CHAPTER 3: Special Diodes

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Nội dung Text: CHAPTER 3: Special Diodes

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 • 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
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
Basic Zener Circuit Key points: • Vin > Vz • Iz = (Vin – Vz)/Rs > Load Current
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 1. 2. Find total current: IT = IL + IZ = (10 + 15) = 25 mA Find drop across R: VR = 12V – 5.1V = 6.9 V 3. 4. Find R: R = VR / IT = 6.9 V / 25 mA = 276 Ohms 5. Select standard value resistor: R = 270 Ohms
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. 1. Find total current: IT = IBIAS + ILOAD = 20mA + 50mA = 70 mA 2. Find power in zener (Pz) at a current (Iz) = 70 mA: Pz = Vz Iz = 10V  70ma = 700 mW 3. Double value for reliability: Use a zener rated for 1.5 Watts or higher
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.
Varactor Diodes A reverse-biased PN junction makes a voltage-controlled capacitor
Varactor Capacitance Capacitance range: from 50 pF to 500 pF
Calculation: C & fR If the varactor of figure 3-12 is biased at VR =5 V. A) Find the capacitance from the graph. B) Find the resonant frequency with a 253 uH inductor. From the graph, C = 100 pF. Resonant frequency fR = 1/(2LC) = 1.0 MHz
Varactor Tuner Similar tuners are used in TVs, cell-phones, etc.
The PIN Diode • Usable at high-frequencies • Shining light on the I region will generate electron-hole pairs
Schottky Diodes • Not a PN junction • Fast, but reverse breakdown voltage less than 50 V
LEDs: Light Emitting Diodes • Brightness proportional to current • Colors: red, white, blue, green, orange, yellow • Drop across an LED is about 1.5 Volts
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
The 7-Segment Display • Bright, but consumes a lot of power • Typically multiplexed to conserve power
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!
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
Symbols for Special Diodes