CHAPTER 3

Special

Diodes

OBJECTIVESDescribe 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 CharacteristicsSome 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 RSuppose 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 mA2. Find total current: IT = IL + IZ = (10 + 15) = 25 mA 3. Find drop across R: VR = 12V – 5.1V = 6.9 V4. Find R: R = VR / IT = 6.9 V / 25 mA = 276 Ohms5. Select standard value resistor: R = 270 Ohms

Calculation: Find PMAXA 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/(2LC) = 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 LEDHow 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 DisplayHow 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 PowerSuppose 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.