CHAPTER 3: Special Diodes potx

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 z

CHAPTER 3

Special 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

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

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

<insert igure 3-21 here>

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