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: I L = 5.1V / 510  = 10 mA 2.Find total current: I T = I L + I Z = ( ) = 25 mA 3.Find drop across R: V R = 12V – 5.1V = 6.9 V 4.Find R: R = V R / I T = 6.9 V / 25 mA = 276 Ohms 5.Select standard value resistor: R = 270 Ohms

Calculation: Find P MAX 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: I T = I BIAS + I LOAD = 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 & f R If the varactor of figure 3-12 is biased at V R =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 f R = 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: P LED = V  I = 2V .01A = 20 mW Assume all segments are lit, then: Power in a Digit: P D = 7  P LED = 7  20mW = 140 mW Total Power: PT = 4  P D = 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

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.