---
title: "Short note: Regulated power supply"
subject: "Basic Electronics Engineering"
chapter: 7
key: "semester_2/basic_electronics_engineering:short-note-regulated-power-supply"
source: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/short-note-regulated-power-supply
site: BCT Engineering
---

# Short note: Regulated power supply

Basic Electronics Engineering · Chapter 7: Application of Electronic System

- Asked in: 2075 Baisakh, 3 marks; 2074 Bhadra, 3 marks; 2073 Bhadra, 3 marks; 2070 Magh, 3 marks; 2068 Bhadra, 3 marks; 2068 Magh, 4 marks (asked 6 times)
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/basic_electronics_engineering/short-note-regulated-power-supply
- Whole chapter: https://www.bctengineer.com/md/semester_2/basic_electronics_engineering/chapter-7

## Question

Write a short note on regulated power supply (using IC).

## Short answer

A regulated power supply maintains constant DC output despite input or load variations, typically using a transformer, rectifier, filter capacitor and a three-terminal IC regulator like 7805. The IC provides fixed voltage (e.g., +5V) with built-in current limiting and thermal protection, simplifying design.

## Answer

A **regulated power supply** gives a constant DC output voltage even when the AC input voltage or the load current varies. An IC regulator (a fixed three-terminal 78xx for positive or 79xx for negative output) is the simplest way to do this.

#### Block diagram

```text
 AC --[Transformer]--[Rectifier]--[Filter C]--[IC 7805]-- DC out
 mains  (step down)  (bridge)     (smoothing)  regulator   +5 V
```

#### Circuit with IC 7805

```text
             +-----------------+
 Vin ---+----| IN   7805   OUT |----+--------+---- Vout = +5 V
 (7-25V)|    +--------+--------+    |        |
        C1           GND            C2      Load
     0.33uF           |           0.1uF      |
        |             |             |        |
 -------+-------------+-------------+--------+---- GND
```

- The transformer steps the 220 V mains down to a lower AC voltage.
- The bridge rectifier converts AC to pulsating DC, and the capacitor filter smooths it.
- The 7805 holds the output at a steady 5 V as long as the input is at least 2 to 3 V above the output (about 7 to 25 V).
- C1 and C2 improve stability and remove high-frequency noise.

The 78xx series gives fixed outputs such as 5, 6, 8, 9, 12, 15 and 24 V, at up to 1 A. The LM317 gives an adjustable output. Advantages: simple, cheap, with built-in current limiting and thermal shutdown.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What does a regulated power supply provide?
   A. A higher AC voltage
   B. A constant DC output despite changes in AC input or load current
   C. A pulsating DC output
   D. An output that follows the input voltage
   Answer: B. Regulation keeps the DC output voltage constant even when the input voltage or the load varies. (Key point: Purpose of regulation)

2. What is the correct order of blocks in the supply using IC 7805?
   A. Rectifier, transformer, filter, regulator
   B. Transformer, filter, rectifier, regulator
   C. Transformer, rectifier, filter, IC regulator
   D. Filter, transformer, regulator, rectifier
   Answer: C. Mains is stepped down, rectified, smoothed by the filter capacitor, then regulated by the IC. (Key point: Block order)

3. For the 7805 to hold exactly 5 V output, the input must be:
   A. Equal to 5 V
   B. Below 5 V
   C. Negative with respect to ground
   D. At least 2 to 3 V above the output
   Answer: D. The 7805 needs an input about 2 to 3 V above its output (about 7 to 25 V) to regulate properly. (Key point: 7805 input requirement)

## Related in Basic Electronics Engineering

- [Fixed bias: IB, IC, VCE (20 V)](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Afixed-bias-20v-200k-80k) — Basic Electronics Engineering, chapter 3, asked 2×
- [Fixed bias: VBE, IB, IC, VCE (12 V)](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Afixed-bias-12v-200k-2k2) — Basic Electronics Engineering, chapter 3, asked 2×
- [Find RB and RC for given IC, VCE](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Afind-rb-rc-ic-1-2ma) — Basic Electronics Engineering, chapter 3

## Related in other subjects

- [Gamma: power supply inadequate](https://www.bctengineer.com/question.md?key=semester_5%2Fprobability_and_statistics%3Agamma-power-supply-12-million) — Probability and Statistics, chapter 3, asked 2×
- [Types of power supply system for industrial plant](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aindustrial-supply-system-types) — Industrial Power Distribution and Illumination, chapter 1, asked 2×
- [Emergency power supply system for industry](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Aemergency-supply-system-industry) — Industrial Power Distribution and Illumination, chapter 9, asked 2×
- [Industrial power supply structure; industrialization in Nepal](https://www.bctengineer.com/question.md?key=semester_7%2Futilization_of_electrical_energy%3Aindustrial-supply-structure-industrialization-nepal) — Utilization of Electrical Energy, chapter 1