EXPERIMENT 6 BRIDGE RECTIFIER AND RECTIFIER WITH FILTER CAPACITOR (SIMULATION AND EXPERIMENT) Reference – Sedra and Smith (Chapter 4) I. OBJECTIVES- We want to covert ac voltage into dc using bridge rectifier circuit and measure ripple voltage. Subsequently, we would reduce the ripple voltage by putting a suitable capacitor in parallel with the resistor. The theoretical values of ripple voltage (V r), dc (average) value of the output (VO) and conduction angle (∆t) would be compared to simulation and experimental result. Theory – A bridge rectifier circuit, shown in Fig. 1 below rectifies ac into a dc voltage. In II. the positive cycle of the waveform, diodes D1 and D2 conduct. In the negative cycle of the waveform, diodes D3 and D4 conduct. The output V0 is measured across R. However there are two limitations. The ripple voltage is large. The mean value of the signal is (dc value) is reduced by almost half. Fig. 1(a) – A bridge rectifier circuit, (b) Input and output waveform For example if the amplitude of the AC signal is Vs, then the ripple voltage is given by the relation, = − (1) The dc component is given by the relation = ⁄ − 39 (2) Here is the typical diode drop of about 0.7V. In order to reduce Vr and increase VO, usually a capacitor is attached in parallel with R. Fig. 2 shows this circuit. Assume that the time-period of the signal is T. D3 D1 VOFF = 0 VAMPL = 10V FREQ = 40 Hz C R Vs D2 D4 Fig. 2 - Bridge rectifier with filter capacitor For CR>>T/2, Equations (1) and (2) change to the following. = (3) = − (4) Furthermore, the conduction angle is given by ∆ = √ ⁄ (5) Simulation work – D3 VOFF = 0 V D1 R VAMPL = 10V 4.7k FREQ = 40 Hz D2 D4 Fig. 3 Bridge rectifier circuit 1. Assemble the circuit in Fig. 3 using PSPICE. For the diodes choose D1N4002 from EVAL library. Please make sure that the Ground point is set to 0. For the voltage source, set the values as shown. 2. Click on PSpice at the top of Capture screen and then click “New Simulation Profile”. A new window opens. In this window select “Time Domain (Transient)” analysis. Set the following values. Run to time = 100ms Maximum step size = 10us Put a tick mark on “Skip the initial transient bias point calculation” 40 Click on “Apply” and click OK to close this window. 3. Run this simulation. A new screen opens and you can see the rectified wave-form. Using the curser, measure the peak of the rectified signal. Verify that it is Vp-2VD. 4. Now add a 10μF capacitor in parallel with the resistor R. The resulting circuit is shown in Fig. 4. D3 VOFF = 0 V D1 V R VAMPL = 10V 4.7k C FREQ = 40 10u D2 D4 Fig. 4 Bridge rectifier circuit with filter capacitor 5. Click on PSpice at the top of Capture screen and then click “New Simulation Profile”. A new window opens. In this window select “Time Domain (Transient)” analysis. Set the following values. Run to time = 100ms Start saving data after = 50ms Maximum step size = 10us Put a tick mark on “Skip the initial transient bias point calculation” Click on “Apply” and click OK to close this window. 6. Run this simulation. You will see that a new window opens that gives you the result. Now measure ripple voltage (Vr) and the conduction time (∆ ) using the cursers. The icon for the curser is located on the simulation window in the menu bar. There are many of them and you should play with them to familiarize yourself. 7. Now change the capacitor C to 22 μF and repeat steps 5 and 6. 8. Finally, change the capacitor C to 47 μF and repeat steps 5 and 6. 9. Using equations (3) and (5) calculate Vr and ∆ for the 3 different values of the capacitor and summarize your result in the following table. 41 C Vr(Theoretical) Vr(Simulation) ∆ (Theoretical) ∆ (Simulation) 10μF 22 μF 47 μF Experimental Work – The chip DF01M is shown below. It consists of 4 diodes arranged in such a manner that it is a bridge-rectifier. This chip is shown in Fig. 5. The two points denoted by tilde (~) are meant for applying ac signal. The other two points shown as ‘+’ and ‘-‘ are meant for load (RL and C). Fig. 5 Bridge-rectifier chip Follow the following procedure. 1. Assemble the circuit in Fig 3 using chip shown above. You don’t need to use 4 separate diodes. Load RL of 4.7 KΩ should be connected between the +ve and –ve terminals. The oscilloscope should be connected to the same two terminals. 2. The Function generator (FG) should have an amplitude and frequency shown in Fig. 3 and applied to the two terminals denoted by tilde (~). Observe the output under DCcoupling mode of the oscilloscope and verify that the output has a peak Vp-2VD. Don’t try to observe input and output simultaneously on the oscilloscope as this creates a grounding loop and thus introduces error. 3. Attach a 10μF capacitor in parallel to RL (as in Fig. 4) and observe the output on the oscilloscope in the AC-coupling mode. Now measure ripple voltage (Vr) and the conduction time (∆ ) using the cursers. There are two cursors, voltage and time. Measure the mean-value of the output in DC-coupling mode. 42 4. Attach a 22μF capacitor in parallel to RL and observe the output on the oscilloscope in the AC-coupling mode. Now measure ripple voltage (Vr) and the conduction time (∆ ) using the cursers. Measure the mean-value of the output in DC-coupling mode. 5. Attach a 47μF capacitor in parallel to RL and observe the output on the oscilloscope in the AC-coupling mode. Now measure ripple voltage (Vr) and the conduction time (∆ ) using the cursers. Measure the mean-value of the output in DC-coupling mode. Present your experimental results in the table below. C Vr(Theoretical) Vr(Experimental) ∆ (Theoretical) ∆ Experimental) 10μF 22 μF 47 μF Compare the theoretical and experimental value. Questions 1. Explain how the bridge-rectifier circuit works. 2. Why does 47μF capacitor give smallest amount of ripple? 3. Which capacitor gives the largest mean value in the output? Explain your answer. 43

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