ELECTRONICS LABORATORY PRACTICAL NO :P02
EMC 2082 MEASUREMENTS &
INSTRUMENTATION
DESIGN OF REGULATED POWER SUPPLY
NAME : M.J.M.IRFAN
DEPARTMENT : CET
INDEX NO : EGT/23/462
GROUP : EET – C01
DATE OF PRACTICAL : 2025/07/18
OBJECTIVES
➢ To understand the construction, working principle, and voltage transformation of a step-
down transformer through both theoretical and practical analysis.
➢ To build and compare half-wave and full-wave rectifier circuits, and observe their output
using an oscilloscope.
➢ To examine the role of filtering components like capacitors in reducing ripple in rectified
outputs.
➢ To implement voltage regulation using the LM7805 and study its effectiveness in providing
a steady DC output.
➢ To analyze load regulation by measuring voltage and current across different loads,
highlighting how AC is converted and stabilized to DC in real-world applications.
INTRODUCTION AND THEORY
Power supplies are essential in virtually all electronic systems, as most electronic components
require direct current (DC) to operate, whereas the electric grid delivers alternating current (AC).
This lab aims to explore and understand the process of converting AC into usable, stable DC voltage
through various stages: voltage transformation, rectification, filtering, and regulation.
Step-Down Transformer
A step-down transformer is an electromagnetic device used to decrease high AC mains voltage
(typically 230V in Sri Lanka) to a safer and lower voltage suitable for electronic applications. It
consists of two windings—primary and secondary—wound on a common magnetic core. The
transformation of voltage is governed by the turns ratio of these windings:
: Where:
• Vp and Vs are the primary and secondary voltages respectively,
• Np and Ns are the number of turns in the primary and secondary windings
respectively.
This stage is crucial because applying high-voltage AC directly to electronic circuits can cause
immediate damage. A suitable transformer must be selected considering both secondary voltage
and maximum current capacity based on the load requirements.
Rectification
Rectification is the process of converting AC (which alternates direction) into DC (which flows
in a single direction). This is achieved using diodes, which are semiconductor devices that allow
current to flow in only one direction. Two main rectifier types are used:
• Half-Wave Rectifier: Uses a single diode and allows only one half (positive or negative)
of the AC waveform to pass through. It's simple but inefficient due to large voltage ripples
and low average output voltage.
• Full-Wave Rectifier: Utilizes either two diodes (in a center-tap configuration) or four
diodes in a bridge rectifier setup. This design allows both halves of the AC waveform to
be used, effectively doubling the frequency of the output and significantly reducing
ripple. It produces a higher average voltage and is widely used in power supply systems.
Filtering
Even after rectification, the output contains ripple—a residual AC component superimposed on
the DC output. To smooth out these fluctuations and obtain a more stable voltage, filtering is
employed using capacitors (typically aluminum electrolytic capacitors with values like 1000μF
or 2200μF).
When the rectified voltage rises, the capacitor charges; when the voltage drops, the capacitor
discharges, filling in the gaps and producing a smoother DC output. The effectiveness of filtering
depends on the capacitance value and the load current—higher capacitance leads to lower ripple.
Voltage Regulation
Even with filtering, the output voltage may still vary with changes in input voltage or load
current. Voltage regulators are used to maintain a constant output voltage. The most common
are:
• Fixed Regulators (e.g., LM7805): Provide a steady, preset output voltage such as +5V,
regardless of changes in load current or input fluctuations.
• Adjustable Regulators (e.g., LM317): Allow users to set a custom output voltage (e.g.,
between 1.25V to 12V) using a voltage divider or potentiometer.
Voltage regulators operate using internal feedback mechanisms that adjust resistance or current
paths to stabilize the output. Proper regulation is crucial for sensitive electronic components that
require precise voltage levels to function correctly.
Load Regulation
Load regulation refers to a power supply’s ability to maintain its output voltage as the load varies.
It is typically expressed as a percentage:
Where:
• V0 is the no-load output voltage,
• VL is the output voltage under load.
A well-designed power supply should have load regulation below 1%, ensuring reliable
performance even under varying operating conditions. This lab will investigate the load
regulation by measuring output voltage and current across multiple load resistances.
DIAGRAMS
Activity 02
Figure 1 : half -wave rectifier
Activity 03
Figure 2 : full-wave (center-tap) rectifier
Figure 3 : full-wave (bridge) rectifier
Figure 4 : full-wave rectifier with capacitor
Activity 04
Figure 5 : full-wave rectifier with valtage regulator
Figure 6 : valtage regulator pin diagram
PROCEDURE
Experiment 01 – Step-Down Transformer Analysis
The primary and secondary windings of the step-down transformer were first identified. All
electrical specifications such as the rated input voltage, output voltages, and maximum current were
obtained by reading the label affixed to the transformer—no measurements were made using a
multimeter at this stage. Using the label data, the theoretical maximum power and the RMS values
of the secondary voltage were calculated. The AC output voltages from the secondary winding were
then measured using a multimeter (input voltage was not measured). An oscilloscope was employed
to observe and sketch the waveform at one of the AC outputs of the transformer. Finally, the
calculated, measured, and observed values were compared to assess any discrepancies and evaluate
the performance of the transformer.
Experiment 02 – Half-Wave Rectification and Filtering
A half-wave rectifier circuit was constructed on a breadboard as shown in Figure 01. It was
connected to one of the secondary outputs of the step-down transformer. The DC output voltage of
the circuit was measured, and any differences between the input AC and output DC voltages were
analyzed. The output waveform of the rectifier was visualized and recorded using an oscilloscope.
Subsequently, a 2200μF electrolytic capacitor was connected across the rectifier output terminals to
function as a smoothing filter. The voltage measurement and waveform observation were repeated
to analyze the effect of the capacitor on ripple reduction and waveform smoothness.
Experiment 03 – Full-Wave Rectification and Filtering
A full-wave rectifier circuit, either using a center-tap configuration or a bridge rectifier as illustrated
in Figure 02 or 03, was assembled on a breadboard and connected to the transformer’s secondary
side. The output DC voltage was measured and compared with the input to understand voltage drop
behavior. The waveform of the output signal was observed using an oscilloscope to identify its shape
and frequency characteristics. A 2200μF capacitor (as in Figure 04) was then added to the output of
the rectifier to reduce ripple. Both voltage measurements and waveform observations were repeated
to analyze the capacitor’s filtering effectiveness.
Experiment 04 – Voltage Regulation Using LM7805
In this stage, a voltage regulator IC (LM7805) was connected to the output of the previously
constructed full-wave rectifier circuit (see Figure 05). The regulated output voltage was measured
to verify the IC’s performance. An oscilloscope was used to observe and sketch the waveform at
the output of the regulator, confirming the stability of the DC voltage and the reduction in ripple.
Experiment 05 – Load Regulation Analysis
Initially, the regulated output voltage of the LM7805 was measured with no load connected. A
10Ω resistor was then connected as a load, and both the voltage across and the current through the
resistor were measured and recorded. This procedure was repeated for different load resistances
including 30Ω, 10Ω, 6.8Ω, and 4.7Ω. For each case, the voltage, current, and load regulation
percentage were documented in a results table. A graph was plotted with Load Regulation (%) on
the Y-axis and Load Current (A) on the X-axis. From the graph, the maximum load current
corresponding to a load regulation of 1% or less was determined.
EQUIPMENT LIST
• Step-down transformer
• Breadboard
• 1N4007 diodes
• Electrolytic capacitors (e.g., 2200μF)
• LM7805 voltage regulator IC
• LM317 adjustable voltage regulator IC
• Multimeter
• Cathode Ray Oscilloscope (CRO)
• Resistors (various values)
• jumper wires
• DC power supply
DISCUSSION
In the initial part of the experiment, the primary and secondary windings of a step-down
transformer were identified using a digital multimeter. Resistance measurements confirmed the
windings' continuity. The primary winding showed lower resistance due to its thicker wire and
fewer turns, which aligns with transformer design principles for stepping down voltage while
increasing current. Label data provided the rated input and output voltages, and theoretical output
values such as RMS voltage and power were calculated for reference.
Next, a half-wave rectifier circuit was constructed using a single diode. This basic configuration
allowed only the positive half-cycle of the AC input to pass through, producing a pulsating DC
output with high ripple. Although simple to implement, the unfiltered output was unsuitable for
sensitive devices. The oscilloscope waveform confirmed significant ripple and intermittent current
flow, emphasizing the need for filtering or regulation in practical applications.
The full-wave rectifier circuit was then built using either a center-tapped transformer with two
diodes or a four-diode bridge configuration. This circuit converted both halves of the AC waveform
into pulsating DC, doubling the output frequency and reducing ripple. Compared to the half-wave
circuit, this arrangement produced a smoother and higher average DC voltage. Oscilloscope
readings clearly demonstrated the increased efficiency and effectiveness of full-wave rectification.
To further improve the output quality, a 2200μF filtering capacitor was introduced. This capacitor
charged during voltage peaks and discharged during drops, effectively smoothing the waveform.
The oscilloscope display confirmed a significant reduction in ripple, and the DC voltage became
more stable. However, small deviations remained due to factors such as diode forward voltage
drop, capacitor leakage, and real-world component tolerances.
A voltage regulator IC (LM7805) was connected to the full-wave filtered output to obtain a
constant +5V DC. This regulator maintained a steady voltage output despite input fluctuations and
varying load conditions. Load regulation tests were conducted by connecting different resistive
loads and measuring output voltage and current. Results showed minimal variation in output
voltage, confirming the regulator’s effectiveness. A graph of load current versus load regulation
helped determine the maximum current that could be drawn while maintaining regulation within
1%.
CONCLUSION
This practical experiment provided a comprehensive understanding of AC to DC power
conversion, beginning with transformer voltage reduction and extending through rectification,
filtering, and voltage regulation. We successfully identified transformer windings using a
multimeter and constructed both half-wave and full-wave rectifier circuits. The performance of
these circuits was observed, measured, and compared with theoretical expectations.
The use of filtering capacitors was shown to effectively reduce ripple, and the LM7805
regulator was able to produce a stable 5V output, demonstrating the importance of voltage
regulation in power supply design. Challenges such as voltage drops, ripple effects, and
component limitations provided realistic insights into circuit behavior under non-ideal
conditions.
Overall, this lab enhanced our practical knowledge of basic power electronics and deepened
our understanding of how AC power is converted into reliable, regulated DC—an essential
function in nearly all modern electronic devices.
REFERENCES
➢ All About Circuits, 2025. Transformers - Basic Theory. Available at:
https://www.allaboutcircuits.com/textbook/alternating-current/chpt-5/transformer-basics/
[Accessed 20 July 2025].
➢ Electronics Tutorials, 2025. Half Wave Rectifier. Available at: https://www.electronics-
tutorials.ws/diode/diode_3.html [Accessed 20 July 2025].
➢ Electronics Tutorials, 2025. Full Wave Rectifier. Available at: https://www.electronics-
tutorials.ws/diode/diode_6.html [Accessed 20 July 2025].
➢ Texas Instruments, 2016. LM7805 Datasheet – 3-Terminal Positive Voltage Regulator.
Available at: https://www.ti.com/lit/ds/symlink/lm7805.pdf [Accessed 20 July 2025].
➢ National Instruments, 2025. Understanding Ripple and Its Effects. Available at:
https://www.ni.com/en-us/innovations/ripple-in-power-supplies.html [Accessed 20 July
2025].
➢ Lecture notes and lab manual, 2025. EMC2112 – Applied Electronics, Dr. Buddhika Amila,
Faculty of Technology, University of Sri Jayewardenepura. [Accessed 20 July 2025].
