I need a help to design a circuits and to do this.

1. An 18V, 50W zener diode is to be used to maintain a constant DC voltage across a load which varies from 10-100ohms. The input voltages is 90V.

2. An unregulated supply varies between 20 and 25V. A 10V zener diode is to regulate this voltage for use in a tape recorder. The recorder draws 30mA when recording and 50mA when playing back. The zener can dissipate a maximum power of 80mW.

 

Read this:
http://knol.google.com/k/electronic-circuits-design-for-beginners-chapter-7#
Or hear
http://www.onsemi.com/pub_link/Collateral/HBD854-D.PDF (on page 35)
And show as your attempt solving the problem