Zener diode as voltage regulator — circuit diagram and working

Question

Explain how a Zener diode is used as a voltage regulator with a circuit diagram. A Zener diode has a breakdown voltage of 6 V. If the input voltage is 10 V and the series resistance is 200 $\Omega$ , find the current through the Zener diode when the load resistance is 1 k $\Omega$ .

(NCERT Class 12, Chapter 14 — frequently tested in CBSE and NEET)

Solution — Step by Step

The Zener diode is connected in reverse bias across the load resistance $R_L$ . A series resistance $R_s$ is connected between the input voltage and the Zener-load combination.

When the input voltage exceeds the Zener breakdown voltage $V_Z$ , the Zener diode conducts in reverse and maintains a constant voltage $V_Z$ across itself (and the load). Any excess voltage drops across $R_s$ .

The voltage across the load equals the Zener voltage: $V_L = V_Z = 6$ V.

Voltage across $R_s$ :

V_{R_s} = V_{in} - V_Z = 10 - 6 = 4 \text{ V}

Total current through $R_s$ :

I = \frac{V_{R_s}}{R_s} = \frac{4}{200} = 0.02 \text{ A} = 20 \text{ mA}

Current through load:

I_L = \frac{V_Z}{R_L} = \frac{6}{1000} = 0.006 \text{ A} = 6 \text{ mA}

Current through Zener diode (by Kirchhoff’s current law):

\boxed{I_Z = I - I_L = 20 - 6 = 14 \text{ mA}}

Why This Works

In the breakdown region, the Zener diode’s V-I curve is nearly vertical — a large change in current produces almost no change in voltage. This is the key to regulation. If the input voltage increases, the extra current flows through the Zener (not the load), keeping $V_L$ constant. If the load resistance changes, the Zener adjusts its current to compensate.

The series resistance $R_s$ is essential — it limits the total current and provides the voltage drop needed when $V_{in} > V_Z$ . Without $R_s$ , the Zener diode would draw unlimited current and burn out.

Alternative Method

You can also analyse this using the current-divider approach. The Zener diode and load are in parallel (both have $V_Z$ across them). The total current from the source splits between them. The Zener absorbs whatever the load does not need.

For CBSE boards, the circuit diagram is worth 2 marks on its own. Draw it neatly with the Zener symbol (note the bent line at the cathode), label $R_s$ , $R_L$ , $V_{in}$ , and $V_Z$ . For NEET, remember: the Zener acts as a constant voltage source in breakdown — treat it like a battery of EMF $V_Z$ for circuit calculations.

Common Mistake

Students often confuse the direction of the Zener diode. In a voltage regulator circuit, the Zener is always in reverse bias. If you connect it in forward bias, it behaves like an ordinary diode (0.7 V drop) and provides no regulation. Also, some students write $I_Z = I_L$ — that is wrong. The Zener current and load current are different; they add up to give the total current through $R_s$ .

Question

Solution — Step by Step

Why This Works

Alternative Method

Common Mistake

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