Course 7: Electronics

Course 7: Electronics

Electronics Topic 6: Diodes All copyright and intellectual property rights in respect of materials developed by the service provider during this project will vest in the Department of Higher Education and Training, which will have the right to allow any individual, company, agency or organisation to use or modify the materials for any purpose approved by this Department, including selling the materials or releasing them as Open Educational Resources (OER) under an appropriate copyright license. Assumed prior learning 05_01_00 05_01_02 05_02_01 05_03_01 05_04_01 Outcomes By the end of this unit the learner will be able to:

1. Identify and explain Zener breakdown voltages 2. Draw and interpret a typical Zener diode I-V curve 3. Explain the purpose of a series resistor in voltage regulation circuits 4. Calculate the resistance of the series resistor required for a voltage regulation circuit Unit 6.5: Voltage Regulation Introduction We learnt about Zener diodes in unit 2 of this topic when we were introduced the different types of diodes. We said then that Zener diodes are very useful for voltage regulation.

In this unit we will explore what voltage regulation is in more detail. Zener recap Before we continue, lets quickly recap Zener diodes. 1 Zener diodes are designed to be reverse biased When the voltage exceeds the Zener voltage, 2 the diode conducts. In the reverse direction The voltage remains constant at the rated 3 value, irrespective of the reverse current Click the button to see an interactive I-V Zener diode curve.

I-V curve Important Zener diodes are primarily used to regulate voltage, in other words, to keep the voltage steady. Zener diode experiment Using this schematic as a guide, build the circuit to explore what Zener diodes do. Rollover or touch each symbol in the schematic to find out more. Watch the video if you need help.

Watch how to build the circuit Review How To Use a Multimeter We now need to take some readings from our circuit and do some calculations. Refer to the multimeter guide saved on your device or click the image to see it again. Img02 Step 1 Measure the current flowing through the circuit.

mA Enter the current Check Img03 Step 2 Measure the voltage across the Zener diode. Img04 V

Enter the voltage Check Step 3 Now, replace the 5V Zener diode with a 10V, 1W diode. It is part number 1N4742A. Measure the current through the circuit and voltage across the diode. Enter the current mA Enter the voltage

V Check Img05 Step 4 Now add a second 9V battery to your circuit. Make sure to connect the positive terminal of one to the negative terminal of the other. What is the circuit voltage now? Measure the current through the circuit and voltage across the diode. Enter the current

mA Enter the voltage V Img06- Check Experiment results We have just seen that, unless the circuit input voltage exceeds the Zener breakdown voltage of a Zener diode, it does not allow current to flow. But as soon as the Zener breakdown voltage is exceeded, the Zener allows current to flow and drops its Zener

breakdown voltage. Watch the video to see each step in the experiment. Vid02 Zener voltage Lets explore Zener diodes and voltage regulation in more detail by using a virtual circuit. Download and work through the worksheet. Then watch the video to make sure you completed everything correctly. Download the worksheet Vid03

Draw a Zener diode I-V curve Click on the button to see the completed tables of values from the virtual circuits. Plot your own I-V curve for this 5V Zener diode, then take a photo of it and upload it to you portfolio. You can download an empty axis if you would like. Reverse biased Forward biased Axis Choose image Upload

Reverse biased results VS 1V 2V 3V 4V 5V 6V

7V 8V 9V 10V VZ (V) -1 -2 -3

-4 -4.92 -4.99 -5.02 -5.04 -5.05 -5.07 IT (mA)

0 0 0 0 -0.09 -10.1 -19.8 -29.6

-39.5 -49.3 Forward biased results VS 0.2V 0.4V VZ (V) 0.2

0.4 IT (mA) 0 0 0.6V 0.8V 1V 2V

0.599 0.715 0.745 0.795 0 0.85 2.55 12 4V 6V 8V

10V 0.84 0.872 0.9 0.926 31.6 51.3 71

90.7 Voltage regulation We have confirmed that, unless the circuit input voltage exceeds the Zener voltage of a Zener diode, it does not allow current to flow. But as soon as the Zener voltage is exceeded, the Zener allows current to flow and drops its rated Zener voltage. Watch the video to see how to create a voltage regulator with a Zener diode. YT01 Zener voltage We know that we can use Zener diodes to drop a specific

and constant voltage in a circuit. But what happens if the load current in a circuit changes? Lets explore this with a virtual circuit. Download and work through the worksheet. Then watch the video to make sure you completed everything correctly. Vid04 Download the worksheet Draw a Zener diode I-V curve Take pictures of your graphs of Zener Voltage vs Load Resistance and Zener current vas Load Resistance and upload these to your portfolio. Click the buttons to see the results again or download blank axes.

Results VZ Axis IZ Axis Choose image Upload Load current results RL ()) VZ (V) 10K

9.98 4K7 9.98 2K2 8.96 470 8.17 330 7.2 220 6V

150 4.86V IZ (mA) 8.18 8.08 4.73 0 0

0 0 IL (mA) 0.1 2.12 4.53 17.4 21.8

27.3 32.4 Zener voltage and current In order for a Zener diode to drop its full breakdown voltage, it needs a certain minimum current (IZmin) to flow through it. If IZ is below IZmin, the Zener will NOT drop its full breakdown voltage. Most Zeners have a stated IZmin of between 5mA to 10mA. The Izmin is usually indicated on the Zener diode I-V curve. Img11

Protecting Zener diodes You may be wondering why we always add a resistor when experimenting with Zener diodes. This is to limit the current through the Zener diode to a safe value and to drop excess circuit voltage. We need to check the power ratings of Zener diodes to make sure that they will not get destroyed in our circuit by excessive current. Remember: P = I x V Watch the video to learn more about power ratings. Zener Power ratings Zener power ratings

All Zener diodes have a power rating. The most common are 0.5W and 1W. We can use the power rating to work out the maximum current rating (IZmax) of a diode as well as its safe operating range. Watch the video to see how this is done. Vid05 For Zener diodes with a power rating of less than 1W assume IZmin = 5mA For Zener diodes with a power

rating of 1W or more, assume IZmin = 10mA What resistor should we use (example 1)? If you know the ISOR of a Zener diode and the supply voltage of the circuit, you can work out what resistor you will need to protect your Zener diode. See if you can work out the answer first and then watch the video to see if you are correct. Worked Solution

The midpoint of the SOR of a 5V, W Zener diode is 15mA. The Zener will be used in a circuit with a 12VDC supply. What resistor must be used to protect the Zener diode? What resistor should we use (example 2)? Here is another example to try on your own first. See if you can work out the answer first and then watch the video to see if you are correct.

Worked Solution A 10V, 1W Zener diode will be used in a circuit with a 25VDC supply. What resistor must be used to protect it? What resistor should we use (example 3)? Here is another example to try on your own first. See if you can work out the answer first and then watch the video to see if you are correct.

Worked Solution A 12V, 1W Zener diode will be used in a bridge rectification circuit with a 12VAC supply. What resistor must be used to protect it? Test Yourself We have come to the end of this unit. Answer the following questions to make sure you understand the basics of rectification and rectifiers. Question 1 In which circuit, will the Zener diode act as a voltage

regulator? a) b) c) d) Question 2 Which of the following statements is true for a reverse biased 12V Zener diode? a) If the supply voltage is 11V, the voltage dropped by the Zener will be 12V. b) The voltage dropped by the Zener diode will decrease as the supply voltage increases.

c) If the supply voltage is 14V, the voltage dropped by the Zener diode will be 12V. d) None of the above statements are true. Question 3 True of False: If the supply voltage exceeds the Zener breakdown voltage, the Zener will drop its rated Zener breakdown voltage irrespective of the load connected in parallel to it? a) True b) False Question 4 What is the IZmax rated value for a 20V, 11W Zener diode? a) b)

c) d) 55A 550A 550mA 55mA Question 5 What is the IZ Permissible values for a 22V, 12W Zener diode? a) b) c) d)

109.09mA 436.36mA 545.45mA 4.36A Question 6 What is the minimum current that must be passed through a 22V, 12W Zener diode for it to drop a constant voltage? a) b) c) d) 545.45mA 100mA 10mA

5mA Question 7 Which is the best choice of resistor to be connected in series with a 6V, W Zener diode if the supply current is 12VDC? a) b) c) d) 150 330 470 2K2

Question 8 Which is the best choice of resistor to be connected in series with a 10V, 1.5W Zener diode if the diode is connected to a bridge rectifier with an input voltage of 23.4VAC? a) b) c) d) 150 330 470 2K2 Video Briefing Vid01

Create a video presented by an expert presenter showing step-by-step how to build the circuit on a breadboard. The presenter needs to explain the following: 1. Add resistor and Zener to board esp. reverse biasing zener 2. Connect end to end with a jumper 3. Connect to power rail 4. Connect power rail to battery Video Briefing Vid02 Create a video presented by an expert presenter showing step-by-step how to take the measurements and do the calculations of the Zener diode experiment. The presenter needs to explain the following: 1. Measure the current and voltage with 9V battery and 5V Zener 1. Explain why current flows and why the voltage across the diode is 5V 2. Measure the current and voltage with 9V battery and 10V Zener 1. Explain why current does not flow and voltage across diode is 9V 3. Measure voltage and current with 2 x 9V batteries and 10V Zener 1. Explain why current flows and why voltage across diode is 10V

Document Briefing Doc01 (1 of 2) Create an annotated PDF worksheet with the following steps. Make sure you have downloaded the EveryCircuit App from your app store. If you are working on a computer, visit http://everycircuit.com/app/. Open the EveryCircuit app and signup. After your trial, you will still have access to EveryCircuit and other peoples circuits. You will just not be able to create your own circuits. Go to the community space and search for the circuit called NOC_Zener diodes reverse biased Open the circuit. You will see that we have a 1VDC supply connected to a 100 resistor and a 5V Zener diode. At the moment, no current is flowing. Why is this the case? Complete the first column of the table (VS = 1V) with the values of the total circuit current (IT) and the voltage across the Zener diode VZ. Remember that your VZ values will be negative Reverse Biased

VS VZ (V) 1V 2V 3V 4V 5V 6V 7V

8V 9V 10V Document Briefing Doc01 (2 of 2) 1. 2. 3. 4. 5. Now increase the supply voltage to 2V and complete the 2nd column of the table. Complete the rest of the table, changing the supply voltage as necessary each time.

What do you notice about the IT at VS = 5V? What do you notice about IT and VZ from VS = 6V to VS = 10V? Go to the community space and search for the circuit called NOC_Zener diodes forward biased. This is the same circuit, except that the diode is not in the forward biased direction. 6. Complete this table. Forward Biased VS VZ (V) IT (mA) 0.2V 0.4V 0.6V

0.8V 1V 2V 4V 6V 8V 10V Video Briefing Vid03

A screencast video presented by an expert electrician working through Doc01. 1. Explain why no current is flowing initially 2. Complete both tables of values. 3. Explain what the values in the tables tell us about the operation of Zener diodes. 4. Use the graph of the results available at https://www.desmos.com/calculator/vfcsxxxy6f to help show that in the forward bias direction the diode behaves like a normal silicon diode with a forward voltage of about 0.7V but that it also has a Zener voltage (reverse voltage) of -5V. Using the shape of the graph, show how the voltage drop at the Zener breakdown voltage is slightly more constant the line is more vertical than at the forward voltage. Image Briefing Img08 Download from - https://www.desmos.com/calculator/hu5sgkzqek. Format for A4. Document Briefing Doc02 (1 of 2) Create an annotated PDF worksheet with the following steps. 1. Make sure you have downloaded the EveryCircuit App from your app store.

2. If you are working on a computer, visit http://everycircuit.com/app/. 3. Open the EveryCircuit app and signup. After your trial, you will still have access to EveryCircuit and other peoples circuits. You will just not be able to create your own circuits. 4. Go to the community space and search for the circuit called NOC_Zener and load current 5. Open the circuit. 6. You will see that we have a 12VDC supply connected to a 220 resistor in series with a 10V Zener diode. 7. Connected across the Zener is a load resistor which has a resistance (RL)of 10K. 8. Complete the table by changing the resistance of the load resistor each time. For each value of the load resistor, measure the voltage across the load and Zener diode (VZ), the current through the Zener diode (IZ) and the current through the load resistor (IL). To measure IZ and IL, click on the Zener diode and the load resistor and the click on the eye icon. RL ()) VZ (V) 10K

4K7 2K2 470 330 220 150 Document Briefing Doc02 (2 of 2) 1. 2. 3.

4. 5. 6. 7. What do you notice about the relationship between VZ and RL? What do you notice about the relationship between IZ and RL? What do you notice about the relationship between IL and RL? Sketch a graph of IZ vs RL. How does the load resistance affect IZ? Now, sketch a graph of VZ vs RL. How does the load resistance affect VZ? How does the current through the Zener diode affect its ability to drop a constant voltage? What is the minimum current required through the Zener diode for it to drop its rated Zener breakdown voltage? Video Briefing Vid03 A screencast video presented by an expert electrician working through Doc02. 1. Show how to take the readings and complete the table.

2. Describe and explain the relationship between VZ and RL? 3. Describe and explain the relationship between IL and RL? 4. Describe and explain the relationship between IZ and IZ? 5. Sketch a graph of IZ vs RL and explain how the load resistance affects IZ? 6. Sketch a graph of VZ vs RL and explain how the load resistance affects VZ? 7. Explain that a Zener voltage cannot hold its defined breakdown voltage unless a specific amount of current is flowing through it. As the load resistance decreases, more current flows through the load and less through the Zener diode, until the Zener is unable to hold is Zener breakdown voltage. 8. IZ has to be a certain minimum for VZ to be constant. In this case the minimum IZ for this is about Image Briefing Img09 Download from - https://www.desmos.com/calculator/y5zqboma7b. Format for A4. Image Briefing Img10 Download from - https://www.desmos.com/calculator/egjbfhxtib. Format for A4.

Video Briefing Vid05 (1 of 2) A screencast video presented by an expert electrician showing the following: How to calculate IZmax from PZ for a 1W Zener. But we normally like to remain within 80% of this theoretical maximum Therefore the practical IZmax = 80% x 100mA = 80mA We also usually assume that Zeners with a PZ < 1W have IZmin = 5mA and where PZ > 1W, IZmin = 10mA With this we can work out the mid-point of the functional or safe operating (SOR) range for a 1W Zener Video Briefing Vid05 (2 of 2) Show these values on this graph (Img12) Video Briefing Vid06

A screencast video presented by an expert electrician showing how to solve the problem. We know that the Zener will drop 5V so the voltage drop across the resistor will be VR = VS VZ = 12V 5V = 7V We also know that we need about 15mA to flow through the Zener and the resistor. Therefore, R = V/I = 7V/0.015A = 466.67. Se we will need to use a 470 resistor to protect the diode. Check your answer Video Briefing Vid07 A screencast video presented by an expert electrician showing how to solve the problem. We can calculate the Izmax: 1W/10V = 0.1A = 100mA. But Iz Permissible = 80% x 100mA = 80mA As a 1W Zener, we assume that Izmin = 10mA. Therefore, the SOR is 10mA 80MA. Therefore the midpoint is (10 + 80)/2 = 45mA. We want 45mA flowing through the Zener. The Vs = 25V so VR = VS VZ = 25V 10V = 15V R = V/I = 15V/0.045A = 333. So we should use a 330 resistor.

Video Briefing Vid08 A screencast video presented by an expert electrician showing how to solve the problem. Things are a bit more complicated because of the AC supply and the bridge rectifier. Firstly the supply is a 12VAC. Therefore the RMS value of the supply is 12V. So the peak voltage (Vp) = 12V/0.707 = 16.97. We need to work with Vp to make sure that the Zener is not damaged by any peak current. But the 2 diodes in the bridge rectifier will drop about 1.2V. Therefore the Vp the Zener will see is 16.97V 1.2V = 15.77V. The voltage drop over the resistor will be VR = Vp Vz = 15.77V 12V = 3.77V. A 1W Zener will have a Izmax = 100mA and a Izmin = 10mA. But Iz Permissible = 80% of Izmax = 80mA. Therefore mid point of ISOR = 45mA. So the current needs to be limited to 45mA.

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