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FOUNDATION EXP 5 – DIODES
EXPERIMENT 5
DIODES
1.0 INTRODUCTION
A diode is a semiconductor device that has a non-linear characteristic and
exhibits a low resistance to current flow when connected in one direction, and
a high resistance in the opposite direction. It has no single value for its
voltage:current ratio, and is thus significantly different from a linear resistor.
The symbol for a diode is shown below in Figure 1.1
(+) Anode
(-) Cathode
Figure 1.1
The cathode of a diode is normally indicated by a band around one end of the
device. If the positive terminal of a power supply is connected to the anode
and the negative terminal to the cathode, then the diode will be forward biased
and a current will flow. In practice this current may be very large and a series
resistor must normally be used to limit it to a safe value.
If the power connections were reserved (i.e. positive to cathode and negative
to anode) then ideally no current will flow. In practice a very small leakage
will flow. If the reverse voltage is made too large, then the diode will break
down and a very large reverse current will flow. Again if this is not limited,
the diode will be damaged and possibly destroyed.
2.0 TESTS
2.1 PROCEDURE 1
The first test is to observe that a diode has no single, meaningful value of
resistance because of its nonlinear characteristic. Set the DMM to measure
resistance, and use it to measure the resistance of the diode in both directions
(forward and reversed biased). Draw circuit diagrams in your lab book for
both directions, noting down the diode resistance next to each.
Next connect up the circuit as shown in Figure 2.1. The value of R1 should be
between 310 and 400. Use the LCD display to the left of the power supply
terminals to take measurements for VS, which should be varied from 0V up to
a maximum of 10V. Use the DMM set up as an ammeter (break and make
method) to measure ID, and then reconnect and reconfigure it to take
Dr. Daniel Nankoo
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FOUNDATION EXP 5 – DIODES
respective readings of VD, where it is set up as a voltmeter (in parallel to the
diode).
ID
R1
+
VS
-
+
VD
-
Figure 2.1
Draw a table in your lab books with three columns, headed by VS, VD, and ID.
As you increase VS, take the corresponding readings for VD and ID. Once you
have enough readings (you decide on an appropriate number), draw a control
graph of ID versus VD. Once you have drawn your graph, examine closely the
points where there is the greatest change in the slope, and decide whether you
need to take finer readings at these points to obtain a smoother curve.
2.2 PROCEDURE 2
ID
R1
+
VS
-
+
VD
-
Figure 2.2
Repeat the procedure outlined in Section 2.1, but for the reversed biased
connection, as shown in Figure 2.2 and again plot ID versus VD, where VD has
negative values. Then combine both sets of results for both the forward and
reversed biased connections and plot the results on one single ID versus VD
graph. When plotting graphs, do not forget to clearly label your axes, and to
use the appropriating scaling.
2.3 PROCEDURE 3
You will now conduct tests on another type of diode called a Zener diode. A
Zener diode is very similar to a conventional diode, except that the reverse
breakdown voltage is adjusted to occur at some lower voltage than normal.
The voltage remains almost constant once breakdown has occurred, which
makes Zener diodes extremely useful in circuits where a voltage reference
source is needed (e.g. in power supplies).
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FOUNDATION EXP 5 – DIODES
Replace the conventional diode in the circuit of Figure 2.1 with the Zener
diode provided. Measure and record the values of VS, IZD and VZD (Zener
current and voltage respectively) in tables drawn in your lab books, and thus
repeating the procedure outlined in Sections 2.1 and 2.3.
Once you have drawn the three control graphs for the Zener diode (forward
bias, reverse bias and a combined graph), estimate the Zener (reverse
breakdown) voltage for your diode from the results.
2.4 PROCEDURE 4
You will now use the oscilloscope in “COMPONENT TESTER” mode. On
the older CRT scopes, there was a COMPONENT TESTER button which was
used to display the voltage-current characteristic of a passive device.
However, on the digital storage scopes in the lab, this function does not exist,
and thus the scope will have to be configured manually in order for this mode
to be operational.
Using the appropriate cables, build the circuit shown in Figure 2.3 on your
breadboard.
CHANNEL 1 OF
SCOPE FOR X
RLOAD 680Ω
1KhZ
MAX AMPLITUDE
PLACE DEVICE TO BE
TESTED HERE
CHANNEL 2 OF
SCOPE FOR Y
RSHUNT
10Ω → 100Ω
Figure 2.3
Set the Function Generator to produce a sine wave of 1kHz, with maximum
amplitude (i.e. the AMP dial pushed in and turned fully clockwise). Ensure
that the 50Ω output is selected. Set up the scope by first pressing the
DISPLAY button, and switching the “Format” to “XY”. This disengages the
horizontal time axis. Next, place a resistor (330  R  820 ) in the part of
the circuit where a device can be tested. Adjust the VOLTS/DIV CH1 for the
X axis and VOLTS/DIV CH2 for the Y axis to obtain suitable traces. Once a
suitable trace has been achieved, draw what is displayed into your lab books.
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FOUNDATION EXP 5 – DIODES
Now connect a conventional diode, and then a Zener diode, again drawing
what you see for both cases of biassing. Do the traces look familiar? What are
they the same as?
2.5 DIODES IN SERIES-PARALLEL
Connect four conventional diodes in the series-parallel combination as shown
in Figure 2.4.
Figure 2.4
Carry out a voltage/current test, following the same procedure as laid out in
Sections 2.1 and 2.2, tabulating your results in your lab books, and drawing
the relevant control graphs of diode current against diode voltage. Then
connect the full diode combination to the circuit of Figure 2.3, and observe
and draw the trace that you see.
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FOUNDATION EXP 5 – DIODES
3.0 COMMENTS AND CONCLUSIONS
As usual it is important to examine your results and see how and why they do
or do not correspond, e.g. are there, or are there not, any connections between
the tests on a single diode and on the series-parallel combination? What is the
range of voltage:current ratios for points on the forward biased part of the
characteristic for a single diode? What is the largest power dissipation in the
diode and the resistor used in the circuits you have investigated?
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