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VCE PHYSICS UNIT 3 FURTHER ELECTRONICS SAC NAME: _________________________________________ 37 marks in total Winston is building a circuit to convert the power from the mains supply into a 12 V DC signal which is to be placed across a DC motor. The motor has a resistance of 180 Ω. The mains supply has an RMS voltage equal to 240 V with frequency of 50 Hz. Winston connects the circuit as shown below. The step down transformer has a turns-ratio of 20:1 and the diode has a switch-on voltage of 0.70 V. RMS 240 V 50 Hz Figure 1 Question 1 (2 + 1 = 3 marks) Calculate the peak-to-peak voltage and the period of the mains supply. (show working) ___________________________ V ___________________________ s Question 2 (4 marks) On the axes below, sketch a graph of voltage against time, for two periods, on the secondary side of the transformer. Include a scale for both voltage and time axes. Note: the time axis is in millisecond. V (V) t (ms) Question 3 (3 marks) With the switch to the capacitor in Figure 1 open, sketch a graph of the voltage across the 180 resistor against time. Include two periods. V (V) t (ms) A full wave rectifier is shown in the diagram below. active neutral Figure 2 Question 4 (2 marks) Onto Figure 2, indicate with an “X” the two diodes which conduct current when the active voltage to the rectifier is negative relative to the neutral line. Winston removes the single diode from the circuit shown in Figure 1 and replaces it with a full wave bridge rectifier shown in Figure 2. Question 5 (4 marks) Draw a full circuit diagram showing the full wave bridge rectifier replacing the single diode shown in Figure 1. _________________________________________________________________________________ The switch to the 100 µF capacitor is still open. Question 6 (3 marks) Sketch on the axis below a graph of the voltage across the 180 resistor versus time after the full wave rectifier has been correctly added. V (V) t (ms) The switch to the capacitor is now closed. Question 7 (2 marks) Calculate the time constant for the circuit. Give your answer in millisecond. ________________________ ms Question 8 (3 marks) On the axis below sketch the voltage across the 180 Ω resistor against time with the switch to the capacitor closed. Label this graph Q8. V (V) t (ms) Question 9 (2 marks) Winston replaces the 180 Ω resistor with a 720 Ω resistor. Onto the axes above, sketch the trace of the voltage across the 720 Ω versus time. Label this sketch Q9. _________________________________________________________________________________ Question 10 (4 marks) Alex wishes to reduce the size of the peak to peak ripple voltage in a rectified and smoothed voltage supply. There are two components in the circuit he can change to achieve this. What are the two components and what changes would he need to make? 1) ________________________________________________________________________________ ________________________________________________________________________________ ________________________________________________________________________________ 2) ________________________________________________________________________________ ________________________________________________________________________________ ________________________________________________________________________________ _________________________________________________________________________________ Ben has a toy that requires a constant 15 V DC supply. He decides that he will use a Zener-diode approach to obtain a steady supply voltage Voutput for his toy. He chooses a Zener diode with a 15 V avalanche point and sets up the circuit shown in Figure 3. He arranges for the supply voltage Vs to have an average value of 18 V but with a peak to peak ripple voltage of 1.6 V. Vs =18 ± 0.80 R Voutput Figure 3 Question 11 (2 marks) Describe the voltage variation across the resistor R. Use numerical values in your description. _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ Question 12 (3 marks) At the instant when the supply voltage is Vs = 17.7 V, the current in the Zener-diode is 50 mA. What is the size of the resistor R? _________________ Ω Ben finds that there is an insufficient current in his toy for it to operate when he connects it across the output Voutput of the circuit shown in Figure 3. Monique suggest he use Toshiba 7815 IC voltage regulator whose output is 15 V provided the input voltage to the chip exceeds 15.5 V at all times. Ben attaches the supply voltage of 18 V with 1.6 V peak to peak ripple voltage and now finds that his stupid little toy now operates. However, the IC-chip gets hot, too hot in fact for Ben to handle. Question 13 (2 marks) Give two different suggestions of what Ben can do to resolve this situation while still having his toy operate. 1) _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ 2) _________________________________________________________________________________ _________________________________________________________________________________ _________________________________________________________________________________ 14 transformer action V1 N1 V2 N 2 15 AC voltage and current VRMS 16 voltage; power V RI 17 resistors in series RT R1 R2 18 resistors in parallel 1 1 1 RT R1 R2 19 capacitors time constant : RC 1 2 2 Vp - p I RMS P VI 1 2 2 I p-p