ITT Technical Institute. ET215 Devices I Chapter 2 Sections

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1 ITT Technical Institute ET215 Devices I Chapter 2 Sections

applications that exploit the fact that a diode is a one-way conductor.

2 Chapter 2 Section 2.8 Special-Purpose Diodes The preceding discussions of diodes has focused on applications that exploit the fact that a diode is a one-way conductor. A number of diodes are designed for other applications. We will now consider zener diodes, varactor diodes, photodiodes, and light-emitting diodes.

3 Zener Diodes The zener diode is a silicon pn junction device that differs from the rectifier diode in that it is designed for operation in the reverse-breakdown region.

4 Zener Diodes As a regulator, zeners have limitations: They do not have the high ripple rejection of integrated circuit regulators. They cannot handle large load-current changes. By combining a zener diode with a transistor or op-amp, better regulators can be constructed.

5 Notice that as the reverse voltage (V R ) is increased, the reverse current (I R ) remains extremely small up to the knee of the curve. At this point breakdown occurs; the internal zener ac resistance begins to decrease as the reverse current increases rapidly (shown on data sheets as Z Z ). A minimum value of reverse current, I ZK, must be maintained in order to keep the diode in regulation.

6 Zener Equivalent Circuit Ohms Law: the ratio of ΔV Z to ΔI Z is the zener impedance: Δ Z Z = ΔV Z / ΔI Z

7 Zener Equivalent Circuit Ohms Law: the ratio of ΔV Z to ΔI Z is the zener impedance: Z Z = ΔV Z / ΔI Z

8 EXAMPLE 2-8: A certain zener diode exhibits a 50 mv change in V Z for a 2mA change in I Z on the linear portion of the characteristic curve between I ZK and I ZM. What is the zener impedance? Solution: Z Z = ΔV Z / ΔI Z = 50 mv / 2 ma = 25 Ω

9 Zener Voltage Regulation Notice as the DC Power Supply voltage changes, the output across the zener diode remains constant.

10 EXAMPLE 2-9: Figure 2-50 shows a zener diode regulator designed to hold 10 V at the output. Assume the zener impedance is zero and the zener current ranges from 4 ma minimum (I ZK ) to 40 ma maximum (I ZM ). What are the minimum and maximum input voltages for these currents? Solution: For the min current, the voltage across the 1.0 KΩ resistor is: V R = I ZK R = (4 ma)(1.0k) = 4 V = 50 mv / 2 ma

11 EXAMPLE 2-9: Solution: For the min current, the voltage across the 1.0 KΩ resistor is: 4 V Since V R = V in V Z then V in = V R + V Z or V in = 14V For the max zener current, the voltage across the 1.0 KΩ resistor is V R = (40 ma)(1.0 KΩ) = 40 V therefore V in = 40 V + 10V = 50 V = 4V + 10V = 14 V

12 Varactor Diodes Varactor diodes are also known as variablecapacitance diodes because the junction capacitance varies with the amount of reverse bias voltage. Varactors are specifically designed to take advantage of this variable-capacitance characteristic. A varactor is basically a reverse-biased pn junction that utilizes the inherent capacitance of the depletion region. They are usually used in communications tuning circuits.

Because they are typically used in tuning circuits we can recall

13 Varactor Diodes Varactors capacitance typically range from a few picofarads to a few hundred picofarads. Because they are typically used in tuning circuits we can recall that the resonant frequency of the tack circuit is: f r 1 / (2π LC) With an approx value of Q > 10.

Determine the tuning range for the circuit if L = 10 mh.

14 EXAMPLE 2-10: The capacitance of a certain varactor can be varied from 5 pf to 50 pf. The diode is used in a tuning circuit similar to that shown in figure. Determine the tuning range for the circuit if L = 10 mh. C T(min) = (C 1(min) C 2(min) ) / (C 1(min) + C 2(min) ) = ((5pF)(5pF)) / (5pF + 5pF) = 2.5 pf The max resonant freq is f r 1 / (2π LC) = 1 / ( mh)(2.5pf) = 1 MHz C T(max) = (C 1(m) C 2(max) ) / (C 1(max) + C 2(max) ) = 25 pf

EXAMPLE 2-10: The max resonant freq is: f r 1 / (2π LC) = 1 / ( 6.

15 EXAMPLE 2-10: The max resonant freq is: f r 1 / (2π LC) = 1 / ( mh)(25 pf) = 1 MHz The min resonant freq is: f r 1 / (2π LC) = 1 / ( mh)(25 pf) = 318 khz

16 Light-Emitting Diodes As the name implies, light-emitting diodes emit light. Electroluminesence is the term used when one layer of a semiconductor material permits the photons to be emitted as visible light.

17 Light-Emitting Diodes As the name implies, light-emitting diodes emit light. Electroluminesence is the term used when one layer of a semiconductor material permits the photons to be emitted as visible light.

18 Photodiodes The photodiode is a pn junction that operates in reverse bias. The reverse current increases with the light intensity at the pn junction. When there is no light, the reverse current (I λ ) is almost negligible and it called the dark current.

19 Photodiodes

20 Photodiodes

21 Photodiodes Self-Study Section The Diode Data Sheet Review parameters pages 94-98

Troubleshooting Diodes 1. Ask Questions: 1. When did it fail? 2. How do you know it failed? 2. Power Check: 1. Is it plugged in? 2. Is a fuse blown? 3. Sensory Check: 1.

22 Troubleshooting Diodes 1. Ask Questions: 1. When did it fail? 2. How do you know it failed? 2. Power Check: 1. Is it plugged in? 2. Is a fuse blown? 3. Sensory Check: 1. Things you see; is it burnt? 2. Things you smell? 4. Isolate the Failure: 1. Half-splitting process of starting in the circuit middle. 2. This will isolate what is working and the failing area.

Troubleshooting Diodes 1. Open Fuse or Circuit Breaker: 1. Check for the cause before replacing? 2. If it is the fuse, replace with identical rating fuse. 2. Open Diode: 1.

23 Troubleshooting Diodes 1. Open Fuse or Circuit Breaker: 1. Check for the cause before replacing? 2. If it is the fuse, replace with identical rating fuse. 2. Open Diode: 1. Know the expected waveforms; i.e., the full wave rectification vs. a half-wave waveform 2. Diode will have very high resistance in both directions. 3. Use an oscilloscope when required. 3. Shorted Diode: 1. Diode will have very low resistance in both directions. 2. Things you smell?

Troubleshooting Diodes 4. Shorted or Leaky Filter Capacitor: 1. Lack of filtration as seen by ripple on waveform. 5. Troubleshooting a Regulated Power Supply 1. Is it repairable?

24 Troubleshooting Diodes 4. Shorted or Leaky Filter Capacitor: 1. Lack of filtration as seen by ripple on waveform. 5. Troubleshooting a Regulated Power Supply 1. Is it repairable? The exact strategy for troubleshooting depends on what is found at each step. The key is that the technician use a series of logical tests to reduce the problem to the exact cause.

25 Let s Look at Some Faulty Circuits

26 Troubleshooting Diodes

27 Troubleshooting Diodes

Chapter 3 SPECIAL PURPOSE DIODE

Chapter 3 SPECIAL PURPOSE DIODE 1 Inventor of Zener Diode Clarence Melvin Zener was a professor at Carnegie Mellon University in the department of Physics. He developed the Zener Diode in 1950 and employed