Lab Experiment 11 on Power Supplies and Filters | ECE 225, Lab Reports of Electrical Circuit Analysis

Download Lab Experiment 11 on Power Supplies and Filters | ECE 225 and more Lab Reports Electrical Circuit Analysis in PDF only on Docsity! Boise State University Department of Electrical and Computer Engineering ECE225L – Circuit Analysis and Design Lab Experiment #11: Power Supplies and Filters 1 Objectives The objective of this laboratory experiment is: • To investigate the performance characteristics of a DC power supply with various filters and to reduce its AC voltage ripple. 2 Theory By definition, a DC power supply produces a voltage that will generate a unidirectional current in a resistor. Hence, a DC supply can be created using an AC supply and modifying the output of the AC supply. Mathematically, if it is assumed that the AC signal is given by vs(t) = Vp sinωt, then the corresponding DC signal can expressed as vr(t) = Vm| sinωt|, where | · | is the absolute value operator. Note that in this particular case, the resulting DC signal is time varying. The reversal of the negative portions of the signal without a change in the positive portions of the signal is a process called full-wave rectification. (As an aside, in half-wave rectification, the negative portions of the signal are eliminated while the positive portions of the signal are not affected.) As shown in Figure 1, full-wave rectification can be implemented using four diodes connected in what is known as a diode bridge. Figure 2 depicts a full-wave rectified sinusoid (the signal shown in dashed lines) assuming that the diodes of Figure 1 are ideal. The DC signal shown in Figure 2 (i.e., the signal which is shown with dashed lines) is not very useful as a DC signal since it has a large ripple. Here the ripple is defined as the peak-to-peak variation in the voltage. (In the case of the full-wave rectified sinusoid, the ripple is from 0 to Vm.) To reduce the ripple, this experiment investigates using various filters at the output of the diode bridge. One such filter is the C-filter shown in Figure 1. Using the C-filter, the ripple voltage is reduced from the full-wave rectified sinusoid shown in Figure 2 to the charging and discharging ripple voltage (i.e., the curve shown with a solid line) also seen in Figure 2. For a practical filter, T1, the time that the capacitor charges, must be much smaller than T2, the time that the capacitor discharges, to reduce the magnitude of the ripple. As an example, if T1 = 0 and T2 = T , the ripple would be zero, while if T1 = T and T2 = 0, the ripple would be the full-wave recti- fied sinusoid shown in Figure 2. Also note that the input signal to the 12.6 Vrms transformer is at 60 Hz (the frequency of the line voltage), and, therefore, the output signal of the diode bridge, which is a full-wave rectified sinusoid, is at 120 Hz. Hence, for this experiment, use T ∼= 8.33 msec. From Figure 2, if we let Vr be the peak-to-peak ripple voltage of the C-filter, then the DC voltage of the C-filter can be written as Vdc ∼= Vm − Vr2 (1) 1 C−Filter Transformer Variable Load Resistor + − + − + − + − V L I L R Full−WaveStep−Down L V dc V m Figure 2: Output of Full−Wave Rectifier with Ripple Voltage from a C−Filter I o I L I L ∆ V L V∆ L Figure 3: Typical DC Regulation Curve r v (t) v (t) v (t) p s r C 1 Bridge Rectifier Figure 1: Full−Wave Rectifier Circuit with C−Filter and Variable Load Resistor t T T 1 T 2 v (t) C V 2 µ470 F 470 Fµ Figure 4: C−Filter 1 C µ470 F − + C L = 330 mH − + C µ940 F Figure 6: LC−Filter πFigure 5: −Filter R = 10 − + − + Ω 2 C 1 5 5 Procedure 1. This part of the experiment uses the power supply with no filter. Measure the no-load output voltage (with IL = 0). You may need to add a large temporary 500-kohm resistor at the output of the full-bridge rectifier. After you are done, remove the 500-kohm resistor. Insert a benchtop DC ammeter in the circuit and attach a 1-kohm 2-W variable resistor set at its maximum at the output of the full-bridge rectifier. Record the DC (average) load voltage using your handheld multimeter for load currents varying from 20 to 80 mA in steps of 10 mA. Also, record the peak-to-peak ripple voltages at each current step. (Use the AC coupling feature on the scope. Be sure to use only one probe in this experiment.) Save a copy of the ripple voltage waveform across the load at 50 mA. 2. This part of the experiment uses the C-filter shown in Figure 4. Be sure to observe the proper polarity on the capacitor. Repeat Step 1 without the 500-kohm resistor at no load. 3. This part of the experiment uses the π-filter shown in Figure 5. Be sure to observe the proper polarity on all capacitors. Repeat Step 1 without the 500-kohm resistor at no load. 4. This part of the experiment uses the LC-filter shown in Figure 6. Be sure to observe the correct polarity on the capacitor. Repeat Step 1 without the 500-kohm resistor at no load. 6 Report Questions 1. Plot the DC voltage regulation curves for each of the four configurations. 2. Compute the apparent internal DC resistance at IL = 50 mA for all four configurations using R = −∆VL ∆IL = − VL2 − VL1 IL2 − IL1 (9) where IL2 = 60 mA and IL1 = 40 mA and VL2 and VL1 are the load voltages corresponding to these two currents, respectively. 3. Comment on the peak-to-peak voltage ripples for each of the four configurations. (Compare how the ripple voltage varies with increasing load current in each configuration and compare the relative magnitude of the ripple voltage for each of the four configurations at 50 mA.) 6 Boise State University Department of Electrical and Computer Engineering ECE225L – Circuit Analysis and Design Lab Experiment #11: Power Supplies and Filters Date: Data Sheet Recorded by: Equipment BSU Tag Number or Serial Number HP/Agilent 54810A Infinium Oscilloscope HP/Agilent 34401A Benchtop Multimeter Fluke 111 True RMS Multimeter Table 1: Power Supply with No Filter Table 2: Power Supply with C-Filter IL (mA) VL (V) Vr,pp (mV) 0 20 30 40 50 60 70 80 IL (mA) VL (V) Vr,pp (mV) 0 20 30 40 50 60 70 80 Table 3: Power Supply with π-Filter Table 4: Power Supply with LC-Filter IL (mA) VL (V) Vr,pp (mV) 0 20 30 40 50 60 70 80 IL (mA) VL (V) Vr,pp (mV) 0 20 30 40 50 60 70 80