DAR ES SALAAM INSTITUTE OF TECHNOLOGY

DEPARTMENT OF ELECTRONICS AND TELECOMMUNICATION

ETT 05213: ELECTRONIC SYSTEM COMPUTER AIDED DESIGN

PILI JAMES 140627193619.

TELECOM 1.

AIM

To design a Dual DC power supply of DC output power Po=290 mV, DC output

voltage Vo(avg) =+12V and -12V, with AC mains Vs=240V/60Hz with the following

functionality;

I. On/Off switch

II. Current flow direction

III. Output results displayed on Oscilloscope

IV. Output results displayed on Graph

V. Voltmeter/Ammeter measurement.

THEORY.

Bellow is the block diagram for the Regulated DC Power Supply.

The operation of power supply circuits is built using AC power source , step down

transformer, rectifiers, filters and then voltage regulators.

An Ac voltage from Ac source is stepped down by transformer .Then the bridge rectifier

circuit convert the Ac voltage into pulsating DC voltage. The capacitor filter circuit filter

out the ripple voltage from rectified voltage to get non-pulsating voltage. In order to get

the steady DC voltage regardless the variation in AC power source ,the Voltage regulator

is required for the same purpose. LM7812 LM7912 regulators are used to get the fixed

+12V and -12V output voltage respectively.

MATHEMATICAL ANALYSIS

The following data were used during designing

1.AC Power Source.

The AC source should be 240V/60Hz.

Editing the AC source properties:

Amplitude=maximum voltage(vm)

Vm=1.414Vrms

Vm=1.414×240V=339.36V

2. Centre tapped transformer.

Transformer output = 30V.

Transformer ratio = Vp/Vs =240/30 =8:1

Editing transformer properties in proteus.

Parameters to consider are ;

1.Primary inductance Lp

2. Secondary inductance Ls

3. Primary voltage VP

4. Secondary voltage Vs

Then;

Lp =( Vp/Vs)2×Ls let Ls =1, Lp =(240/15)2 ×1 = 256.

NOTE. Vs is taken as 15V since it is the centre tapped transformer and Ls is

edited as 2 due to the same reason. The results is as shown in the diagram

bellow

V1VSINE

TR1

TRAN-2P3S

SW1

SW-SPST

AC Volts

+15.0

3. Bridge Rectifier.

The output from bridge rectifier is the pulsating DC voltage and is calculated

as follows;

Vdc = 0.9Vrms

= 0.9×15v

= 13.5V

NOTE; Vrms is taken as 15V since is from the half part of the centre tapped

transformer.

Therefore the calculated output value is 13.5V but the measured value is

14.4V as shown bellow.

4. Selecting capacitor as the low pass filter.

BR1

BRIDGE

R30.68k

AC Volts

+14.4

Given that; Output power Po = 290mW.

And Po = Vo2/RL, where ,RL is the load resistance. Then,

RL = Vo2/po

RL = 13.52/290mW = 628.45Ω or 0.628kΩ.

R10.68k

C214.0uF

From, Vdc = Vm÷[1+(5/RLC)] for filtration.then,

C = 5Vdc ÷ [RL(Vm-Vdc)] where ,

C is the capacitance in microfarads and

RL is the load resistance in kilohms.

Also ,Vm = 1.414Vrms.

Vm = 1.414×15V

= 21.21V . then,

C =( 5×13.5)÷[0.628(21.21-13.5)]

= 67.5÷4.842

= 13.94uF.

From the calculated value of resistor and capacitor, then the selected standard

value for RL is 0.68kΩ and for capacitor is 14.0uF.

5. Selecting DC voltage regulator.

The DC voltage regulators are LM7812 and LM7912 for +12V and -12V fixed

voltage respectively. C2 and C4 are for noise isolation.

C114.0uF

R10.68k

VI1

VO3

GN

D2

U17812

C214.0uF

VI2

VO3

GN

D1

U27912

Volts

+12.0

Calculating DC output current.

From , current I = V/R,

= 12/0.68k = 0.02A.

C314.0uF

C414.0uF

R20.68k

VI2

VO3

GN

D1

U27912

Volts

-12.0

The measured values are shown bellow;

R10.68k

C214.0uF

Amps

+0.01

C414.0uF

R20.68k

Amps

-0.01

List of components used are as follows

S/N Components Quantity Values

1 AC power source 1

2 Centre tapped transformer 1 3 Bridge Rectifier 1 4 Capacitors 4 14.0uF 5 Voltage Regulators 2 LM7812, LM7912

6 Resistors 2 0.680k 7 Switch(SPST) 1 8 DC Voltmeter 1 9 AC Voltmeter 1

THE PROPOSED CIRCUIT DIAGRAM

Results from oscilloscope.

V1VSINE

TR1

TRAN-2P3S

SW1

SW-SPST

BR1

BRIDGE

C114.0uF

R10.68k

VI1

VO3

GN

D2

U17812

C214.0uF

C314.0uF

C414.0uF

R20.68k

VI2

VO3

GN

D1

U27912

+12Vo

-12Vo

Output from the transformer.

After rectification

After Filtration and voltage regulation respectively

The table for comparison between measured and calculated values

Quantity Calculated value Measured value Pulsating Vdc 13.5V 14.4V Output filtered Vdc 13.5V 13.4V Regulated output Vdc - +12V ,-12V Output dc current 0.02A 0.01A