FOUNDATION EXP 6 – RECTIFIERS
EXPERIMENT 6
RECTIFIERS
1.0 INTRODUCTION
Diodes are widely used in switching circuits to clamp or clip input voltages to
ensure that they do not go negative. Diodes used for this purpose are known as
small signal diodes and are designed to switch small currents at high speed.
Diodes are also used extensively in power supply circuits where they are used
to rectify the alternating mains supply to produce a DC output.
2.0 PROCEDURES
2.1 HALF WAVE RECTIFIER
For this part of the investigation, you will require one conventional diode and
a 10kΩ resistor for R1. You will also need to use the Function Generator and
oscilloscope for all the inclusive tests.
A
B
IL
+VD+
VS
-
R1
+
VL
-
C
Figure 2.1
Connect up the half-wave rectifier circuit as shown in Figure 2.1. Switch on
the scope, and prior to making any connections, use the AUTO SET button to
initialise both channels.
Set up VS using the Function Generator, and use the appropriate controls and a
BNC-BNC cable to obtain a sine wave signal with a frequency of 50Hz and a
peak to peak voltage 4.0V, using the 50Ω output. Ensure that the DC OFFSET
button on the frequency generator control panel has been pushed in. Use the
SEC/DIV and VOLTS/DIV dials to obtain a suitable display on the screen, so
that you can adequately see two cycles. Note down the corresponding settings.
Once you have set up VS, draw this signal in your lab book, noting down the
waveform’s dimensions and scope settings.
Apply this signal to your circuit, by connecting a BNC-croc clip cable from
the Frequency Generator to points A (red) and C (black – this is the
earth/common point), and use another lead to observe this signal on CH1 of
the scope. On Channel 2, observe the waveform at B (red) with respect to
earth at C (black). Use the same VOLTS/DIV as you used when setting up VS.
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FOUNDATION EXP 6 – RECTIFIERS
Draw VBC in your lab book, noting down the dimensions (peak-to-peak
voltage, period) and scope settings. Note how VBC is periodic (i.e. repetitive at
equal time intervals). Based on your observations, can you explain why this is
called half wave rectification? You should have observed that only the positive
half cycle of the sine wave appears across R1, hence the circuit being known as
a half wave rectifier.
2.2 FULL WAVE RECTIFIER
A full wave rectifier may be produced by using a second diode, as shown in
Figure 2.2.
A
There is no
connection
at this point
+VDRA
+
VS
-
C
RB
R1
B
+VD-
D
Figure 2.2
For this circuit, you will need to set up VS by using the function generator so
that it produces a sine wave from its 50Ω output, with a frequency of 50Hz,
and a peak to peak voltage of 12V. Use the appropriate controls and leads to
set up and test the required waveform, which should be displayed on Channel
1. Draw the sine wave in your lab book, noting down its dimensions and the
corresponding scope settings.
Because the earth of both the oscilloscope and Function Generator are not
isolated, scope measurements for the circuits of Figures 2.2 and 2.3 are not
possible using the known conventional techniques. Thus, the scope has to be
set up to function in differential mode. A consequence of this set up is that you
will only be able to use the scope to measure one waveform, which in this case
will be the output of the circuits. Therefore make sure you have set up your
input waveform correctly before making the following adjustments to the way
the scope operates.
To set the scope up in differential mode, follow these steps:
1. Set both Channel 1 and Channel 2 to DC coupling
2. Enable the red trace, i.e. press the red MATH MENU button
3. Set the scope to display CH1 – CH2
4. Connect BNC-croc clip cables from both channels
5. Connect the black ends of the cables together
6. Set both channels to 2V/DIV
7. Connect the red cable from Channel 1 to point D
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FOUNDATION EXP 6 – RECTIFIERS
8. Connect the red cable of Channel 2 to point C (earth point).
If you are unsure about the above procedure ask a member of staff for help.
Construct the circuit of Figure 2.2 on your breadboard, where RA = RB = 560Ω
and are resistors that provide a mid-point reference node at C. R1 = 10kΩ, and
C is the earth point. Connect VS to your circuit, but do not use the scope to
measure it, as the scope’s earth will effect what is displayed on the output, so
it is only necessary to measure the output, using the procedure outlined above.
To measure the output, use steps 7 and 8 as mentioned above. Draw the
waveform into your lab book, noting the dimensions and scope settings. Based
on your observations, can you explain why this circuit performs full wave
rectification?
2.3 BRIDGE RECTIFIER
In practice it is more common to use a four-diode bridge rectifier for power
supply applications. Such a circuit is shown in Figure 2.3, where R1 is a 10kΩ
load resistor and point C is the earth point. Prior to carrying out the tests,
ensure that both channels of the scope have been centred, and that DC
coupling has been selected for both channels. Once this has been done,
carefully construct the circuit of Figure 2.3 on your breadboard. For this part
of the investigation, you do not need to include the capacitor.
A
+
VS
-
D
+
-
R1
B
C
Figure 2.3
For the same scope set up as described in the previous section, observe the
output and the diode (BC) voltage waveforms VDC and VBC on the
oscilloscope. Use the scope controls to obtain clear waveforms, and draw both
waveforms in your lab books, not forgetting to note the dimensions and
settings.
2.4 BRIDGE RECTIFIER WITH SMOOTHING CAPACITOR
The output from the circuit in Figure 2.3 (without the capacitor) would not be
much use for power supply purposes, where a smooth DC output would
normally be required.
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FOUNDATION EXP 6 – RECTIFIERS
To smooth the output, a capacitor is frequently employed as it can be
connected so that it charges up quickly from the supply and then slowly
(depending on the CR time constant) releases charge to the load. The time
constant (remember Experiment 4) delays the change in the output, resulting
in a change from the input. If CR is made large enough, the voltage across the
load will not have fallen by very much before the next half cycle of the input
arrives to quickly restore the charge on the capacitor back to its peak value.
(i)
Switch off the function generator (i.e. disengage VS), and use an
electrolytic capacitor, 47μF or 100μF, (long leg is positive, short is
negative) so that it is connected across points D-C of your bridge
rectifier circuit, as in Figure 2.3.
(ii)
Take care to ensure that the positive end of the capacitor is connected
to point D, and the negative end to point C.
You must take particular care with the set up and recording for the following
tests, as the traces will be needed for measurement purposes.
(iii)
Firstly, using the same VS as in Section 2.3, turn on the function
generator, and observe the voltage across D-C using the oscilloscope in
differential mode. Draw this waveform in your lab book as accurately
as possible, noting scope settings and dimensions.
(iv)
Try to improve the resolution of the scope by adjusting the
VOLTS/DIV for both channels (make sure they are the same settings
for both channels). Observe and sketch the waveform across D-C
again, recording your VOLTS/DIV and TIME/DIV settings and the
waveform’s dimensions.
(v)
Observe and draw the diode voltage waveform VBC.
(vi)
Turn off the power supply, and replace the 10kΩ resistor with a 680Ω
resistor. Repeat step (iv) again, and drawing the resulting waveform.
3.0 COMMENTS AND CONCLUSIONS
1. Explain the operation of the circuits by identifying the times during which
the diode(s) are ‘forward’ or ‘reversed’ biased, in particular indicating where
evidence of the forward drop of 0.7V can be observed on your waveforms.
2. Use your waveforms derived for the half wave rectifier (Section 2.1) to
determine the following:
a) The period and the frequency of the output signal.
b) The maximum value of the load current.
c) The value VD at the time instants when VS is a positive maximum, and when
VS is a negative maximum. You will need to use Kirchoff’s Voltage Law
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FOUNDATION EXP 6 – RECTIFIERS
(KVL) for Figure 2.1 (VS = VD + VL) and take measurements from the
waveforms drawn in Section 2.1 (take care over the ± signs).
d) The % of the period for which VS is positive and the % for which it is
negative. It should be 50%:50%.
e) The % of the period for which VL is positive and the % for which it is zero.
Explain how the 0.7V diode drop affects this change from a 50:50 ratio.
3. For the bridge rectifier, and by taking into consideration the circuit when
the 10kΩ resistor and capacitor (Section 2.4) are connected, determine the
following:
a) Apply Ohm’s Law to the DC coupled waveform for the 10kΩ resistor load
with the capacitor connected to estimate the average (or DC) load current.
b) Using the waveforms drawn in Section 2.4, part (v), determine the gradient
of the waveform as the voltage is reducing (volts-per-second).
c) Check that the current and gradient values are consistent with the equation
for a capacitor, stated below
iC
dV
dt
4. Finally, investigate some possible uses of the different types of rectifiers
studied above.
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