1 / 21

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.

kathleen-mann
Download Presentation

CHAPTER 3

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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.

More Related