Question

(a) Explain the construction, working principle, and characteristics of a Zener Diode. Discuss its applications in electronic circuits. (8)

(b) What is a Zener Barrier? With the help of a diagram, explain how a Zener Barrier works in an intrinsic Safe Circuit and discuss its importance in hazardous environments. (8)

Answer 1

Part (a)

Zener diode - construction, working principle, characteristics and applications:

Construction: a Zener diode is a heavily doped p-n junction diode designed to operate in the reverse breakdown region. The heavy doping gives a very narrow depletion region, so the breakdown voltage is low and well defined.
Working principle: in the forward direction it behaves like a normal diode. In the reverse direction, when the reverse voltage reaches the Zener (breakdown) voltage Vz, the diode breaks down and conducts heavily, but the voltage across it remains almost constant at Vz over a wide range of reverse current. This is due to Zener breakdown (for low voltages) or avalanche breakdown (for higher voltages).
Characteristics: the reverse characteristic shows a sharp knee at the Zener voltage; beyond this the current rises steeply while the voltage stays nearly constant. The forward characteristic is like a normal diode. The Zener voltage is specified at a particular test current, and the dynamic resistance (dV/dI) is small in the breakdown region.
Applications: voltage regulation (a Zener diode maintains a constant output voltage across a load despite variations in input voltage or load current); as a voltage reference; in voltage clamping and protection circuits (limiting overvoltage); in waveform shaping; and in power supplies as a shunt regulator.

Part (b)

Zener barrier in an intrinsically safe circuit:

A Zener barrier is a safety device used to limit the energy (voltage and current) reaching a hazardous area so that it cannot ignite a flammable atmosphere. It is fitted between the safe area (control equipment) and the hazardous area (field instruments).
Construction: the barrier contains Zener diodes, a series resistor (current-limiting), and a fuse, all encapsulated in a flameproof housing and earthed to a high-integrity earth.
Working principle: under normal operation the Zener diodes do not conduct and the signal passes through the series resistor to the hazardous area. If a fault causes the voltage to rise above the Zener voltage, the Zener diodes conduct and clamp the voltage to a safe level, and the fuse blows to protect the diodes. The series resistor limits the current to a safe value. Thus the energy (voltage and current) reaching the hazardous area is limited to intrinsically safe levels, below the minimum ignition energy of the gas.
Importance in hazardous environments: it prevents the ignition of flammable gases or vapours by limiting the electrical energy in the circuit. It allows standard (non-intrinsically safe) control equipment to be used with intrinsically safe field instruments, providing safety in tankers, gas carriers, and other hazardous areas. The barrier must be correctly earthed and certified for the hazardous area classification.

✓ Verified Model Answer (Text Solution)

Zener diode - construction, working principle, characteristics and applications:

Zener barrier in an intrinsically safe circuit: