AReS

PART7Basic Circuit of Semiconductor

Experiment 3 :Filter(Smoothing) Circuit

Theory

We’ve found out from previous experiment that the output voltage of rectifier changes according to time. In most rectification circuits, it is necessary to reduce the size of AC component that is small compared to average DC voltage or that of rectification output’s ripple. The device or circuit that executes this operation is called a filter.
There are three types of filter circuits that are used generally. They are; inductor input type(LC), capacitor input type(LCπ), and resistance-capacitor type(RCπ).

Fig.7-11 shows the inductor input type filter circuit.

fig7-11

Fig. 7-11Inductor Input Type Filter

The inductor L is connected in series to the load. The reverse electromotive force to the change of current occurs through the inductor and the flow of AC is cut off, and constant current is maintained through the load. Eventually, it becomes more constant voltage to the load. To make the output voltage more constant, the capacitor C shwos very small impedance to the ripple frequency.
The percentage of rippler voltage of inductor input type filter is expressed as below.

$V _{r} %= {144` TIMES 10 ^{4}} over {f ^{2} LC}$

f = ripple frequency
L = inductance [H]
c = capacitance [uF]

In full-wave rectification output, the ripple frequency is double the power frequency so the formula can be simplified as below.

$V _{r} %= {100} over {LC}$

Fig.7-12 indicates the capacitor input type filter circuit.

fig7-12

Fig. 7-12Capacitor Input Type Filter

The capacitor C is connected in parallel to the laod, and charged in the maximum value of the input voltage. If the load is not infinite, the capacitor discharges the charged voltage through the load. The time constant of discharge is bigger than the period of the ripple, so the output voltage is more constant to the load.
The percentage of ripple voltage is as below.

$V _{r} %= {10 ^{8} sqrt {2}} over {2 pi f _{r} R _{L} C} = {2245 TIMES 10 ^{4}} over {f _{r} R _{L} C}$

Fig.7-13 shows a good circuit to improve the efficiency of this filter.

fig7-13

Fig. 7-13LC π Type Filter

The percentage of this circuit’s ripple voltage is as below.

$V _{r} pi %= {V _{r} %} over {10 ^{-6} TIMES (2 pi f _{r} ) ^{2} TIMES LC 1{}}-1$

In fig.7-13, the inductor can be replaced with the resistance R as in fig.7-14.

fig7-14

Fig. 7-14RC π Type Filter

The output voltage of RCπ filter is smaller than that of LC filter because of the voltage drop of the resistance.
The percentage of this circuit’s ripple voltage is as below.

$V _{r} pi %= {V _{r} % TIMES 10 ^{6}} over {2 pi f _{r} RC1}$

Experiment Process

fig7-15

Fig. 7-15LC Smoothing Circuit Experiment

1. Using Block b of M07 board, make a connection as in fig.7-15.

2. Input AC 7V to the positive power using AC power supply.

3. Draw AC input waveform in table 7-5 using the oscilloscope, and draw the output between R4’s both ends in the relevant column of table 7-5. Measure the ripple voltage and calculate and record its percentage.

tab1

Experiment 7-3.1 LC Smoothing Circuit Experiment

1.Connection(Block b of M07)

1.Circuit Connection

Connect between the right terminal of D4 of Block b and the terminal connected to the right of D2 with yellow line.

Connect between the left terminal of L of Block b and its left terminal with yellow line.

Connect between the upper terminal of R4 of Block b and its upper terminal with yellow line

Connect between the upper teriminal of C2 of Block b and its upper terminal with yellow line.

2.Power Connection

Connect between V1 terminal of Variable Power on M07 board and left terminal of D2 of Block b with red line, and between V2 terminal and the left terminal of D5 of Block b with black line.

Connect between COM terminal and the earthing terminal with black line.

3.Measuring Instrument Connection

Connect between A+ terminal of Signal Input CH A on front panel and the left terminal of D2 of Block b with red line, and between A- terminal and COM terminal with black line.

Connect between B+ terminal of Signal Input CH B on front panel and the upper terminal of R4 of Block b with red line, and between B- terminal and the earthing terminal with black line.

2.Wiring Diagram

flash

3.Measurement

1Choose variable power at Touch LCD panel and click Advanced 3 Phase AC Power tab.
Click button_arrow at the right of AC Voltage[%] V1 display and make it as 50%(7V).

Click button_arrow at the right of AC Voltage[%] V2 display and make it as 50%(7V).

Set up the phase of Phase Shift V2[DEG] as 180.

Click on to output V1, V2 voltages of Advanced3 Phase AC Power.
2Measure the input/output waveform.
Choose analog input on front panel.

In case of output waveform, measure by setting up the coupling as Ac and Volt/Div as 0.2V.

Draw the waveform in oscilloscope screen in the relevant column of table 7-5.
3After the measurement, click on red of Advanced 3 Phase AC Power to cut off the output.