Download Assessment Task for Further Electronics

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.
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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.
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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)
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2)
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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.
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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)
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2)
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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