Equipment Required:
You should submit your laboratory write-up electronically at: https://loop.dcu.ie/mod/assign/view.php?id=1157951 There is no template document available for this lab. Please use the same format and style as was used for Laboratory 1. |
Introduction:
In this experiment you are going to examine how a diode works. We are going to study the properties of a junction diode and we are going to obtain the characteristics of a silicon diode and some LEDs.
Before the laboratory begins you need to watch the video on Adders for the second part of the laboratory:
YouTube Video
Procedure
Introduction
For this part of the experiment we are going to set up the circuit as shown in Figure 3. There is one difficulty with this assignment in that you must use at least two Digital Multimeters - one to measure diode voltage (voltmeter) and one to measure diode current (ammeter). It is useful to place a third voltmeter across the supply voltage.
Figure 3. The setup we are using for this experiment. You will need to wire this on your breadboard
What we are trying to achieve in this experiment is to demonstrate that a diode exhibits a behaviour as described by Figure 4. This graph illustrates that when a diode is forward-biased it allows current to pass freely once we achieve a fairly low 'knee-voltage'; however, when the diode is reverse-biased it does not allow current (negative current) to flow until we reach a fairly high 'breakdown-voltage'. The breakdown-voltage for a diode like this is typically from 50V to 1,000V. In effect, a diode only allows current to flow in one direction (like a one-way water valve). This property means that it can be used to design circuits such as AC to DC converters.
Connect the Silicon Diode (Figure 1) on your breadboard as shown in Figure 3, using the Mini-lab regulated power-supply for VS. This diode has a band at the cathode end (see Figures 1 and 2).
Now answer these questions:
(a) Is the characteristic that will be obtained using the circuit of Figure 3 the forward or the reverse characteristic of the diode? Explain.
(b) Why is there a resistor in the circuit?
Meter Settings
First decide which Meter will act as the Voltmeter and which will act as Milli-ammeter. Set the Voltmeter to the 2V (2000mV) DC range and the Milli-ammeter to its ´20mA range.
(NB: if using the TENMA Auto Power-Off Digital Multi Meter (DMM) to measure current insert the positive lead into the mA /mA Terminal and set the rotary selector to mA!). Further on in the experiment you will be required to change the ranges.
Forward-Bias Measurements
Perform the measurements by making fine adjustments to the Power-supply as per the left half side of Table 1 (Forward Bias) and record your results in the table below:
Please remember to take photographs of your setup for your write-up.
Forward Bias |
Reverse Bias |
||
VF [V] |
IF [mA] |
VR [V] |
IR [μA] |
0.05 |
|
1 |
|
0.1 |
|
2 |
|
0.15 |
|
3 |
|
0.2 |
|
4 |
|
|
1 |
5 |
|
|
2 |
6 |
|
|
3 |
7 |
|
|
4 |
8 |
|
|
5 |
9 |
|
|
6 |
10 |
|
|
7 |
15 |
|
Table 1: Silicon Diode Measurements.
Reverse Polarity of the Diode
Perform the measurements according to the right half of Table 1 (Reverse Bias). To apply the higher voltages, use Blue and Red terminals of the Regulated Power Supply. Pay special attention to the Meter ranges. Record your results in the table. Again, remember the photographs.
Analysis of Results Part 1:

When you are doing your laboratory write-up: Plot the results for the Silicon diodes on a graph, with IF going from 0 to 10mA, VF from 0 to 2V, IR from 0 to -50μA and VR from 0 to –30V. (You may alter any of the dimensions if they don’t suit your results). Label your graph carefully and write each diode type next to its characteristic.
Using your noted source voltage, see if you can plot a load-line for the silicon circuit on your graph. Does your load-line intersect your I-V characteristic where you expect it to?
You can hand draw a plot and photograph it, or plot the data in Excel, Google Sheets (or any on-line plotting package).
Light emitting diodes (LEDs) are junction diodes made from more complicated semiconductors such as GaAs or GaAsP. You will have to choose a smaller resistor than the 1k resistor.
Part 3 - The Zener Diode Voltage Regulator
Voltage Supply (Volts) | Rs = 100 Ohm Voltage Out (Vo) (Volts) | Rs = 1k Ohm Voltage Out (Vo) (Volts) |
0.0 | ||
0.5 | ||
1.0 | ||
1.5 | ||
2.0 | ||
2.5 | ||
3.0 | ||
3.5 | ||
4.0 | ||
4.5 | ||
5 | ||
6 | ||
7 | ||
8 | ||
9 | ||
10 | ||
11 | ||
13 | ||
15 |
Half Adders and Full Adders
Now, in the video at the beginning of the lab description I use and XOR gate (exclusive OR gate), but that is not available to you in this lab, rather you must build the circuit using NAND gates, which is more representative of a real-world half/full adder. Check the notes to see the equivalent circuit for an XOR gate and think about how this could be implemented using NAND gates only, as illustrated in Figure 10.

Figure 10. 7400: Quad 2-input NAND
Part 4 - The 1-bit Half-adder
Figure 10. 7400: Quad 2-input NAND
Part 4 - The 1-bit Half-adder
Construct the circuit as shown in Figure 11 using 7400 NAND Gates.

Figure 11: A 1-bit Half-adder circuit using NAND gates
Vary the inputs A
and B
(i.e., 0 and +5V) to obtain all the possible combinations and complete a truth table for the sum output S
and the carry output C
. You should wire up LEDs on your output for S and C to make this step more straightforward.
Comment in your report on whether this is as you expected.
Could we combine multiple half-adders to implement arbitrary width (n-bit) addition operations? Explain your answer in your write up.
Construct the circuit as shown in Figure 11 using 7400 NAND Gates.
Figure 11: A 1-bit Half-adder circuit using NAND gates
Vary the inputs
A
andB
(i.e., 0 and +5V) to obtain all the possible combinations and complete a truth table for the sum outputS
and the carry outputC
. You should wire up LEDs on your output for S and C to make this step more straightforward.Comment in your report on whether this is as you expected.
Could we combine multiple half-adders to implement arbitrary width (n-bit) addition operations? Explain your answer in your write up.
Part 5 - The Integrated 4-bit Full-adder
In the video at the beginning of this lab I used DIP switches, but this is not necessary for you implementation. Instead, just move the input wires between the high (+5V) and GND voltage rails or wire up simple buttons using a button and a pull-up/pull-down resistor. This is slightly more manual but is fine for this assignment. Just remember that a floating wire (a wire left disconnected) is not necessarily the same as a wire that is connected to GND (You saw this in the first laboratory)
Figure 12 shows the pinout of the 74HCT283, which is an integrated high speed 4-bit Full-adder. It accepts two 4-bit words (A1
to A4
) and (B1
to B4
) and a carry input (C0
). It generates the binary sum outputs (S1
to S4 These are sigma on the chip
) and the carry output (C4
) from the most significant bit. The carry input (C0) is there to allow you to connect two 4-bit adders together to create a 8-bit adder. For the moment, you can connect this input to GND. Be careful, it you leave it disconnected it will probably be received as a 1.
Use 5 LEDS (4 for the sum and 1 for the carry) on the output side of the adder. Remember to use appropriate resistors (use 100 ohm if in doubt) to limit the current through the LEDs.
Make sure that you know the order of your bits from MSB to LSB. So for the Sum outputs S4 should be your MSB and S1 should be your LSB - but also remember that A4 is your MSB and A0 is your LSB when you are inputting a value.
Consider the following 4-bit binary additions:
0110 + 0101
1011 + 0011
1110 + 0100
1111 + 1111
For each expression, predict what the sum bits and the carry-out bit should be, both for the case that the carry-in bit is 0
and that it is 1
. Record your predictions in your write-up.
Now test all these predictions, using a 74HCT283. Record the results in your write-up. Comment on whether or not your predictions were satisfied.
Make sure that you know the order of your bits from MSB to LSB. So for the Sum outputs S4 should be your MSB and S1 should be your LSB - but also remember that A4 is your MSB and A0 is your LSB when you are inputting a value.
Consider the following 4-bit binary additions:
0110 + 0101
1011 + 0011
1110 + 0100
1111 + 1111
For each expression, predict what the sum bits and the carry-out bit should be, both for the case that the carry-in bit is 0
and that it is 1
. Record your predictions in your write-up.
Now test all these predictions, using a 74HCT283. Record the results in your write-up. Comment on whether or not your predictions were satisfied.
Conclusions:
In your own words, state what conclusions you draw from the experiment.
- State briefly, but clearly, what you have learned from this assignment.
- How did you split the work between yourself and your partner?
- What was the most difficult aspect of the assignment?
- State one thing you enjoyed about the assignment.
- State one thing you disliked about the assignment.
- Add any final comment of your own.