Essential Electronics Laboratory Experiments and Procedures

Experiment 4: V–I Characteristics of a P–N Junction Diode

Apparatus Required

Breadboard, diode, DC power supply, multimeter, resistor, and connecting wires.

Theory

When a p-type semiconductor is joined with an n-type semiconductor, electrons from the n-region and holes from the p-region diffuse across the junction and recombine. This results in a region near the junction that is depleted of free charge carriers, known as the depletion region. A diode is a two-terminal electronic device that allows current to flow mainly in one direction.

Forward Characteristics

The circuit arrangement for obtaining the forward characteristics of a diode consists of a diode connected to a variable DC power supply and a load resistance (RL). The resistance is included to limit the current through the diode. A voltmeter is connected across the diode to measure the voltage, and an ammeter (multimeter) is connected in series to measure the current. In forward bias, the positive terminal of the DC supply is connected to the anode, and the negative terminal is connected to the cathode. The applied voltage is increased gradually in small steps (about 0.1 V), and the corresponding diode current is noted.

Reverse Characteristics

For obtaining the reverse characteristics, the diode is connected in reverse bias by connecting the negative terminal of the voltage source to the anode and the positive terminal to the cathode. The reverse voltage is increased gradually in suitable steps, and the corresponding diode current is recorded. In reverse bias, only a very small current flows through the diode until breakdown occurs.

Result

The V–I characteristics of the p–n junction diode were studied and plotted.

Precautions

• All connections should be tight and proper.
• Follow the procedure carefully.
• Take readings accurately.
• Do not touch live wires.

Experiment 5: Zener Diode as a Voltage Regulator

Apparatus Required

Zener diode, DC power supply, multimeter, breadboard, connecting wires, and resistor (680 Ω).

Theory

A p–n junction diode normally does not conduct when it is reverse biased. However, when the reverse bias voltage is increased beyond a certain value, the diode starts conducting heavily. This voltage is called the breakdown voltage. If excessive current flows through the diode, it may get permanently damaged. To limit the current, a resistor is connected in series with the diode.

Once the Zener diode operates in the breakdown region, it maintains an almost constant voltage across its terminals irrespective of the current flowing through it. This property indicates that the Zener diode has very low dynamic resistance in the breakdown region. A Zener diode is a specially designed p–n junction diode meant to operate safely in the breakdown region and is widely used for voltage regulation.

Procedure

1. Connect the circuit as shown in the circuit diagram.
2. Apply the DC power supply to the circuit.
3. Observe and note the readings in both forward bias and reverse bias modes.
4. Tabulate the voltage and current readings.
5. Plot the V–I characteristics of the Zener diode on graph paper.
6. Calculate the forward resistance and reverse resistance from the graph.

Result

The V–I characteristics of the Zener diode were plotted, and the Zener diode was successfully studied as a voltage regulator.

Precautions

• All connections should be tight and proper.
• Switch off the power supply when not in use.
• Take readings carefully and accurately.

Experiment 6: Half Wave and Full Wave Rectifiers

Apparatus Required

Step-down transformer (5–8 V), diode(s), resistor (1 kΩ), breadboard, connecting wires, and AC power supply.

Theory

A rectifier is an electronic device that converts alternating current (AC) voltage into direct current (DC) voltage. A p–n junction diode is commonly used for rectification because it allows current to flow in only one direction. Depending on the circuit configuration, rectification can be achieved using one, two, or four diodes. Rectifiers are mainly classified as Half Wave Rectifiers and Full Wave Rectifiers (Centre-tap).

Half Wave Rectifier

A half wave rectifier consists of a single semiconductor diode connected in series with a load resistance (RL) across the secondary winding of a transformer. During the positive half cycle of the AC input, the diode is forward biased and conducts, producing a positive output across the load resistance. During the negative half cycle, the diode is reverse biased and does not conduct. Hence, only one half of the input waveform appears at the output, resulting in a pulsating DC voltage.

Full Wave Rectifier

In a centre-tapped full wave rectifier, the AC input is applied through a transformer having a centre-tapped secondary winding. Two diodes (D1 and D2) are connected to the opposite ends of the secondary winding. The cathodes are connected together to the load resistance (RL), which is further connected to the centre tap. During the positive half cycle, D1 is forward biased and conducts. During the negative half cycle, D2 is forward biased. Thus, both halves of the AC input waveform are utilized, resulting in a pulsating DC output with less ripple compared to a half wave rectifier.

Result

The waveforms of the Half Wave Rectifier and Full Wave Rectifier were successfully studied and traced.

Precautions

• All connections should be tight and secure.
• Waveforms should be traced carefully.
• Switch off the power supply when not in use.
• Handle the transformer with care.

Experiment 7: OP AMP as Adder and Subtractor

Apparatus Required

IC 741, resistance (10 Ω), breadboard, power supply, multimeter.

Result

The OP AMP was successfully studied as an adder and subtractor circuit.

Precautions

• Perform connections properly.
• Take readings accurately.
• Switch off the power supply when not in use.

Experiment 8: Truth Table of Basic Logic Gates

Apparatus Required

AND Gate IC-7408, NOT Gate IC-7404, OR Gate.

Result

The basic logic gates were studied and verified.

Precautions

• Connect the IC properly.
• Ensure no loose connections.
• Verify the gate operations.

Experiment 9: Truth Table of Universal Gates

Apparatus Required

NAND Gate IC-7400, NOR Gate IC-7402.

Result

The universal gates were studied and verified.

Precautions

• Connect the IC properly.
• Ensure no loose connections.
• Verify the circuit functionality.