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•BLOCK DIAGRAM•FUNCTIONS

•TYPES

LINEAR DC POWER SUPPLY

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OBJECTIVES

At the end of the lesson students able to:

• i. State the importance of DC power Supply units in electronic appliances

• ii. Draw the block diagram of DC power supply

• iii. Explain function of each block.

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The Basic DC Power Supply

Most are designed to convert high voltage AC mains electricity to a suitable low voltage supply for electronics circuits and other devices.

The importance of DC power supply: a) Most electric equipment requires DC voltage. b) Dry cells unable to accommodate the needs of electrical equipments and not economical. c) Power supplied to the house in AC but electrical equipment needs DC voltage.

A power supply can by broken down into a series of blocks, each of which performs a particular function.

Block Diagram of DC Power Supply

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Function of each block;a) Transformer - convert AC electricity from one voltage to another with little loss of power. - steps down high voltage AC mains to low voltage AC. b) Rectifier - converts AC to DC, but the DC output is varying. c) Filter - smooth the DC from varying greatly to a small ripple. d) Regulator - eliminates ripple by setting DC output to a fixed voltage. e) Voltage divider – distributes DC output voltage to load

Transformer Rectifier Regulator

Filter Voltage

divider

§ Half-wave

§ Full-wave

§ Bridge

§ RC filter

§ LC filter

§ π filter

§ Zener diode

§ Serial transistor

§ Integrated circuit

§ Step-up

§ Step-down Fixed

Variable

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Block diagram of a typical linear DC power supply

Peak to peak AC signal

Average voltage with AC ripple

Pulsating DC

DC signal

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Transformer

Transformers convert AC electricity from one voltage to another with little loss of power.

Transformers work only with AC and this is one of the reasons why mains electricity is AC.

Step-up transformers increase voltage, Step-down transformers reduce voltage. Most DC power supplies use a step-down transformer to reduce the dangerously high mains voltage (240V ) to a safer low voltage.

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Transformer

*to Step Down high voltage AC main to low voltage AC. Example: 120Vac to 12Vac.

*The low voltage AC output is suitable for lamps, heaters and special AC motors. It is not suitable for electronic circuits unless they include a rectifier and a smoothing capacitor.

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Rectifierto convert AC to DC, but the DC output is varyingThere are several ways of connecting diodes to

make a rectifier to convert AC to DC. i. single diode can be used as a rectifier but it only

uses the positive (+) parts of the AC wave to produce half-wave varying DC.

ii. bridge rectifier is the most important and it produces full-wave varying DC.

iii full-wave rectifier can also be made from just two diodes if a centre-tap transformer is used, but this method is rarely used now that diodes are cheaper.

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Rectifier

to convert AC to DC, but the DC output is varying

There are several ways of connecting diodes to make a rectifier to convert AC to DC.

i) half wave rectifierii)Full wave rectifieriii)Bridge rectifier

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Diode as half wave rectifier

A single diode can be used as a rectifier but this produces half-wave varying DC which has gaps when the AC is negative.

It only uses the positive (+) parts of the AC wave to produce half-wave varying DC. 

i. Half-wave Rectifier  

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During +ve half-cycle :The diode D is forward bias. Diode D act as

short circuit, the current flow through the diode M, D, RL and N circuit. So that the output waveform is +ve voltage.

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During -ve half-cycle :The diode D is reverse bias. Diode D act as

open circuit, there is no current flow through the circuit . So that the output waveform is zero volt.

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Input & Output waveform of Half-wave Rectifier

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ii. Full-wave Rectifier

A full wave rectifier is exactly the same as the half wave, but allows unidirectional current through the load during the entire sinusoidal cycle.

Average value of output becomes twice that of the half wave rectifier output: VAVG= 2Vp/π

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ii. Center-tapped Full-wave Rectifier

Two diodes connected to the secondary of a center-tapped transformer can form a full-wave rectifier that allows both half-cycles of the AC waveform to contribute to the DC making it smoother than a half-wave rectifier

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ii. Center-tapped Full-wave Rectifier

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ii. Center-tapped Full-wave Rectifier

During +ve half-cycle :The diode D1 is forward bias, diode D2 is

reverse bias. Diode D1 act as closed circuit and D2 as open circuit, the current flow through M, D1, RL and G. So that the output waveform is +ve voltage.

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ii. Full-wave Rectifier

During -ve half-cycle :The diode D1 is reverse bias, diode D2 is

forward bias. Diode D1 act as open circuit and D2 as closed circuit, the current flow through N, D2, RL and G. So that the output waveform is +ve voltage.

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Input & Output waveform of Full-wave Rectifier

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Bridge Rectifier

A bridge rectifier can be made using four individual diodes.

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No center tap results in full secondary voltage reaching the load.

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SUMMARY

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FILTER

FILTER

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- During the +ve quarter cycle of the input, the diode is forward biased allowing the capacitor to charge to within a diode drop of the input peak, as figure (a)

-When the input begins to decrease below its peak (b), the capacitor retains its charge and the diode becomes reverse biased

During the first quarter of the next cycle, the diode again will become forward biased when the input voltage exceeds the capacitor by approximately a diode drop, as ilustrated in (c)

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FILTER

A capacitor-input filter will charge and discharge such that it fills in the “gaps” between each peak. This reduces variations of voltage. This voltage variation is called ripple voltage.

The smaller the ripple voltage, the better the filtering action. The advantage of a full-wave rectifier over a half-wave is quite

clear. The capacitor can more effectively reduce the ripple when the time between peaks is shorter.

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The higher value of capacitor will remove ripple voltage more effective. The higher value of capacitor will take more times to discharge (Time constant longer).

  A larger capacitor will give less ripple. The capacitor value

must be doubled when smoothing half-wave DC. 

Time Constant, RLC1

Vk20

15

5

0t

62 84 10A

B

C

X

Z

C1

0.1uFC2

1uFRipple Voltage

Time Constant, RLC2

10

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Three (3) types of filter:

RC FILTERLC FILTERπ FILTER

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LC Filter

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VOLTAGE REGULATOR

The regulated DC output is very smooth with no ripple. It is suitable for all electronic circuits.

The main purpose was to reduce change to zero or at least to value that most minimum.

Types of voltage regulator circuits:i. Zener diodeii. Serial transistoriii. Integrated circuits

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ZENER DIODE’S VOLTAGE REGULATOR

Zener Diode will be operational as voltage regulator during reverse bias.

Zener Diode having own specially namely could arrange voltage if operating inside zener area. To operate in zener area, input voltage must be greater than zener voltage load resistance does not cause current become zero.

Rectifier Filter Dz

R

RL

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Transistor that connected serial with load would be handle entry voltage value that permitted to product.

Voltage take out will sampled by one circuits which supplies reverse biased voltage that will be compared with reference voltage

If output voltage decrease, addition in VBE will cause transistor to generate more value stream that are going to rise output voltage and maintain output voltage.

Zener diode going to act as reference voltage. Process that same happens if output voltage increase. Transistor would be minimized value stream, cause output voltage decrease and maintain output voltage.

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IC Voltage Regulator

Eliminates ripple by setting DC output to a fixed voltage

Voltage regulator ICs are available with fixed (typically 5, 9 and 12V) or variable output voltages.

Many of the fixed voltage regulator ICs have 3 leads and look like power transistors, such as the 7805 +5V 1A regulator shown on the right. They include a hole for attaching a heatsink if necessary.

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Linear DC Power Supply Complete Circuit

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TRANSFORMER

RECTIFIER

FILTER

VOLTAGE REGULATER

VOLTAGE DIVIDER

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Transformer + Rectifier + Filter + Regulator

The regulated DC output is very smooth with no ripple. It is suitable for all electronic circuits.

Filter

Filter