Laboratory Session 2 - The Diode and Adders - EE223 - Digital and Analogue Electronics

Laboratory Session 2 - The Diode and Adders

Objectives:

To obtain the characteristics of silicon diodes.
To study the basic characteristics of LEDs.
To build a basic Zener diode regulator
To build and test a 1-bit half-adder
To test an integrated (pre-built) 4-bit full-adder

Equipment Required:

Laboratory Instrument Station.
Lead Kit.
Silicon Diode, 1N4001.
1k Resistor.
Breadboard.
Tenma (DMM).
3.3V Zener Diode (1N746A).

You should submit your laboratory write-up electronically at: http://moodle.dcu.ie/mod/assign/view.php?id=27686

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.

Figure 1. A 1N4001 Silicon Diode - The lighter grey line on the diode indicates the direction that it should be placed in the circuit

Figure 2. The symbol of a diode, where the line on figure 1 aligns with the straight line to the right of this symbol

Before the laboratory begins you need to watch the video on Adders for the second part of the laboratory:

YouTube Video

Now, in this video 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.

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.

Figure 4. The Volt/Ampere Characteristics of a typical diode.

Part 1 - The Silicon Diode

Connect the Silicon Diode (Figure 1) on your breadboard as shown in Figure 3, using the Mini-lab regulated power-supply for V_S. 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

To apply the higher voltages, use Blue and Red terminals of the Regulated Power Supply.

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
V_F [V]	I_F [mA]	V_R [V]	I_R [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

Switch off the power supply. Reverse the polarity of the diode shown in Figure. 5. Why have the meters been rearranged? Connect to Blue and Red Terminals of Power-Supply to give 30V range. Use the TENMA DMM to measure the current (mA Range).

Figure 5. Circuit for Obtaining Reverse Diode Characteristics

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 I_F going from 0 to 10mA, V_F from 0 to 2V, I_R from 0 to -50μA and V_R 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).

Part 2 - Light-Emitting Diodes (LEDs)

A Light-emitting Diode (LED) is a semiconductor-based light source that was traditionally used as a state indication light in all types of devices. Today, high-powered LEDs are being used in car lights, back lights for monitors and even in place of filament lights for general purpose lighting (e.g. home lighting, traffic lights etc.) due to their extremely high efficiency. We are going to be using very low power LEDs in our circuits, usually to indicate if a state is true or false. Your kit contains several different LED colours, but they all have similar electrical properties.

Figure 6(a) The symbol for an LED, and (b) An actual LED noting the polarity

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.

The symbol for an LED is illustrated in Figure 6(a). The anode(+) is usually connected to a more positive source than the cathode (-). Figure 6(b) shows an LED, which has one leg longer than the other. The longer leg is the anode(+) and the shorter leg is the cathode (-). The plastic surrounding of the led often has a flat edge, which indicates the -ve leg of the LED.

Our LEDs have certain requirements. For example one of our LEDs requires a maximum voltage of 3V and a working current of 20mA. A LED does not have a significant resistance, so if we were to connect the LED across our 5V supply we are exceeding the voltage and the current requirements (it should be okay for short periods of time). However, to do this properly we need a resistor. So, if our supply is 5V and we wish to have a drop of 3V across the LED, we would like 2V to drop across our series resistor. We would also like to limit the current to 20mA, so we need a resistor of value:

R = V/I (as V = IR, Ohm's law)

R = 2V/0.02A = 100 ohms

This is the required resistance for one particular LED - you will have to calculate this for each LED type. If the resistance was calculated at 109 ohms, a 100 ohm or 120 ohm resistor would be fine.

So, our a circuit to light an LED would look like Figure 7. Here we place our resistor in series with the LED. The resistor forces a current of 20mA through the LED and has a voltage drop of 2V, thereby limiting the voltage across our LED to be 3V - meeting the specification requirements.

Figure 7. An LED circuit designed to meet the properties of our LEDs

NOTE: Please ensure that the voltage supply is set to zero before you start this, or you will burn out the LED instantly! Also, be careful only to forward bias a LED; they can be badly damaged in reverse breakdown!

Try to determine the “turn-on” or knee voltage for a red LED and a blue LED. Turn the voltage supply very slowly (it should be quite low - certainly less than 3V if the forward voltage is 3V).

Why is this value different for different LEDs?

Please note that this experiment could also be performed using only the 9V battery, diodes, and some resistors from your component kit. As a battery is a fixed voltage, you would need to use a varying series resistor - potentiometer (instead of a variable lab voltage supply) to give different diode currents.

Part 3 - The Zener Diode Voltage Regulator

The previous diodes work in forward bias mode; however, zener diodes are designed to operate in the breakdown region. Zener diodes (as illustrated in Figure 8) are low cost devices - these diodes cost about 1c each. The data sheet is attached to this page. The 3.3V zener diode you are using has a resistance of 28 Ohms and a reverse current of 10uA. It can dissipate up to 500mW with a Z-current of up to 110mA. The zener diode is orange and black and has 3V3 written on it - meaning 3.3 Volts.

Figure 8. 3.3V Zener Diode BZX55C3V3

Figure 9 illustrates a very simple voltage regulator circuit that provides almost constant voltage output when supplied from a variable voltage supply. The Resistor Rs limits the current flowing through the zener diode.

Figure 9. The Zener Diode Circuit

Wire up the circuit as illustrated in Figure 9. In the first case set Rs to be 100 Ohms and measure the Output Voltage (Vo) for a varying Supply Voltage. Remember that the Zener diode circuit behaves like a simple regulator circuit and will try to keep the output voltage Vo as constant as possible, at the 3.3V level in this case. For this circuit Vo = -VD and Vsupply + Rs.iD + VD = 0, so we will look at the effect of changing the resistor Rs on the output characteristics of the circuit.

Step 1. Set Rs = 100 Ohm and use the variable voltage supply on the bench supply to apply a voltage range as illustrated in Table 2. Measure the input voltage supply and the output voltage using two multimeters set to measure DC voltage. Fill in the values in Table 2.

Step 2. Change Rs = 1000 Ohm (1k Ohm) and use the variable voltage supply to apply the voltages as listed in Table 2 again.

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

Table 2. The Zener Diode Circuit Behaviour

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

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.

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 `S₄These 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.

Figure 12. The 74HCT283 Four Bit Adder

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.

You should submit your laboratory write-up electronically at: http://moodle.dcu.ie/mod/assign/view.php?id=27686

1N4001.pdf

(170k)

Derek Molloy,

23 Oct 2012, 06:26

v.1

3.3VZenerDiode.pdf

(266k)

Derek Molloy,

26 Oct 2013, 08:40

v.1

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