Diodes & Power Supplies
Hazard Identification:
AIMS
1.
To construct and observe a half wave rectifier
2.
To construct and observe a full wave bridge rectifier
3.
To observe the effect of a simple capacitor output filter
4.
To construct and observe a simple zener regulator
5.
To use oscilloscope DC and AC input coupling as appropriate
APPARATUS
Rectifier module:
 diodes
 470  resistor 5W
 470 F capacitor
Diode board:
 8.2 V zener diode
 4x resistors
Also:
 Variac transformer
 RMS volt meter
 Oscilloscope
 Jumper leads
EXPERIMENTAL
Set the output of the Variac to 20Vpk. What value RMS voltage is this? __________Vrms
PART A: Half-wave rectifier
1.
Using one rectifier diode, construct a half-wave rectifier as shown.
0V
470R
Vout
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2.
Use an oscilloscope to observe the output waveform. Ensure that the scope input
coupling is set to DC.
Note. If the peak value is greater than 20V, recheck the wiring.
Do not proceed until the circuit is wired correctly.
Sketch the output waveform, (and record peak values and the frequency).
Channel 1 sensitivity: ......... V/div
Time base: ................. ms/div
PART B: Half-wave rectifier + reservoir capacitor
1.
2.
Add a 470 F capacitor in parallel with the
load resistor. Don't forget about polarity!!
+
0V
470R
470  F
Vout
Observe the output waveform with an oscilloscope.
Superimposed on the sketch in Part A/2, sketch the observed waveform.
Page 2
3.
"Zoom up" the ripple by changing the input coupling to AC and increasing the
sensitivity.
Carefully sketch the ripple waveform, and record the peak to peak amplitude.
Channel 1 sensitivity: ......... V/div
Time base: ................. ms/div
PART C: Full-wave bridge rectifier
1.
Construct the full-wave bridge rectifier circuit shown below:
+
470R
Vout
0V
_
2.
Ensure the scope coupling is set back to DC!!
Sketch the output waveform, recording peak values and the frequency.
Channel 1 sensitivity: ......... V/div
Time base: ................. ms/div
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PART D: Full-wave rectifier + reservoir capacitor
+
Add a 470 F capacitor in parallel with the
load resistor.
1.
+
470
Vout
0V
_
2.
Observe the output waveform with an oscilloscope.
On the same sketch in Part C/2, sketch the observed waveform.
3.
As in part B, use AC coupling to "zoom in" to observe the ripple.
Examine the ripple waveform, and record the peak to peak amplitude.
Vripple(p-p) = ..............
PART E: Using a Zener Diode as a Simple Regulator
1.
2.
Turn off the power to your circuit. Change the input AC voltage to 10 Vrms.
Remove the 470  load resistor and construct the zener shunt regulator as shown
below. Use the 8.2 V zener.
 DON'T FORGET TO INCLUDE THE 150  VOLTAGE DROPPING RESISTOR 
3.
To the output of this supply, connect a load resistor
(combination of resistors; minimum 400)
4.
150R
R load
Full Wave
Bridge
Rectifier
Complete the following table. (Use the scope to read Vout.)
Table 1. Zener regulated supply: load regulation
R load
V out
(pk-pk)
Page 4
5.
Present these results as a graph of Vout versus RL (load regulation).
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PROBLEMS
1.
Use measured values of the frequency (PARTS A2 and C2) to verify the
relationship between fripple for a half-wave rectifier and a full-wave rectifier.
2.
For the full-wave rectifier, how did the measured ripple compare with the
theoretical value as calculated in PART C?
3.
a) Explain why the zener regulation fails if the load resistance becomes too
small.
b) For what load resistance is the zener dissipating the most power?
EXTRA
A.
Complete the following statements for the power supply shown in Figure 1.
Transform er
230:10+10
+
Figure 1.
Centre tapped DC supply:
Mains
230V(rms)
470  F
Vout
10V(rms)
10V(rms)
(a) This power supply uses ............. wave rectification.
aluminium electrolytic.
(b) The capacitor type is most likely to be ceramic.
mylar.
regulated
(c) This is a
power supply.
non-regulated
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B.
(a) For Figure 1, the output wave shape with no load (as shown) is approximately:
Vp
(i)
Vp
(ii)
0V
Vp
(iii)
0V
Vp
(iv)
0V
0V
(b) The value of Vp is approximately ................. volts.
C.
If a 470  load resistor is connected to the output of the power supply in Figure 1,
calculate an approximate magnitude for the ripple voltage.
Ripple voltage (peak to peak)  .......................
D.
Sketch a simple shunt regulated DC supply comprising of:
a transformer with turns ratio 230:10
a bridge rectifier
a 470 F reservoir capacitor
a 470  resistor
a zener diode
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