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CHAPTER 3

CHAPTER 3. Special Diodes. OBJECTIVES. Describe and analyze the function and applications of: surge protectors varactors switching diodes LEDs & photodiodes trouble-shooting techniques for special diodes. Zener Diodes.

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CHAPTER 3

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  1. CHAPTER 3 Special Diodes

  2. OBJECTIVES Describe and analyze the function and applications of: • surge protectors • varactors • switching diodes • LEDs & photodiodes • trouble-shooting techniques for special diodes

  3. Zener Diodes • A zener is a diode with a defined value of reverse breakdown voltage • A zener is used in reverse breakdown mode • The voltage across a zener is more or less independent of the current through it • The function of a zener is to provide a voltage reference in a circuit

  4. Zener Characteristics Some important zener characteristics: • Nominal Zener Voltage : 5.1V zener, 12V zener, etc. • Nominal Bias Current: the Iz to get the nominal Vz • Tolerance on zener voltage, e.g. : 12V  5%, Maximum Power: 1Watt zener, 5 Watt zener, etc. • Temperature coefficient: by what % does zener voltage change as diode temp. changes 1OC • Dynamic Resistance (Rd): the change in zener voltage (V) caused by a change in zener current (I): Rd = V/ I

  5. Basic Zener Circuit Key points: • Vin > Vz • Iz = (Vin – Vz)/Rs > Load Current

  6. Calculation: Find R Suppose a 5.1 Volt zener is connected to a 12 Volt supply through a resistor. The zener requires a 15 mA bias, and the load is 510 Ohms. Find the required resistor value. • Find load current: IL = 5.1V / 510 = 10 mA • Find total current: IT = IL + IZ = (10 + 15) = 25 mA • Find drop across R: VR = 12V – 5.1V = 6.9 V • Find R: R = VR / IT = 6.9 V / 25 mA = 276 Ohms • Select standard value resistor: R = 270 Ohms

  7. Calculation: Find PMAX A 10 V zener has 20 mA of bias current. The load resistor Across the zener is 20 Ohms. What power rating should the zener have? Remember: if the load is removed, all current is in the zener. • Find total current: IT = IBIAS + ILOAD = 20mA + 50mA = 70 mA • Find power in zener (Pz) at a current (Iz) = 70 mA: Pz = Vz Iz = 10V  70ma = 700 mW • Double value for reliability: Use a zener rated for 1.5 Watts or higher

  8. Voltage Surge Protectors • Fast, high-voltage transients, called “spikes”, on AC power lines can damage electronic equipment. • Back-to-back zeners can clip off the spikes.

  9. Varactor Diodes A reverse-biased PN junction makes a voltage-controlled capacitor

  10. Varactor Capacitance <insert figure 3-12 here> Capacitance range: from 50 pF to 500 pF

  11. Calculation: C & fR If the varactor of figure 3-12 is biased at VR =5 V. • Find the capacitance from the graph. • Find the resonant frequency with a 253 uH inductor. From the graph, C = 100 pF. Resonant frequency fR = 1/(2LC) = 1.0 MHz

  12. Varactor Tuner Similar tuners are used in TVs, cell-phones, etc.

  13. The PIN Diode • Usable at high-frequencies • Shining light on the I region will generate electron-hole pairs

  14. Schottky Diodes • Not a PN junction • Fast, but reverse breakdown voltage less than 50 V

  15. LEDs: Light Emitting Diodes • Brightness proportional to current • Colors: red, white, blue, green, orange, yellow • Drop across an LED is about 1.5 Volts

  16. Calculation: Power in an LED How much power does an LED consume if it requires 25 mA and has a forward drop of 2.0 Volts? P = V  I = 2V  .025A = 50 mW

  17. The 7-Segment Display • Bright, but consumes a lot of power • Typically multiplexed to conserve power

  18. Power in a 7-Segment Display How much power would a 4-digit 7-segment LED display consume if each LED required 10 mA and had a forward drop of 1.5 Volts? Power in one LED: PLED = V  I = 2V  .01A = 20 mW Assume all segments are lit, then: Power in a Digit: PD = 7  PLED = 7  20mW = 140 mW Total Power: PT = 4  PD = 4  140 mW = 560 mW That’s over half a Watt!

  19. Multiplexing to Reduce Power Suppose a 4-digit display requires 400 mW if all segments are lit. If the display is multiplexed so that each digit is lit in a continuous sequence (1,2,3,4,1,2,3,4...) how much power would the display use? Since each digit is on for only 25% of the time, P = 0.25  400 mW = 100 mW

  20. Symbols for Special Diodes <insert igure 3-21 here>

  21. Troubleshooting • Silicon diodes can be checked for opens and shorts by measuring their resistance with a DMM or a VOM • Zener diodes are checked by measuring their voltage either in-circuit or in a test fixture. • LEDs can be checked out of circuit with a DC voltage source and a resistor. Put 10 to 20 milliamps through the LED and see if it lights. • Other special diode require special test fixtures, such as an oscillator circuit and frequency counter for a varactor.

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