AReS

PART10Power Supply Circuit

Experiment Purpose

1.Investigate characteristics of power supply circuit using bridge rectifier.
2.Investigate characteristics of voltage regulator through experiment.
3.Investigate characteristics of transistor of variable voltage regulator and power supply circuit using OP Amp method through experiment

Fig. 10-1AReS-EO-M10 Power Supply Module

Experiment 1 :Bridge Rectifier and Smoothing Circuit

Theory

Half-Wave Rectifier

When changing AC to DC, diode is used and this is called rectifier and the process is called rectification. The most basic way of rectification is the half-wave rectifying circuit in fig. 10-2. When the secondary voltage of transformer is + half cycle(V_AB is +), Diode D1 becomes forward bias, and it indicates very low resistance value to the voltage source so most of the secondary voltage is shown at the both ends of load resistance RL. Forward biased diode shows forward voltage drop of 0.5~1.0V in case of silicon, and of 0.2~0.6V in case of germanium. The voltage drop is mostly ignored to simplify the circuit interpretation, and especially when the supplied voltage is high, the forward voltage drop of the diode becomes very small proportion to the output voltage.

fig10-2

Fig. 10-2Half-wave Rectification Circuit

Fig. 10-2 (b) explains the operation of half-wave rectifier. Here, note that the output V_CD becomes 0 when the transformer voltage V_AB is minus(-). This is becomes the diode becomes reverse bias(the positive polarity is added to the negative polarity). Ideally, this is same as the open circuit. The average DC voltage(V_dc) is same as 0.318 times of the maximum value(0.318 = 1/π). Most voltmeters indicate average value so it indicates 0.318 of the maximum voltage in case of half-wave circuit. However, to calculate the power, effective value should be used. The effective voltage of half-wave rectification circuit is 0.5 times of the maximum value.

In case of half-wave,

V_dc=V_av=1/π V_P=0.318V_P

V_rms=0.5V_P

These two way of voltage indication can be confusing. Fortunately, in DC we normally use, the average value and effective value are almost same so there is no need to worry. The current earned by dividing the average voltage of load by the load resistance is called the average voltage I₀.

I_o=I_av=V_av/R_L

In case of forward bias, the voltage drop is very small. However, In case of reverse bias, the maximum input voltage is appeared as the voltage drop at both ends of the diode. This is called peak reverse voltage(PRV). All diodes have maximum permitted PRV which cannot be exceeded, and if it is exceeded, the element will be burnt. In case of the reverse biased diode, the diode voltage diode V_AC of the diode in fig. 10-2(b) follows V_AB so the diode has very big resistance value. Also, note that when D1 is forward biased, the voltage drop(V_AC) is not “0” and it has small defined value. This is the forward voltage drop of diode, which is smaller than 1V.

Full-Wave Rectifier

Generally, more useful and effective way to transform AC to DC is using both + and - range of AC input signal. There are two circuits used for this purpose, and one of them is indicated in fig. 10-3. This method uses whole input wave form as DC input and is known as full-wave rectification.

The center-tap rectifier in fig. 10-3 uses secondary coil that has the center-tap. If the polarity of voltage is same as in the fig., the positive electrode has + polarity to the negative electrode, so D1 is forward biased and conducted, while D2 is reverse biased and non-conducted. Therefore, only D1 produces electric current to the load.

fig10-3

Fig. 10-3Center-tap Rectifier

In the next half cycle of AC, the polarity of the secondary voltage of transformer is reversed so the situation becomes the opposite. Therefore, D1 is reverse biased, D2 is forward biased and D2 produces electric current to the load. Each diode is conducted only during the half cycle(half cycle in turn) so the load current which is double of half-wave rectification current is produced.

The output wave form of this is indicated in fig. 10-3. Here, note that the frequency is actually doubled at the output. This is because the period T of the output wave form is 1/2 of the AC input signal. Remember that the frequency is the reciprocal of the cycle (f = 1/T).

In most DC power supplies, the center-tap circuit was the most universal full-wave rectification circuit until the tube diode was replaced with silicon diode. However, with the advent of silicon diode which is cheap, credible and small, bridge circuit became the most universal form.

fig10-4

Fig. 10-4Input and Output of Full-wave Rectifier

That is because the size of transformer needed to gain the output which is as big as that of center-tap circuit can be reduced. In the center-tap circuit, during each half cycle of + and -, the electric current flows in turn at the half of secondary part of the transformer.

Bridge Rectifier

Compared to half-wave rectifier, the bridge rectifier in fig. 10-5 can flow double DC load current with same transformer voltage. In the half cycle in which the transformer voltage V=V_ac is +, the electric current flows in the direction indicated in fig. 10-5(c), and in the half cycle in which V is -, it flows in the direction indicated in (d), so the electric current of same direction always flows at R_L. Therefore, this kind of rectifier is called full-wave rectifier, and the electric current is defined as below.

I_dc=〖2V〗_m/〖πR〗_L =〖2I〗_m/π

Meanwhile, the shortcomings of bridge rectifier are that it needs 4 diodes, and that power consumption occurs at two diodes in both + and - cycles, and that the load voltage is as low as the voltage drop of two diodes.

fig10-5

Fig. 10-5Bridge Full-wave Rectifier

Smoothing Circuit

To change AC as DC, after rectified by the diode, the electric current must go through low pass filter(smoothing circuit). In other words, to make the rectified pulsating current as DC without ripple, the pulsating current should be removed by smoothing circuit. Below are most commonly used methods and their strengths and weaknesses.

1.Smoothing Circuit Using Choke and Condenser

This method has lower output voltage fluctuation rate by the load than RC Smoothing method and has higher output efficiency. This is because the DC resistance of choke is extremely small so the DC voltage drop by load current is nearly 0 and only the voltage fluctuation rate for the pulsating part exists. However, the AC reactance of choke is big so it can hugely decrease the ripple.

Reference

Voltage Fluctuation Rage= (No Load Output voltage-Load Output Voltage)/(Load Output Voltage)×100%

a. In the smoothing circuit using choke(inductor), the condenser input type can gain the DC voltage which is higher(less than √2 times) than AC voltage of rectifier input. However, choke input type rather outputs slightly lower voltage(less than 1/√2 times).

b. Choke input type has smaller output voltage fluctuation rate than condenser input type, so it is used when voltage stability is needed or in case of DC power supply circuit of over thousands volt which cannot use constant voltage.

c. Smoothing circuit using choke has higher output efficiency than the circuit using the resistance. This is because the choke indicates big reactance to the ripple but only the DC resistance of coil affects DC so pure DC voltage drop by the load current is extremely small.

d. When the values of L and C or of R and C are mutually proper, maximum ripple removal efficiency and output efficiency without waste can be raised.

Reference

In case of full-wave,

Ripple Rate= (Ripple Voltage(V_rms))/(DC Output Voltage(V_AV))×100%

Here,

Average Voltage(V_AV )=Maximum Voltage(V_PK)×2/π

Effective Value Voltage(V_rms )=Maximum Voltage(V_PK)×1/√2

2. Smoothing Circuit Using Resistor and Condenser

RC smoothing circuit is cheap and the attaching location is small. However, when the load current is big, the voltage drop at the series filter resistance of the circuit occurs frequently so the voltage fluctuation rate is big and the output efficiency is low. Therefore, it is used in the small load current circuit of under hundreds mA.

One thing to be noteworthy at RC smoothing circuit is that the rate power of R should be reflected in the design. That is, when the current flowing at series resistor is I_R, the rate power is W=R∙I_R2 and in the actual usage, it becomes double of this value.

a. In the same voltage circuit, as the load current gets bigger, the condenser capacity rather than smoothing choke or resistance should be increased. This is because the choke or the resistance is applied to the ripple rate only and it is worked as disadvantage to the voltage fluctuation rate according to the load current increase..

b. The resistance smoothing circuit is used for the load of small power consumption for which the power efficiency do not have to be considered. This is because the circuit is minimized.

c. In the smoothing circuit composed of L and C, if the value of L is increased and that of C is increased than proper L and C, the output voltage fluctuation rate increases.

fig10-6

Fig. 10-6Smoothing Circuit

Reference-1

Acceptable Design of L and C Value of Smoothing Circuit which is composed of L and C(C is the minimum value)

Design of L(Inductance) Value

X_L=1/K∙(E(Choke Input Volate V))/(I(Current Flowing through Choke A))

Here, if L is calculated,

L(Henry)= 1/K∙(E(Volt))/(2πf∙I(Ampere))≒(E(Volt))/(I(mA))

• Design of C(Capacitance)

C(uF)≒[(I_L t)/〖0.5r〗_R ]∙〖10〗^4

Here, I_L = Load Current (A), r_R = Ripple Rate(%), Pulsating Cycle t is 60Hz full-wave
current so

t= 1/120Hz=8.3×〖10〗^(-3) sec.

Reference-2

Acceptable Design of R and C Value of Smoothing Circuit which is composed of R and C(C is the minimum value)

• Design of Resistor Value

R(Ω)=K_R∙(E(R Input Voltage V))/(I(Load Current A))

Here, K_R=25/100 This means that the voltage drop between both ends of smoothing resistance R is set up as 25% of R input voltage.

• Design of Capacitance Value

C(uF)≒[(I_L∙t)/〖0.12r〗_R ]∙〖10〗^4

Here, I_L = Load Current (A)

r_R = Ripple Rate (%)

t= 1/120=8.3×〖10〗^(-3) sec.

Experiment Process

fig10-7

Fig. 10-7Bridge Rectification Circuit

tab1

Experiment 10-1.1 Bridge Rectification Circuit Experiment (In Circuit-1 of M10, compose a circuit as in fig. 10-7.)

1.Connection(Circuit-1 of M10)

1.Circuit Connection

Capacitor C1 Connection; Connect between 1g terminal and 1k terminal of Circuit-1 with yellow line.(Wiring diagram. Through 1 step)

2.Power Connection

Connect between Variable Power V1 terminal on the left of M10 board and 1a terminal of Circuit-1 with red line, and between COM terminal and 1b terminal with black line.(Wiring diagram. Through 3 step)

3.Measuring Instrument Connection

Voltmeter Connection

Input Voltage Measurement Connection: - Connect between 1j terminal of Circuit-1 and A+ terminal of Signal Input CH A on the front panel of Multimeter with red line, and between 1n terminal and A- terminal with black line.(Wiring diagram. Through 5 step)

2.Wiring Diagram

flash

3.Measurement

1Choose variable power at Touch LCD panel, and click 3 Phase AC Range Power tab.
Click arrow at the right of AC Voltage indication window and make it as 12.0V.

Click on and apply the output of 3 Phase AC Range Power to the input of Circuit-1.
2Measure the output wave form and effective voltage with no-load.
Choose analog input on the front panel.

Draw the wave form indicated in Oscilloscope screen in table 10-1.

Choose Volt & Ampere Meter tab, click voltage , dc , rms at CH A and record the measured output voltage in the relevant column of table 10-1. (CH B is not used.)
3Measure the output wave form and effective voltage with load.
Load Connection: Connect between 1i terminal and 1m terminal of Circuit-1 with yellow line.(Wiring diagram. Through 6 step)

Draw the wave form indicated in Oscilloscope screen in table 10-1.

Record the measured value in Volt & Ampere Meter screen in the relevant column of table 10-1.s

After the measurement, click variable power and click on red to cut off AC 12V
4Measure by connecting capacitor C2 additionally.
Capacitor C2 Connection; Connect between 1h terminal and 1l terminal of Circuit-1 with yellow line.(Wiring diagram. Through 6 step)

Execute the measurement process 2), 3) above and record the measured result in the relevant column of table 10-1.(Wiring diagram. Through 7 step)

In case of no-load, remove the connection between 1i terminal and 1m terminal of Circuit-1 and measure

Wiring Diagram

flash

After the measurement, choose variable power and click on red to cut off AC 12V output.

tab2

Experiment 10-1.2 Bridge Rectification Circuit Experiment (In Circuit-1 of M10, compose a RC smoothing circuit as in fig. 10-8.)

fig10-8

Fig. 10-8RC Smoothing Circuit

1.Connection(Circuit-1 of M-10)

1.Circuit Connection

RC Smoothing Circuit Connection; Connect between 1j terminal and 1o terminal of Circuit-1 with red line, and between 1n terminal and 1p terminal with black line.(Wiring diagram. Through 2 step)

2.Power Connection

3.Measuring Instrument Connection

Voltmeter Connection

Input Voltage Measurement Connection: Connect between 1q terminal of Circuit-1 and A+ terminal of Signal Input CH A on the front panel of Multimeter with red line, and between 1s terminal and A- terminal with black line.(Wiring diagram. Through 6 step)