CHAPTER 3: Special Diodes potx

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 1 O C • 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. 1. Find load current: IL = 5.1V / 510 = 10 mA 2. Find total current: IT = IL + IZ = (10 + 15) = 25 mA 3. Find drop across R: VR = 12V – 5.1V = 6.9 V 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 <insert figure 3-12 here> Capacitance range: from 50 pF to 500 pF [...]... for Special Diodes 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. .. 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... 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 . CHAPTER 3 Special Diodes OBJECTIVES Describe and analyze the function and applications of: • surge protectors • varactors • switching diodes • LEDs & photodiodes • trouble-shooting. 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. generate electron-hole pairs Schottky Diodes • Not a PN junction • Fast, but reverse breakdown voltage less than 50 V LEDs: Light Emitting Diodes • Brightness