最简单
用一个7812三端稳压IC
http://www.elecfans.com/article/88/131/138/2008/200801127170.html
串联型直铅知流稳压电源设计图
http://hitjpkc.hit.edu.cn/elec/JS/js08/mold/moni/CHAP15/15-2/1.htm
串塌激察联团茄型可调稳压电路
http://www.dzcpkf.com/dlt/sort05/sort0147/25239.html
www.electfans.com