Question

Determine the characteristic curve of Zener diode.

Hint:

The series resistor in the reverse bias circuit is absolutely essential. Without it, the current would become very large after breakdown, destroying the Zener diode. Choose a resistor value that limits the maximum current to the diode's rated value (e.g., for a 5.1V, 0.5W Zener, max current is \(\sim\)100 mA).

Answer 1

Step 1: Understanding the Concept: 
A Zener diode is a special type of p-n junction diode designed to operate in the reverse breakdown region. In forward bias, it behaves like a normal diode. In reverse bias, it allows a very small leakage current until the applied voltage reaches the Zener voltage (Vz). At this point, Zener breakdown occurs, and the current increases sharply while the voltage across the diode remains almost constant at Vz. This property is used for voltage regulation. 
Step 2: Apparatus: 
Apparatus Required: 
A Zener diode, a variable DC power supply (0-15V), a DC voltmeter, a DC milliammeter, a current-limiting resistor (\(\sim\)1 k\(\Omega\)), a key, and connecting wires. 
Step 3: Detailed Procedure: 
Part A: Forward Bias Characteristics 
1. Circuit: Connect the Zener diode in a forward-biased configuration (p-side to positive terminal). The circuit is identical to that of a normal p-n diode. 
2. Readings: Increase the forward voltage in small steps and record the corresponding forward current. 
Part B: Reverse Bias Characteristics 
1. Circuit: Connect the Zener diode in reverse bias (p-side to negative terminal). Importantly, connect a current-limiting resistor in series with the diode to protect it from excessive current after breakdown. Connect the voltmeter in parallel across the Zener diode. 
2. Readings: Slowly increase the reverse voltage from the power supply. Record the reverse voltage (V) and the corresponding reverse current (I). The current will be negligible at first. As the voltage approaches the Zener voltage (Vz), the current will start to increase. Take several readings in this region. Past Vz, the voltage will remain almost constant while the current increases sharply. 
Step 4: Graph and Result: 
1. Plotting: Plot the I-V graph. The forward bias curve is in the first quadrant, and the reverse bias curve is in the third quadrant. 
2. The Curve: The reverse bias curve will be nearly flat until it reaches the breakdown region, where it bends sharply downwards into an almost vertical line. 
3. Finding Zener Voltage (Vz): The voltage at which the current starts to increase rapidly is the Zener breakdown voltage (Vz). This is read from the graph at the "knee" of the reverse characteristic curve. 
The result is stated as: "The Zener breakdown voltage (Vz) for the given diode is ..... V."