Solving Zener Diode Problem - Need Help

[waihung]

Homework Statement

Please refer to the picture
[PLAIN]http://img337.imageshack.us/img337/7118/46864828.jpg

Can anyone help me?
Thanks.

 

[vk6kro]

You can work out the value of the resistor when the load is connected, but there is only a 15 volt supply.

You know the minimum current for the Zener and you know the current in the load (it has 10 volts across it) so you know the total current in the series resistor.
You know the voltage across this resistor so you can work out what size resistor it should be.

So, you can select this resistor and see how it affects the currents when there is a 20 volt supply and/or there is no load resistor.

Notice that the Zener will take whatever current is necessary to maintain the output voltage at 10 volts.

Then check if the Zener power rating is being exceeded.