Q.A Zener diode of breakdown voltage 6 V is connected, through a series resistance of 200 Ω, to an unregulated 10 V supply, to hold the voltage across a load that draws a current of 10 mA. Find the current flowing through the Zener diode.
🔒You're viewing a preview — the full solution, concept, methods & PYQ mapping are locked.
🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Zener Diode Voltage Regulation
Intuition First: Why a Zener Diode Can "Hold" a Voltage
Imagine a pressure relief valve in a water pipe. If the water pressure gets too high, the valve opens just enough to let some water escape, keeping the pressure downstream constant — no matter how much the upstream pressure fluctuates. A Zener diode does the same thing, but with voltage.
In a normal diode, reverse bias blocks current completely — until you push too hard and destroy it. A Zener diode is specially designed so that when you reverse-bias it past a certain voltage (the Zener voltage, VZ), it doesn't break — it conducts steadily without damage. And here's the magic: once it starts conducting in breakdown, the voltage across it stays almost exactly at VZ, even if the current through it changes dramatically.
That flat voltage is what makes it a regulator.
The Precise Statement
A Zener diode, when reverse-biased into its breakdown region, maintains a nearly constant voltage VZ across its terminals over a wide range of reverse currents. In a circuit, this allows it to hold a fixed output voltage despite variations in the input supply or the load current — provided the current through the Zener stays within its safe operating limits.
How It Works in a Circuit
The simplest Zener regulator looks like this:
- A series resistor Rs connects the unregulated input Vin (which is higher than VZ) to the Zener diode.
- The Zener is reverse-biased, with its cathode toward the positive supply.
- The load RL is connected across the Zener diode.
The resistor is the workhorse. It drops the excess voltage (Vin−VZ) and limits the current. The Zener does the regulation.
Case 1: Input voltage rises
If Vin increases, the current through Rs tries to increase. But the Zener is already in breakdown — it simply absorbs the extra current, and the voltage across it stays at VZ. The load sees no change.
Case 2: Load draws more current
If the load resistance decreases (more current needed), that current comes from the Zener branch. The Zener current drops by exactly the same amount. The total current through Rs stays the same, so the voltage drop across Rs doesn't change — and VZ remains constant.
The Zener acts like a variable resistor that automatically adjusts its resistance to keep the voltage fixed. When the load demands more current, the Zener "gives up" some of its own current. When the load demands less, the Zener takes the extra.
The Key Formula
IZ=RsVin−VZ−IL
Where:
- IZ is the current through the Zener diode
- IL=VZ/RL is the load current
- The first term is the total current through Rs
For regulation to work, IZ must always stay between IZ(min) (the minimum current needed to keep the Zener in breakdown) and IZ(max) (the maximum current the Zener can handle without overheating).
The Two Critical Limits
A Zener regulator fails if: …
Total current I=(Vi−VZ)/RS; Zener current IZ=I−IL. …
Given Vi=10 V, VZ=6 V, RS=200 Ω, IL=10 mA:
I=RSVi−VZ=20010−6=2004=0.02 A=20 mA …
Find the total current through RS from (Vi−VZ)/RS, then subtract the given load …
- Using Vi instead of (Vi−VZ) across RS -- only the DIFFERENCE drops across the series resistor, since VZ is held across the diode/load. …
- CBSE 2026Set A1 markMCQQ.Which device is used as a voltage regulator? (A) Transistor (B) Transformer (C) Zener diode (D) Photodiode
›Reveal solutionSolution
A Zener diode maintains a constant voltage across its terminals in reverse breakdown, making it the standard voltage regulator.
A Zener diode is designed to operate in the reverse breakdown region. Once the reverse voltage reaches the Zener voltage, the voltage across the diode stays almost constant even though the current through it varies over a wide range. Connected across a load, it therefore holds the output voltage steady against changes in supply voltage or …
- CBSE 2024Set ANNUAL1 markQ.Draw the symbol of Zener diode.
›Reveal solutionSolution
A Zener diode's circuit symbol is the ordinary diode symbol with the straight cathode line replaced by a bent (Z-shaped) line.
A Zener diode is drawn like an ordinary p-n junction diode — a triangle (representing the p-region/anode) pointing toward a straight bar (representing the n-region/cathode), the arrow showing the conventional (easy, forward) direction of current flow, anode to cathode.
…
- CBSE 2023Set ANNUAL1 markMCQQ.Zener diode functions in(1) forward biased condition(2) reverse biased condition(3) both forward and reverse biased condition(4) none of these
›Reveal solutionSolution
Unlike an ordinary diode, a Zener diode is deliberately operated in reverse breakdown, where the voltage across it stays nearly constant over a range of currents.
…
- CBSE 2023Set ANNUAL1 markMCQQ.The Zener diode is primarily used as :(a) Oscillator(b) Rectifier(c) Voltage regulator(d) Amplifier
›Reveal solutionSolution
Operated in reverse breakdown, a Zener diode holds a fixed voltage across itself regardless of current variations, so it is used as a voltage regulator.
Working
A Zener diode is a specially designed p-n junction diode intended to be operated in the reverse breakdown region without being damaged (heavily doped junction). In this region, the voltage across the diode stays almost constant over a wide range of reverse currents. This property is exploited to hol …
- CBSE 2023Set ANNUAL1 markMCQQ.A Zener diode when biased correctly,(a) overheats(b) acts as fixed resistance(c) has constant voltage across it(d) has constant current across it
›Reveal solutionSolution
A correctly biased Zener diode maintains a constant voltage across itself.
A Zener diode is operated in reverse bias at its breakdown (Zener) voltage. In this region the voltage across it stays almost constant even as the current through it changes widely. This makes it ideal as a voltage regulator. T …
- CBSE 2022Set HE2171 markMCQQ.Which is used as a voltage regulator?(i) Transformer(ii) Transistor(iii) Zener diode(iv) Photodiode
›Reveal solutionSolution
A Zener diode is used as a voltage regulator because its reverse breakdown voltage stays nearly constant.
A voltage regulator must hold its output voltage essentially fixed even when the input voltage or the load current varies. A Zener diode, when connected in reverse bias beyond its breakdown (Zener) voltage VZ, carries a wide range of current while the voltage across it stays almost constant at VZ. By placing the Zener diode in parallel with the load (through a series resistor from the unregulated supply), any excess current is diverted through the diode, keeping the lo …
- CBSE 2017Set ANNUAL1 markQ.A zener diode is always operated in ______ bias.
›Reveal solutionSolution
A zener diode is designed to work in the reverse breakdown region, so it is always connected in reverse bias.
An ordinary p-n junction diode is destroyed if driven into reverse breakdown, but a zener diode is specially doped (heavily doped p and n regions) so that it can withstand a sharp, well-defined reverse breakdown voltage (VZ) without damage. Once the reverse voltage reaches VZ, the current increases sharply while the voltage across the diode stays almost constant at VZ.
…
🎓Unlock everything free for 14 days
- ✓Full step-by-step solutions
- ✓Concept-first explanations
- ✓Methods, shortcuts & mistakes
- ✓PYQ mapping + timed mock tests
Full access for 14 days. No credit card required.