn that pot on the front panel you simply increase or decrease that 1 millisecond pulse a short time to increase the average voltage. If you have a friend with an oscilloscope, he can show you the waveform to verify this. Using a resistor to drop the voltage would be extreemly wasteful of energy and quickly discharge your 12 volt battery. You can find the circuity to build you own on the web at several sources.