In the circuit shown, a 5 V Zener diode is used to regulate the voltage across load R₀ . The input is an unregulated DC voltage with a minimum value of 6 V and a maximum value of 8 V. The value of Rs is 6 Ω . The Zener diode has a maximum rated power dissipation of 2.5 W. Assuming the Zener diode to be ideal, the minimum value of R₀ is ______ Ω .