Physics · Semiconductor Electronics : Materials, Devices And Simple Circuits · NEET
A Zener diode is used in reverse bias because regulation happens only in the reverse breakdown region. In forward bias any ordinary diode conducts and gives about 0.7 V, which is not a useful controlled value. In reverse breakdown, once the input reaches the Zener voltage Vz, the diode allows a large change in current with almost no change in its voltage. That flat V-I curve in breakdown is exactly what holds the output steady. So for regulation the Zener is always reverse biased.
The series resistor Rs is placed between the unregulated input and the Zener-load parallel combination. It absorbs the extra voltage: voltage across Rs = Vin - Vz. When the input rises, more current flows and Rs drops more voltage, so the Zener still sees Vz. Rs also limits the current so the Zener is not damaged. Without Rs the full input would fall on the Zener and burn it. So Rs = current limiter and voltage-drop absorber.
Yes, approximately. In the breakdown region the V-I graph is nearly vertical, so a large change in Zener current causes only a tiny change in Zener voltage. This is why the output (load) voltage stays fixed at about Vz. It is not perfectly constant in a real diode (there is a small dynamic resistance rz), but for NEET-level problems we treat an ideal Zener as holding exactly Vz.
The Zener diode is connected in PARALLEL with the load (across the output), and the series resistor Rs is in series with the input line. Because the Zener is in parallel with the load, they share the same voltage Vz. So the load always gets the regulated Zener voltage. Extra current not needed by the load flows through the Zener.
If Vin drops below the Zener voltage Vz, the Zener no longer reaches breakdown, so it stops conducting and regulation fails. The output then just follows the input (minus small drops) and is no longer constant. Regulation works only while Vin is greater than Vz, so the extra voltage Vin - Vz can appear across Rs.
Consider the following Statements (A) and (B) and identify the correct answer. A. A Zener diode is connected in reverse bias when used as a voltage regulator. B. The potential barrier of a p-n junction lies between 0.1 V to 0.3 V.
Given below are two statements: Statement I: Photovoltaic devices can convert optical radiation into electricity. Statement II: Zener diode is designed to operate under reverse bias in breakdown region. Choose the most appropriate answer.
An ideal Zener diode with breakdown voltage of -3 V is reverse biased with a negative input voltage Vi = -5 V. In the circuit shown (C - [Zener] - B - R - A), find the magnitude of the voltage difference between points B and A.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is a circuit where a reverse-biased Zener diode, placed in parallel with the load and fed through a series resistor, keeps the output voltage fixed at Vz even when the input voltage or the load changes.
In the reverse breakdown region the V-I graph is almost vertical, so a large change in current produces almost no change in voltage. This flat characteristic is what keeps the output at Vz.
Voltage across Rs = Vin - Vz. The current through Rs is (Vin - Vz) / Rs, and this current splits between the load and the Zener.
The Zener breakdown voltage Vz. You choose a Zener whose Vz equals the output voltage you want, and the regulator holds the output near that value.
No. Regulation only works when Vin is greater than Vz so the diode reaches breakdown. If Vin falls below Vz the Zener stops conducting and the output is no longer constant.
