QB_PH_SEC8_006: Semiconductor Devices & Diodes

A $10\text{ V}$ Zener diode regulator circuit is supplied by an unregulated input DC voltage of $V_s = 25\text{ V}$ through a series resistor $R_s = 500\,\Omega$. If a load resistance $R_L = 1\text{ k}\Omega$ is connected across the Zener diode, what is the current through the Zener diode $I_Z$ in mA?

Correct Answer

### Step-by-Step Derivation & Verification: 1. **Circuit Operation in Breakdown**: Since $V_s > V_Z$, the Zener diode regulates the load voltage to $V_L = V_Z = 10\text{ V}$. 2. **Source Current Calculation**: $I_s = \frac{V_s - V_Z}{R_s} = \frac{25\text{ V} - 10\text{ V}}{500\,\Omega} = \frac{15\text{ V}}{500\,\Omega} = 0.030\text{ A} = 30\text{ mA}$ 3. **Load Current Calculation**: $I_L = \frac{V_L}{R_L} = \frac{10\text{ V}}{1000\,\Omega} = 0.010\text{ A} = 10\text{ mA}$ 4. **Zener Diode Current**: $I_Z = I_s - I_L = 30\text{ mA} - 10\text{ mA} = 20\text{ mA}$ 5. **Official Value**: 20 mA.