DEPARTMENT OF BIOMEDICAL ENGINEERINGFACULTY OF ENGINEERING

UNIVERSITY MALAYA

Lab 8 : DC Power Supply And Rectification

Introduction

Direct Current (DC) is the unidirectional flow of electric charge, which always flowed in constant direction, distinguishing it from Alternating Current (AC). In order to obtain DC, a DC power supplies are needed. They usually are the combination of transformer and rectifier. By using this combination, we are able to convert AC line voltage into pulsating DC. As to reduce the resultant “pulsations” or ripple voltage, the output can be further filtered and maintain constant using voltage regulating networks.

This experiment consists of 3 parts. In Part 1, we will deal with the theory and design of the half-wave rectifier while Part 2 is trying to experiment out full-wave rectification process. After that, in Part 3, we will use a capacitor in order to minimize the ripple occurring in the “rectified” output.

Both half-wave and full-wave rectification processes are used to convert AC into DC. By using a diode or sets of diodes, we are able only letting the current flow in one direction. This happen because diode only let current flow through when its cathode is negative and its anode is positive. At the time which current passing through a “load” such as a resistor, it will only flow in one direction and produce a “voltage drop” in pulsating form.

There are some important parameters we needed for the experiment.

Main parameters:

1. Vm=√2Vac( N2N1

)(the peak value of the voltage applied to the diode)

2. Idc=Vdc/RL (the DC current flowing through the diode and the load RL)Parameters for Part 1:1. Vdc=0.318Vm(the average value of the pulsating dc voltage output)2. Vr(rms)=0.385Vm(the rms values of the ripple voltage superimposed on the DC voltage output

of the rectifiers)

3. r=Vr(rms)V dc

=1.21(for half-wave rectifiers)

4. Diode PIV=Vm(the peak-inverse-voltage across a non-conducting diode)Parameters for Part 2:1. Vdc=0.636Vm

2. Vr(rms)=0.305Vm

3. R=0.48(for full-wave rectifiers)4. Diode PIV=Vm\

Parameters for Part 3:The amount of ripple voltage superimposed on the DC voltage output of the rectifiers renders this voltage useless to power electronic equipment. In order to obtain pure DC or less ripple voltage “Vr”, filters are required. The most simple, costless, effective filter is a large value capacitor across the resistor RL. That capacitor will charge to Vm and will discharge gradually during the time the input voltage passes the peak to its minimum value.

1. Vdc=Vm-4.16C 1

Idc(value of Idcalready in mA, value of C1 is already in μF)

2. Vr(rms)=2.4C1

Idc(value of Idcalready in mA, value of C1 is already in μF)

3. %VR=VNL−VFLVFL

x100(percentage of voltage regulation)

4. Diode PIV≥Vm

Equipments1. 1 Oscilloscopes2. 1 Broadboard3. 1 Isolation Transformer4. 1 VCM or DMM5. Cables

Materials1. 1 1KΩ Resistor2. 1 100µF, 200V Electrolytic Capacitor3. 220µF, 200V Electrolytic Capacitor4. 4 Diodes Rectifiers(1N5404)5. Jumper

ProcedurePart 1: Half-wave Rectifiers1. The circuit componentsare set up as shown in the Figure 1. By assuming V AC=70V(rms), the

theoretical values of Vm, Vdc, Idc,and Vrcan be calculated. The values should be recorded into Table 8.1.

2. The meter polarities and scale settings must be selected carefully to obtain probable values of Idcand Vdcbefore applying the power.

3. The variac control should be CCW.4. The output of variac should be increase gradually until VAC reads 70V. After that, readings of

Idc and Vdc can be read and recorded into Table 8.1.5. After taking the readings, the oscilloscope should be connected across the RL.

6. Through step 5, waveform of Vdc will be shown on the oscilloscope. Value of Vm can be measured and recorded into the Table 8.1. Waveform of Vdc should be sketch.

7. The following equations can be verify:

Vm= Vdc0.318

Idc=Vdc/RL

Vr(rms)=0.385Vm

r=Vr(rms)V dc

Figure 8.1 Half Wave Rectifier

Part 2 : Full-wave Rectifiers1. In accordance with Figure 8.2, the values of Vm, Vdc, Idc, Vr(rms), and r can be calculated by

assuming Vac=70V(rms).2. The circuit is being constructed by referring to Figure 8.2. Position and polarities of the

diodes and the meters must be in the correct position. If the diodes are not connected properly, it may result to short circuit where could damage other components or devices.

3. The power applied to the circuit should be gradually increased while observing the Idcuntill values of Vac reached 70V(rms). The values of Vac can be determined by a DMM.

4. Values of Idc and the Vdc are read and recorded into Table 8.2.5. Oscilloscope is connected across the RL, and the value of Vm is measured and recorded.

Waveform of Vdc can be seen on the screen of the oscilloscope and is being recorded.6. The following equations can be verify:

Vm= Vdc0.636

Idc=VdcRL

Vr(rms)=0.305Vm

r=Vr(rms)Vdc

Figure 8.2 Full Wave Rectifier

Part 3 : Capacitor Input Filter1. By using the above equations and assuming Vac=70V(rms), RL=1kΩ, and C1=100μF, the values

of Vm, Vdc, Idc, Vr(rms), r, and %VR can be calculated.2. The circuit is set up as shown in the Figure 3. The meter polarities and scale settings must be

selected carefully to obtain probable values of Idcand Vdcbefore applying the power.3. The variac control should be CCW. The output of the Variac is gradually being increased until

Vac=70V. The values of Idc and Vdc are being recorded. The values of Vm can also be measured across the transformer secondary and its value is recorded.

4. By using oscilloscope in the AC mode, ripple voltage Vracross RL can be measured. The voltage Waveform is drawn and the measurement of peak to peak value of the ripple is being calculated by using the equations:

Vr(rms)=Vr( p−p)2√3

r=Vr(rms)Vdc

5. After that, remove the load RL from the circuit and measure the no-load voltage(VNL) across the C1 by using a DMM.The %Vr is calculated by using the following equation

%Vr=VNL−VFLVFL

x100

6. The supply internal resistance can also be calculated by this equation.

Rint=VmIdc

xRL

7. Step 1 to 7 is repeated by changing the 100μF to 220ΩF. Data of the experiment are collected.

Figure 8.3 Full Wave Rectifier (Capacitor Input Filter)

ResultTable 8.1 : Results for Part 1

Theoretical Experimental Percentage of Error

Vm 8.02V 8.01V 0.12%

Vdc 2.55V 3.30V 29.41%

Idc 2.55mA 0.51mA 80%

Vr(rms) 3.09V 3.08V 0.32%

r 1.21 0.93 23.14%

Given that( N2N1

) = 0.081

Calculations(Theoretical Part) Calculations(Experimental Part)

Vm=√2Vac( N2N1

) Vr(rms)=0.385Vm

=√2(70)(0.081) =0.385(8.01) =8.02V =3.08V

Vdc=0.318 Vmr=Vr (rms )V dc

=0.318(8.02) =3.083.30

=2.55V =0.93Idc=Vdc/RL

= 2.551000

=2.55mAVr(rms)=0.385Vm

=(0.385)(8.02) =3.09

r=Vr (rms )V dc

=3.092.55

=1.21

Table 8.2 : Results for Part 2

Theoretical Experiment Percentage of Error

Vm 8.02V 7.98V 0.50%

Vdc 5.10V 5.94V 16.47%

Idc 5.10mA 0.51mA 90.0%

Vr(rms) 2,45V 2.43V 0.82%

r 0.48 0.41 14.58%

Given that( N2N1

) = 0.081

Calculations(Theoretical Part) Calculations(Experimental Part)

Vm=√2

Vac( N 2N 1 )Vr(rms)=0.305Vm

=√2(70)(0.081) =(0.305)(7.98) =8.02V =2.43V

Vdc=0.636Vm r=Vr (rms)/Vdc=(0.636)(8.02) =(2.43)/(5.94)

=5.10V =0.41

Idc=Vdc/RL

=(5.10)/ (1000 ) =5.10mA

Vr(rms)=0.305Vm

=0.305(8.02) =2.45V

Table 8.3 : Results for Part 3

Vm Vdc Idc Vr(rms) %Vr

C1=100μF (Calculated) 8.02V 8.02V 8.02mA 0.19V -

C1=100μF (Measured) 8.02V 9.38V 1.15mA 0.028V 8.85%

Percentage of Error 0.0% 14.58% 85.66% 85.26% -

C1=220μF (Calculated) 8.02V 8.02V 8.02mA 0.087V -

C1=220μF (Measured) 8.02V 9.52V 1.17maA 0.013V 9.56%

Percentage of Error 0.0% 18.70% 85.41% 85.05V -

Given that( N2N1

) = 0.081VNL(C1=100μF)=10.21V VNL(C1=220μF)=10.43V

Calculations(Theoretical Part)

Given Vdc=Vm-4.16C 1

Idc --------------①

Idc=Vdc/RL -------------------②

ForC1=100μF ForC1=100μF

By substitute ②into①, By substitute ②into①,

Vdc=Vm-4.16C 1

(Vdc/RL) Vdc=Vm-4.16C 1

(Vdc/RL)

Vdc=8.02-(4.16100

)(Vdc/1000) Vdc=8.02-(4.16220

)

(Vdc/1000) Vdc-8.02=4.16x10−5Vdc Vdc-8.02=1.89x10−5Vdc

1.00Vdc=8.02 1.00Vdc=8.02Vdc=8.02v Vdc=8.02v

Idc=8.02/1000 Idc=8.02/1000Idc=8.02mA Idc=8.02mA

Vr(rms)=2.4C1

Idc Vr(rms)=2.4C1

Idc

Vr(rms)=( 2.4100

)8.02 Vr(rms)=( 2.4220

)8.02

=0.19V =0.087V

Calculations(Experimental Part)

Vr(rms)=2.4C1

Idc Vr(rms)=2.4C1

Idc

Vr(rms)=( 2.4100

)1.15 Vr(rms)=( 2.4220

)1.17

=0.028V =0.013V

%VR=VNL−VFLVFL

x100 %VR=

VNL−VFLVFL

x100

%VR=10.21−9.389.38

x100 %VR=

10.43−9.529.52

x100

=8.85% =9.56%

Since Rint=VmIdc

xRL,,

ForC1=100μF ForC1=220μF

Rint=VmIdc

xRL Rint=VmIdc

xRL

=( 8.021.15m

)x1000 =( 8.021.17m

)x1000

=6.97x106Ω =6.85x106Ω

Oscilloscope waveformHalf-wave Rectifiers

Voltage(V)

11.0 Period=19.0ms

Time(ms)

Full-wave RectifiersVoltage(V)

10.0 Period=9.0ms

Time(ms)Full-wave Rectifiers (Capacitor Input Filter, 100μF Capacitor)

Voltage(V)

Vr(p-p)=1.5V

Time(ms)

Full-wave Rectifiers (Capacitor Input Filter, 220μF Capacitor) Voltage(V)

Vr(p-p)≈0V

Time(ms)

DiscussionDeviation (Errors)1. From the data collected, we can see that there aredifferences between the experimental

values and theoretical values. This happens because of few reasons:(a) The experimental values of Vm and Vr (rms) are slightly lower than the theoretical values.

This might due to the ratio of N 2N1

is not exactly equal to 0.081. Thus, there will be an

error.(b) The resistance in the circuit is assumed to be zero for theoretical part. But in fact, in the

real situation, there is resistance in the circuit. The resistance of the circuit will further

increase when the temperature is increase. During the experiment, data cannot be obtained as soon as the circuit is on. Thus, the temperature of the circuit Is increase. Therefore, there will be differences for the experimental values of Vdc and Idc when compared to the theoretical values.

(c) There is current leakage in the cables and the crocodile clips wire. The wires used are not fully isolated. Thus, there is resistance inside the connecting wires. This result in the differences in data obtained compared to theoretical values.

(d) Besides, the electrolytic diode may affect the result as well. This will happen because the diode is not exactly ideal. Thus, the data obtained will be deviated from the theoretical values.

Precaution1. In order to eliminate the errors of the experiments, there are few precautions that

should take into consideration.(a) To reduce the resistance of the circuit, the circuit should be off all the time when no

data is required to take. This is to cool down the circuit as to reduce the resistance. Since resistance of the circuit is directly proportional to the temperature, when temperature is decreased, the resistance will surely to decreased, and this can reduce the errors of the data obtained.

(b) Since there are resistances in the connecting wires, thus we should reduce the usage of the connecting wires. We should try connected the apparatus and the electric devices as simple and efficient as we can.

(c) The experiment should be repeated several times. Through several experiments, we can obtain different set of data and we also can eliminate the wrong data. By getting the average values, the results should be less deviated from the theoretical values.

(d) The high voltage supply must be handle carefully in order to prevent the happened of short circuit.

(e) The electric components must be set up correctly as to prevent the happen of short circuit which could damage other components and apparatus.

ExplanationPart 1

1. From the experiment Part 1, we can see that the Alternating Current (AC) is being converted to Direct Current (DC). This happened because the present of a diode. A diode is an electric device where it only allowed current flow in one direction. When Alternating Current pass through the diode, there are only positive output and no negative output can be obtained. This process is call Half-wave rectification.

2. During the first current cycle, the current will flow in the direction of the diode. Thus, the current can pass through the diode and reached negative terminal. Therefore, there is positive output can be detected. This phenomenon is called forward-biased.

3. In the second current cycle, the current will flow in opposite direction. This is because the current is an alternating current. Since the flow of current now is opposite with the direction of the diode. Therefore, no current will pass through the diode. There is no negative output can be detected. This phenomenon is call reverse-biased.

Part 21. From the experiment Part 2, we can see that the oscilloscope diagram shows that the

current is undergoing full-wave rectification. Full-wave rectification is the process which there is input and output voltage of the circuit.

2. First of all, the current will flow from the positive terminal to the Diode A. After that it will pass through the resistor and then go through the Diode C. the current can flow back to the negative terminal. Thus, there is input voltage.

3. Secondly, the current will flow from negative terminal to the Diode B. Afterwards it will pass through the resistor and then go through the Diode D. The current will be able to flow back to the positive terminal. Thus, there is output voltage.

4. This phenomenon is knew as full-wave rectification, where the current produced will be in sinusoidal form.

Part 31. In Part 3, capacitor is used to smooth the output current. This is because the capacitor is

an electric device which can store charge. During the first cycle, the capacitor is charged as the current pass through it. When second cycle had come, it started to discharge. Therefore, the current will not drop to zero during the second cycle.

2. When a larger capacitor is used, the oscilloscope diagram shows that the current cycle is almost look like the direct current. Larger value of capacitance will increase the amount of charge it can be stored. Thus, capacitor can be used as a full-wave rectification.

3. Output current which almost look like the direct current is very useful. Electronics and devices require direct current to be able to function. If the electronics and devices using alternating current, the devices and the electronics cannot function well as the devices and electronics. The present of capacitor is to prevent the current from dropping back to zero.

ConclusionAs conclusion, the main objective of this experiment is achieved. The rectification processes are the process which can change alternating current to direct current with the help of diode as the rectifier. Full-wave rectifiers are better than half-wave rectifier. With the present of a capacitor with high capacitance, the converted current will almost like a straight line graph. Which mean the current produce will smoothen.

References1. http://en.wikipedia.org/wiki/Rectifier . Accesed on 23 September 20112. http://en.wikipedia.org/wiki/Direct_Current . Accessed on 23 September 20113. http://en.wikipedia.org/wiki/File:Types_of_current_by_Zureks.svg . Accessed on 25

September 2011.4. Hawkins Electrical Guide, 1917, Chapter 54: Rectifiers