In the circuit shown, a $\text{5 V}$ Zener diode is used to regulate the voltage across load $R_{0}$. The input is an unregulated $DC$ voltage with a minimum value of $\text{6 V}$ and a maximum value of $\text{8 V}$. The value of $R_{S}$ is $6\;\Omega$. The Zener diode has a maximum rated power dissipation of $\text{2.5 W}$. Assuming the Zener diode to be ideal, the minimum value of $R_{0}$ is _________________ $\Omega$.