Zener diodes have a zener voltage (avalanche or breakdown voltage) in the range of 3 V to 200 V as

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1 Exercise 2: EXERCISE OBJECTIVE When you have completed this exercise, you will be able to demonstrate the operation of a zener diode by using a dc characteristic curve. You will verify your results with a multimeter. DISCUSSION The forward biased resistance of a diode is low and permits a large current. Its reverse biased resistance is high and permits very little current until the breakdown voltage is reached. Then the reverse current starts to increase rapidly. The breakdown voltage is also called the avalanche or zener voltage. The avalanche or zener voltage of a zener diode is at the a. forward bias voltage. b. breakdown voltage. A zener diode (CR1) is a PN junction diode designed to operate safely at the avalanche or zener voltage. The zener diode symbol has a Z-shaped line at its cathode. Zener diodes have a zener voltage (avalanche or breakdown voltage) in the range of 3 V to 200 V as The forward voltage drop of a zener is about the same as a conventional diode (around 0.6 to 0.9 Vdc). Festo Didactic P0 155

2 The zener diode is a. CR1. b. CR2. Z ) is approached (6.8 V ±10% for this zener diode). 156 Festo Didactic P0

3 The zener region (breakdown point) is at the knee of the curve. At the zener region of a dc characteristic curve for a zener diode, the a. forward current increases very rapidly with a slight increase in voltage. b. reverse current decreases very rapidly with a large increase in reverse voltage. c. reverse current increases very rapidly with a slight increase in reverse voltage. general purpose zener diode. The zener test current (I ZT = 20 ma) is the zener current, (V Z ) is within its tolerance range (6.8 V ±10% for this diode). Festo Didactic P0 157

4 The part of the curve where the current increases slowly with increases in the zener voltage is the soft region. In the stiff region, where V Z zener voltage. The zener test current (I ZT ) is at a point on the characteristic curve where the zener current a. increases slowly with an increase in reverse voltage. b. starts to increase very rapidly with a small increase in the zener voltage. 158 Festo Didactic P0

5 This is a schematic of a typical zener test circuit. The source voltage (V A ) provides a voltage higher than the zener. The resistance of R2 is selected to keep I Z at a value equal to the zener test current (I ZT ). Otherwise, the zener could be damaged by excessive power dissipation. In the circuit shown, the zener current (I Z ) would be a. (V A V Z b. V Z The value of I ZT for the zener diode used in the following procedure is 37 milliamps. In practice, the zener test current varies with the voltage and power rating of the zener diode. ZT Festo Didactic P0 159

6 PROCEDURE Locate the ZENER DIODE REGULATOR circuit block. Forward bias the zener diode (CR1) with an ohmmeter by connecting the red (positive) meter lead to the anode and the black (negative) meter lead to the cathode. The meter indicates that the zener diode is a. conducting. b. not conducting. Reverse bias the zener diode (CR1) with an ohmmeter by connecting the red (positive) meter lead to the cathode. The meter indicates that the zener diode is a. conducting. b. not conducting. 160 Festo Didactic P0

7 Connect the circuit shown. Adjust the negative variable supply for 2.0 Vdc at the test point between R1 and R2. Use a multimeter to measure the voltage. Measure the voltage across zener CR1 (V CR1 ). V CR1 = Vdc (Recall Value 1) Does your measurement of V CR1 ( Vdc [Step 5, Recall Value 1]) indicate that CR1 is forward or reverse biased? a. reverse biased b. forward biased Festo Didactic P0 161

8 Increase the negative variable supply to 2.5 Vdc at the test point between R1 and R2. Measure V CR1. V CR1 = Vdc (Recall Value 2) Based on your V CR1 measurements of Vdc (Step 5, Recall Value 1) and Vdc (Step 7, Recall Value 2) when the supply voltage was increased, forward biased? a. yes b. no Connect the circuit shown. The positive variable supply (V A ) is in the circuit. In the following procedure steps, you will determine the zener diode (CR1) reverse current (I Z ) versus the reverse bias voltage (V CR1 ) relationship by recording measurements and calculations in a table. 162 Festo Didactic P0

9 Set the positive variable supply (V A ) to 6.0 Vdc. V A is set at 6.0 Vdc. a. Measure V CR1. Vdc (Recall Value 3) b. Measure V R3. NOTE: The answer should be in mvdc. mvdc (Recall Value 4) Festo Didactic P0 163

10 Set the positive variable supply (V A ) to 7.0 Vdc. V A is set at 7.0 Vdc. a. Measure V CR1. Vdc (Recall Value 5) b. Measure V R3. mvdc (Recall Value 6) 164 Festo Didactic P0

11 Set the positive variable supply (V A ) to 8.0 Vdc. V A is set at 8.0 Vdc. a. Measure V CR1. Vdc (Recall Value 7) b. Measure V R3. mvdc (Recall Value 8) Festo Didactic P0 165

12 Set the positive variable supply (V A ) to 10.0 Vdc. V A is set at 10.0 Vdc. a. Measure V CR1. Vdc (Recall Value 9) b. Measure V R3. mvdc (Recall Value 10) 166 Festo Didactic P0

13 Complete the following table to show the I Z (in ma) versus V CR1 data that you obtained. V A V CR1 V R3 I = V R3 / Vdc Vdc mvdc (Step 11, Recall Value 3) (Step 11, Recall Value 4) 7.0 Vdc Vdc mvdc (Step 13, Recall Value 5) (Step 13, Recall Value 6) 8.0 Vdc Vdc mvdc (Step 15, Recall Value 7) (Step 15, Recall Value 8) 10.0 Vdc Vdc mvdc (Step 17, Recall Value 9) (Step 17, Recall Value 10) ma ma ma ma The graph shows the reverse bias section of a typical zener dc characteristic curve. Compare your data to the graph. Does your zener current (I Z ) versus voltage drop (V CR1 ) data match a typical zener dc characteristic curve? a. yes b. no I Z is about 0 ma when V CR1 is less than 6.0 Vdc because the zener a. voltage has not been reached. b. is forward biased. Is the zener voltage (V Z ) about Vdc (Step 15, Recall Value 7)? a. no b. yes Festo Didactic P0 167

14 Is the zener test current (I ZT ) between (Recall Value 10 a. yes b. no (Recall Value 8 CONCLUSION A zener is designed to operate safely at the reverse breakdown point. Your measurements of the reverse bias characteristics of a zener diode showed that the diode is turned off until its zener voltage (V Z ) is reached at the breakdown point. At the zener voltage (V Z ), the zener reverse current increases rapidly with very small increases in reverse voltage. In a zener diode circuit, a resistor is placed in series with the zener diode to limit the current to a value equal to the zener test current (I ZT ). REVIEW QUESTIONS 1. a. it is designed to operate at the forward voltage drop. b. it is made from glass instead of ceramic material. c. it is designed to operate at the breakdown voltage. d. 2. At the zener voltage, the reverse current a. decreases very rapidly with small increases in reverse voltage. b. equals the leakage current. c. d. increases very rapidly with small increases in reverse voltage. 3. The zener voltage is the a. reverse breakdown voltage of a zener diode. b. forward voltage drop of a zener diode. c. d. voltage region before the breakthrough voltage. 4. The zener test current (I ZT ) a. occurs at the knee of the reverse voltage and current curve. b. occurs when the zener overheats due to excess power generation. c. sets the zener voltage within its tolerance limits. d. occurs when a forward bias is applied to the zener diode. 168 Festo Didactic P0

15 5. If the voltage drop is 1.0 Vdc across a 50 resistor in series with a zener diode having a zener a. 20 ma. b. equal to I ZT. c. 50 ma. d. 136 ma. Festo Didactic P0 169