LINEAR DC POWER SUPPLY

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i. All electronic circuits need power source to function.

ii. Power Supply Unit is a device used to convert, regulate and transmit the required power to the circuit to be operated.

iii. Electronic circuits made up of semiconductors need specific value of Direct Current (DC) voltage.

iv. Batteries are common DC voltage source for electronic equipment especially portables like cell phones and iPods.

v. Most non-portable equipment uses power supplies that operate from the AC power line but produce one or more DC outputs.

Click to “Play” the Video

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POWER SUPPLY CHARACTERISTICS

i. The input is 240volt 50Hz Alternative Current (AC).

ii. A power supply unit converts the AC into DC and provides one or more DC output voltages.

iii. Some modern electronic circuits need two or more different voltages.

iv. Commonly required DC voltages are 48, 24, 15, 12, 9, 5, 3.3, 2.5, 1.8, 1.5, 1.2 and 1 volts.

v. A good example of a modern power supply is the one inside a PC that furnishes 12, 5, 3.3 and 1.2 volts.

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The operation of a DC power supply circuit

The importance of DC power supply units in electronic appliances

i. Power supply provide one or more fixed voltages with sufficient current to the operating circuit.

ii. Electronic devices such as transistor radio, television and video cd players operate fully or partly on DC power supply in the range of 0 to 24V.

iii. The successful operation of the device depends on the proper function of the DC power supply. The power supply tries to provide a smooth, constant DC voltage, as required by an electronic device.

iv. Major electronic appliances have built-in electronic circuits that take power from a wall socket and convert it to the form and voltages required by the other internal circuits of the appliance.

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Block Diagram of DC Power Supply

To load

VDC

t

Regulated smooth D.C Supply

Regulator

A.C. Mains 230V, 50Hz A.C. voltage

VAC

t

Rectifier Pulsating typeVDC

t

FilterUnregulated

VDC

t

Voltage Divider

Transformer VAC

t

Low A.C. voltage To step down high voltage AC to low voltage AC

To step down high voltage AC to low voltage AC

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

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

To smoothen the DC from varying greatly to a small ripple

To smoothen the DC from varying greatly to a small ripple

To eliminate ripple by setting DC output to a fixed voltageTo eliminate ripple by setting DC output to a fixed voltage

To divide DC voltage which are different needed value circuit

To divide DC voltage which are different needed value circuit

The function for each block DC Power Supply

Click on the Block to display its function

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Transformer

TransformerTransformer

Symbol

Input: High Voltage AC Output: Low Voltage AC

Transformer

i. A transformer is commonly used to step down the input AC voltage to a lower level.

ii. For example, a transformer with a turns ratio of 10 to 1(10:1) would convert the 240 volt 50 Hz input sine wave into a 24 volt sine wave.

iii. The low voltage AC output is suitable for lamps, heaters and special AC motors.

iv. It is not suitable for electronic circuits unless they include a rectifier and a smoothing capacitor.

Symbol Transformer

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Transformer + Rectifier

TransformerTransformer RectifierRectifier

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Transformer + Rectifier

i. There are several forms of rectifiers but all are made up of diodes.

ii. The rectifier converts the AC sine wave into a pulsating DC wave.

Input: Low Voltage AC Output: Varying DC

Diodes

Input: Low Voltage AC Output: Varying DC

Input: Low Voltage AC Output: Varying DC

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Transformer + Rectifier

i. A single diodes rectifier produces half-wave varying DC.

ii. Rectifier with two or four diodes will produce full-wave varying DC.

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Transformer + Rectifier + Filter

TransformerTransformer RectifierRectifier FilterFilter

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Transformer + Rectifier + Filter

i. Rectifier produces a pulsating DC, unlike a constant steady value that is produced by a battery.

Output: Smooth DC

Output: Varying DC

Pulsating (varying) DC outputPulsating (varying) DC output Steady value DC outputSteady value DC output

Input: Varying DC Output: Smooth DC

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Transformer + Rectifier + Filter

ii. A filter is then used to remove the pulsations and create a constant output.

Output: Varying DC

Pulsating (varying) DC outputPulsating (varying) DC output Constant / Smooth DC outputConstant / Smooth DC output

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Transformer + Rectifier + Filter

iii. The most common filter is a large capacitor.

iv. The filtered DC output have small ripples. It is suitable for most electronic circuits.

Input: Varying DC Output: Smooth DC

RectifierRectifier

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Transformer + Rectifier + Filter+ Regulator

TransformerTransformer FilterFilter RegulatorRegulator

Input: Smooth DC Output: Regurated DC

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Transformer + Rectifier + Filter+ Regulator

i. The regulator is a component that helps to maintain a fixed or constant output voltage.

RegulatorRegulator

Input: Smooth DC Output: Regurated DC

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Transformer + Rectifier + Filter+ Regulator

ii. Changes in the load or the AC line voltage will cause the output voltage to vary.

Input: Smooth DC Output: Regurated DC

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Transformer + Rectifier + Filter+ Regulator

iii. Most electronic circuits cannot withstand the variations since they are designed to work properly with a fixed voltage.

iv. The regulator fixes the output voltage to the desired level and maintains that value despite any output or input variations.

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

Input: Smooth DC Output: Regurated DC

DC output is very smooth with no ripple

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Types of Rectifier

RectifierRectifier

Half-wave rectifier

Half-wave rectifier

Full-wave rectifier

Full-wave rectifier

Bridge rectifierBridge rectifier

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Types of Rectifier

RectifierRectifier

Full-wave rectifier

Full-wave rectifier

Bridge rectifierBridge rectifier

Half-wave rectifier

Half-wave rectifier

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HALF-WAVE RECTIFIER

Positive Cycle Operation

During the POSITIVE cycle of the input signal:

i. diode is forward biased.

ii. diode act as a CLOSED switch

iii. current flows through diode

The voltage drop at RL is the same as input signal (at positive cycle magnitude)

The output voltage keeps the shape of the input voltage

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HALF-WAVE RECTIFIER

Negative cycle operation

During the NEGATIVE cycle of input signal:

i. the D diode is reverse biased

ii. Diode act as OPEN switch

iii. no current flow through diode

The voltage drop at RL is zero.

VL = 0 V

I = 0 ANEGATIVE Cycle

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HALF-WAVE RECTIFIER

Calculation of output voltage Output voltage for the half-wave rectifier circuit is for the positive cycle only.

Example : Vin = 20 V Vout = Vin – 0.7V(silicon diode)

= 20V – 0.7V = 19.3V The output signal frequency is same frequency as input.

Vout Vin = 20 V

Vin = 20 V

Vout = 19.3 V

0.7V(silicon diode)

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Types of Rectifier

RectifierRectifier

Half-wave rectifier

Half-wave rectifier

Bridge rectifierBridge rectifier

Full-wave rectifier

Full-wave rectifier

FULL-WAVE RECTIFIER

i. A full-wave rectifier is exactly the same as the half-wave rectifierii. It use 2 diodes, D1 and D2

iii. It allows unidirectional current flow through the load during the entire sinusoidal cycle (as opposed to only half the cycle in the half-wave)

Full-wave Rectifier Output Signal

AC input Signal

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FULL-WAVE RECTIFIER

Positive cycle operation

During the positive cycle:

i. the M terminal becomes positive, G becomes zero (earth) & N becomes negative.

ii. Diode D1 is forward biased while diode D2 is reverse biased

Thus, the current will flow along M,D1,A, RL,G.

A

B

G

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FULL-WAVE RECTIFIER

Negative cycle operation

During the NEGATIVE cycle:

i. M terminal becomes negative, G becomes zero (earth) and N becomes positive.

ii. D2 is forward biased, while D1 is reverse biased

iii. Thus , the current will flow along N, D2, C, B, RL, G.

OUTPUT wave is always POSITIVE because the current that flows through RL (load) is same direction for both the positive and negative cycle.

c

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FULL-WAVE RECTIFIER

Output signal negative cycle

Output Voltage is formed at both cycle.Frequency of output is twice of the input frequency.

Output Voltage

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Types of Rectifier

RectifierRectifier

Half-wave rectifier

Half-wave rectifier

Full-wave rectifier

Full-wave rectifier

Bridge rectifierBridge rectifier

BRIDGE RECTIFIER

Positive Cycle OperationBridge rectifier use four diodes, D1, D2, D3 and D4During the POSITIVE input cycle :i. D1 and D2 diodes are forward biased

ii. D3 and D4 are reverse biased

The current will flow along G, D, D2, C, N, M, A, D1,B, RL,G.

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BRIDGE RECTIFIER

Negative cycle operationDuring the NEGATIVE input cycle:

i. D3 and D4 diodes are forward biased

ii. D1 and D2 are reverse biased.

The current will flow along G, D, D3, A, M, N, C, D4,B, RL,G.

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BRIDGE RECTIFIER

Output voltage

33/50Notes: PIV is Peak Inverse Voltage.

Comparison between two diodes rectifier and four diodes rectifier

FilterFilter

( Vr )p-p

Va.t.

Va.t.

V

t

V

t

Figure 5.5.2 : Pure DC voltage Figure 5.5.3:Ripple DC voltage

The output of rectifier circuit is the pulsating DC voltage . The resulting pulsating dc signal smooth the output voltage, we used filter circuit The main function of filter is to convert pulsating DC voltage to the ripple DC

voltage or as good as pure DC voltage(Figure 1) and free from pulse which is similar to the value obtain by battery.

However, not all the filter can produce pure DC voltage .The output voltage from filter normally is not really stable. This voltage is the mixture of DC voltage with the AC voltage (ripple voltage) as shown in figure 2.

Types FilterTypes Filter

• The best filter circuit is the filter that can decrease as much as the value of Vr p-p produce by ripple DC voltage .The other type of filter are

1. Capacitor filter2. RC filter3. LC filter4. Π- Type Filter

1.5 RC filter

1.6 LC & π filter

1.7 Voltage Regulator1.7.1 The voltage regulator circuits construct by using Zener

diode

1.7 Voltage Regulator

1.7.3 The voltage regulator circuits construct by using Serial transistor

1.7 Voltage Regulator1.7.3 The voltage regulator circuits construct by using Integrated

Circuits

1.8 Voltage DividerVoltage divider rule is a simple way of determining the output voltage across one of two impedances connected in series

1.9 1.9 The schematic diagrams of a simple power supply unit which includes full-wave rectifier, filter and Zener

diode voltage regulator.

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1.9 1.9 The schematic diagrams of a simple power supply unit which includes full-wave rectifier, filter and IC voltage regulator.

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1.9 1.9 The schematic diagrams of a simple power supply unit which includes Bridge rectifier, filter and IC voltage regulator.

EE3110 Oscillator 48

DC Linear Power Supply

Rectifier Circuit

Filter Circuit

Regulator Circuit

Voltage Divider

Transformer

Halfwave Rectifier

Fullwave Rectifier

Bridge Rectifier

Capacitor filter

RC filter

LC filter

π filter

Zener Diode

Serial Transistor

Integrated Circuit (IC)

IC LM78XX

IC LM79XX

AC 230V

DC