In figure, what is the voltage needed at the source, to maintain 15 V across the load resistance RL , assuming that the series resistance R is 200 Ω and Zener requires a maximum current of 10mA to work satisfactory?

Load resistance and zener diode are in parallel ∴ Potential drop across Load = 15 V  Ohm's law across Load : VZ=ILRL                                          ⇒15 =IL (2000)                                              ⇒IL=7.5mAAs per Kirchhoff's Junction Rule :  I=IZ+IL=10+7.5=17.5mAOhm's Law across Series Resistance : VS = IR = (17.5×10-3×200) = 3.5 VHence, Voltage at source = E =VS+ VZ = 3.5 + 15 = 18.5 V