A zener diode of power rating $2 \mathrm{W}$ is to be used as a voltage regulator. If the zener diode has a breakdown of $10 \mathrm{V}$ and it has to regulate voltage fluctuated between $6 \mathrm{V}$ and $14 \mathrm{V}$, the value of $R_s$ for safe operation should be _____ $\Omega$ .