Download Fundamentals of Linear Electronics Integrated & Discrete

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.
1.
2.
3.
4.
5.
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
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.