What i meant is ive fixed voltages, and varied voltages, but 18 volts rectified then filtered is way above 12 volts. Something like 23 volts top end, i cant think of a way to close the gap and stay variable with the 18 volt transformer, then pushed up by capacitor filtering. lower the transformers secondary to 12 volts AC then the final out put will be closer to your 12 volts needed, about 16 volts, but will drop some under load to closer to 12 volts.
Dave.
Dave.