Hi, I started a project a few months ago to replace the voltage regulator on my car's alternator but I had to stop due to lack of knowledge and testing ideas. I've since learned alot more about electronics and have decided to pick it up again. Straight away I've made a design change after reading some of the sections in The Art of Electronics by Horowitz & Hill. Hard going but a good book! Here is my suggested circuit. The left side shows feedback from the stator. This goes through a low pass filter with the 3db point at around 1.75Hz and a charge time to full of 0.5s (time constant of 0.1s). This then overcomes the zener if too high and opens the transistor collector/emitter path and dumps current to earth through a low resistor R3. Normally the rotor coils would be being charged at the same current the existing regulator consumes at idle speed, which is settable by changing the value of R4. I propose to make R1 and R2 very high maybe 100k plus to prevent circuit loading and to keep the signal good. I've arrived at some theoretical values for all components involved I just have one more thing to do before acquiring a second hand alternator and testing this out. So my question is can anyone see any major problems or design flaws with it before I go to testing stage? http://img486.imageshack.us/img486/3997/circuitca2.gif I'd appreciate any feedback offered thanks *Edit* One thing I have thought of is that when the transistor is fully on I should arrange for the parallel resistance of R4 and R3 to be the same as just R4 in series.