need solution for Ac to Dc converter

Hi,
First of all I dont have very much knowledge in electronics.

I want someone to find a good solution to my problem regrding recharging circuit of my bike.My bike alternator has three leads,one for headlight second one for recharging and third one for spark.Head lights are on AC so I used a 12V regulator for the headlight but when rpm increases headlight bulb got blown.I dont know how this happens even though i used a 12v regulator.Now in the recharging part I fixed a fullwave bridge rectifier circuit with its output to a clipper and then to a 7815 IC (custom made by my friend) to limit recharging to 15V. At normal rpm the output voltage is very much low for recharging (when headlight bulb is connected to it its not getting lighted up,only a small red/orange color in the filament) and when i increased the rpm multimeter read 15v then suddenly it showed 18v(just for second) and returned to 15V.

At first I had only bridge rectifier part dint had that 7815 IC,at that time multimeter read about 19-24V during increased rpm and 12v in normal rpm and may be due to that increased voltage battery I used earlier is now damaged.

So I need someone to clear my doubt whether this circuit is sufficient or circuit is not a proper one and also is there any rectifier regulator circutry which gives a constant DC output for recharging the battery at normal rpm.