23a HW p20 Power

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2019 Fall EECS 314 HW problem 21 Student’s name: Htet, Myo (last, first; write legibly) © 2019 Alexander Ganago Page 1 of 8 Last printed 4/10/23 4:15:00 PM File: 23a HW p20 Power.docx Problem 21 (100 points) Diodes, rectifiers, and power supply Part One (15 points) The diode load line On the plot below draw the load lines for (a) ࠵? ! = 3 ࠵? ࠵?࠵?࠵? ࠵? = 300 ࠵? (b) ࠵? ! = 6 ࠵? ࠵?࠵?࠵? ࠵? = 300 ࠵? For each load line, determine the current through and the voltage across the diode; calculate the power absorbed by the diode. Show your work. Current, mA Voltage, V Power, mW a 4 mA 1.8 V 7.2 mW b 14 mA 1.9 V 26.6 mW

2019 Fall EECS 314 HW problem 21 Student’s name: Htet, Myo (last, first; write legibly) © 2019 Alexander Ganago Page 2 of 8 Last printed 4/10/23 4:15:00 PM File: 23a HW p20 Power.docx Part Two (15 points) The offset diode model For the LED whose volt-amp characteristic is given in Part One, use the offset diode model with ࠵? "# = 1.8 ࠵? and assume the load resistance R = 300 Ω. Calculate the current through the diode and the power absorbed by the diode for two source voltages Vs = 3 V and 6 V. Briefly discuss whether you results agree with those in Part One where you used the load lines. According to the manufacturer’s specifications, the power absorbed by this diode should not exceed 50 mW. Use the offset model to calculate the minimal load resistance for two source voltages Vs = 3 V and 6 V. Show your work. -V S + V R + V D0 = 0 -3 + V R + 1.8 = 0 V R = 1.2 V I = V R /R = 1.2/300 * 1000 = 4 mA -V S + V R + V D0 = 0 -6 + V R + 1.8 = 0 V R = 4.2 V I = V R /R = 4.2/300 * 1000 = 14 mA These results agree with the answers in part one. P D = I * V D0 50 = I * 1.8 I = 27.78 mA R Min = V R /I R Min = 1.2/0.02778 = 43.20 ohm R Min = 4.2/0.02778 = 151.19 ohm

2019 Fall EECS 314 HW problem 21 Student’s name: Htet, Myo (last, first; write legibly) © 2019 Alexander Ganago Page 3 of 8 Last printed 4/10/23 4:15:00 PM File: 23a HW p20 Power.docx Part Three (15 points) How to connect the bridge rectifier On the diagram in the left panel below, the rectangle represents a full-wave diode bridge rectifier, which is connected to the secondary coil of the transformer and produces the output voltage sketched on the right. The load resistor is connected to the terminals labeled ࠵? $%& . The diagram in the right panel shows the full-wave diode bridge rectifier. Identify the correct connection: relate M, N, P, R to 1, 2, 3, 4. Write your result in the table: M N P R 2 3 4 1 Suppose that your classmate connected M to 1 and N to 4. Circle the correct statements about that circuit: • It is a good rectifier • The current through the circuit equals zero when ࠵? ’( > 0 • The current through the circuit equals zero when ࠵? ࠵?࠵? < ࠵? • The current through the circuit is dangerously high when ࠵? ࠵?࠵? > ࠵? • The current through the circuit is dangerously high when ࠵? ’( < 0 .

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2019 Fall EECS 314 HW problem 21 Student’s name: Htet, Myo (last, first; write legibly) © 2019 Alexander Ganago Page 4 of 8 Last printed 4/10/23 4:15:00 PM File: 23a HW p20 Power.docx Part Four (15 points) Two ways of connecting the load resistor to the ground The diagrams in the table below show two ways of connecting the load resistor to the ground. ࠵? ’( = +12 ࠵? V LOAD = 10.6 V ࠵? ’( = +12 ࠵? V LOAD = 10.6 V ࠵? ’( = −12 ࠵? V LOAD = 10.6 V ࠵? ’( = −12 ࠵? V LOAD = 10.6 V Power supply that produced negative voltage variable between 0 and –25 V Power supply that produced positive voltage variable between 0 and +25 V On each diagram, draw arrows to show the direction of current through the diodes and through the load resistor. Carefully label with + and – the polarity of voltage across the load resistor (recall that the current always enters the more positive terminal of a resistor). Assume that the voltage drop across each forward-biased diode equals 0.7 V and write on each diagram the voltage in volts across the load resistor. -V IN + V D0 + V LOAD + V D0 = 0 -12 + 0.7 + 0.7 + V LOAD = 0 V LOAD = 10.6 V

2019 Fall EECS 314 HW problem 21 Student’s name: Htet, Myo (last, first; write legibly) © 2019 Alexander Ganago Page 5 of 8 Last printed 4/10/23 4:15:00 PM File: 23a HW p20 Power.docx Recall that in Lab 7 you had to connect your Op Amp circuit to two parts of the triple power supply (basically, to two separate power supplies): • one of which produced positive voltage variable between 0 and +25 V, and • the other produced negative voltage variable between 0 and –25 V. In the blank panels of the table write for which of these power supplies you would use the circuit above. Part Five (15 points) Two grounds in one circuit make it dangerous In their desire to build a safe circuit, some students get tempted to connect to the ground not only the load resistor (see Part Four of this problem) but also the secondary coil of the transformer. This desire backfires, because the resulting circuits may become dangerous (the current does not flow through the load resistor because the two ground terminals are electrically connected). ࠵? ’( = +12 ࠵? ࠵? ’( = +12 ࠵? This circuit is dangerous because the current flows from one ground terminal to another without passing through the load resistor. Therefore, the current can get very high. Lower left diode is short-circuited because it is between the two ground terminals. ࠵? ’( = −12 ࠵? ࠵? ’( = −12 ࠵?

2019 Fall EECS 314 HW problem 21 Student’s name: Htet, Myo (last, first; write legibly) © 2019 Alexander Ganago Page 6 of 8 Last printed 4/10/23 4:15:00 PM File: 23a HW p20 Power.docx Lower right diode is short-circuited because it is between the two ground terminals. This circuit is dangerous because the current flows from one ground terminal to another without passing through the load resistor. Therefore, the current can get very high. On each diagram, draw arrows to show the direction of current through the diodes and through the load resistor. Carefully show whether the current flows through the load resistor; if it does not, the circuit is dangerous. In some cases you can also find that one of the diodes may be connected between two ground nodes; then the current would not flow through that diode. Carefully label such cases: show on the diagram which of the diodes is short-circuited by the grounds. Write a brief conclusion on whether any of these circuits is dangerous. Part Six (15 points) Ripples in power supply output Circuit A The output voltage of the original circuit A has the ripple voltage equal to 5% of ࠵? ’(, -. . This circuit can be modified in 4 ways shown below as B-E. Assume that the nominal values of resistors R and capacitors C are the same in all circuits, and the diode bridge always remains the same and fully functional. For each of the circuits B-E calculate the ripple voltage as % of ࠵? ’(, -. . Which circuit has the smallest ripples? Circuit B Circuit C Circuit D Circuit E

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2019 Fall EECS 314 HW problem 21 Student’s name: Htet, Myo (last, first; write legibly) © 2019 Alexander Ganago Page 7 of 8 Last printed 4/10/23 4:15:00 PM File: 23a HW p20 Power.docx Show your work. V Ripple /V OUT, peak = (T/2)/(R * C) Circuit A V Ripple /V OUT, peak = (T/2)/(R * C) = 5% Circuit B V Ripple /V OUT, peak = (T/2)/(R * 2C) = 0.5 * 5% = 2.5% Circuit C V Ripple /V OUT, peak = (T/2)/(R * C/2) = 2 * 5% = 10% Circuit D V Ripple /V OUT, peak = (T/2)/(R/2 * C) = 2 * 5% = 10% Circuit E V Ripple /V OUT, peak = (T/2)/(2R * C/2) = 1 * 5% = 5% Circuit B has the smallest ripples.

2019 Fall EECS 314 HW problem 21 Student’s name: Htet, Myo (last, first; write legibly) © 2019 Alexander Ganago Page 8 of 8 Last printed 4/10/23 4:15:00 PM File: 23a HW p20 Power.docx Part Seven (10 points) More on the ripple voltages The circuit shown on this diagram includes a transformer connected to the power grid sinusoidal voltage at 60 Hz, a bridge rectifier built of identical diodes, three identical capacitors and two identical resistors in parallel. Diagram B Diagram C Consider the following changes of the circuit and the statements about them. For each of these statements write whether it is true or false. Briefly explain why each of the false statements is false. A. If one of the diodes is removed, the ripple amplitude drops by a factor of 2. False because when a diode is removed, the rectifier becomes half-wave. Therefore, the ripples in the half-wave rectifier increase by a factor of 2. B. If one of the diodes is removed, and one resistor is removed so the rest of the circuit becomes as shown on diagram B, the ripple voltage is the same as it was in the original circuit. True C. If all diodes are intact but one resistor and one capacitor are removed so that so the rest of the circuit becomes as shown on diagram C, the ripple voltage gets smaller than it was in the original circuit. True D. None of the above statements is correct. False because statements B and C are correct.