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Laboratory Session 2 - Diodes

Objectives:

1.  To study the properties of a junction diode, LED and zener diode.

2.  To obtain the characteristics of germanium and silicon diodes.

3.  To compare the properties of germanium and silicon diodes.

4.  To study the basic characteristics of LEDs.

5.  To build a basic zener diode regulator

Equipment Required:

  • Laboratory Instrument Station.         
  • Germanium Diode, OA95.
  • 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 a template document at: Template Document to help you to write-up this laboratory.

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, a germanium diode and some LEDs.


Diode Image
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


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 Germanium Diode

Connect the Germanium diode, OA95 (Figure 5) 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 2 and 5).




                                                                                                                                                            Figure 5. Germanium diode - OA95

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

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: Germanium 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 6. 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.

Part 2 - The Silicon Diode

Switch off the Power-Supply. Reassemble the circuit shown in Figure 3 with the Silicon Diode, 1N4001 in place of the Germanium one. A silicon diode has a band at the cathode end. Repeat the forward-biased measurements using the silicon diode, recording your results. Again reverse polarity, measure and record in Table 2.

For one of the mid-range forward bias diode voltage measurements, note the corresponding value of the source voltage.

Remember your photographs.
 

Forward Bias

Reverse Bias

VF [V]

IF [mA]

VR [V]

IR [μA]

0.2

 

1

 

0.3

 

2

 

0.4

 

3

 

0.5

 

4

 

 

1

5

 

 

2

6

 

 

3

7

 

 

4

8

 

 

5

9

 

 

6

10

 

 

7

15

 

                                     Table 2: Silicon Diode Measurements

Analysis of Results Part 1 and Part 2:

When you are doing your laboratory write-up: Plot both sets of results for the Germanium and Silicon diodes on the same 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 (or some suitable on-line plotting package - please e-mail me if you find a really good one). 


Part 3 - 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 7(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 7(a). The anode(+) is usually connected to a more positive source than the cathode (-). Figure 7(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 8. 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 8. 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 4 - 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 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 9. 3.3V Zener Diode BZX55C3V3

Figure 10 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 10. The Zener Diode Circuit

Wire up the circuit as illustrated in Figure 10. 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 3. Measure the input voltage supply and the output voltage using two multimeters set to measure DC voltage. Fill in the values in Table 3.

Step 2. Change Rs = 1000 Ohm (1k Ohm) and use the variable voltage supply to apply the voltages as listed in Table 3 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 3. The Zener Diode Circuit Behaviour

Analysis of Results Part 4:

Use the data in Table 3 to plot two curves for each of the resistor values, plotting VO against the Voltage Supply. Discuss what this characteristic means we can use this circuit for.

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

There is a template document at: Template Document to help you to write-up this laboratory.

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