PHYS 162 - Chapter 3
Special-Purpose Diodes
CHAPTER 3 SPECIAL-PURPOSE SPECIAL-PURPO SE DIODES 3-1 THE ZENER DIODE -
Zener diodes are commonly used for voltage regulation, that is, they maintain a constant voltage at the output.
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It is designed to work in the reverse breakdown region.
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The breakdown voltage is adjusted by controlling the level of doping.
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The V-I characteristic and zener diode symbol are shown in Figure 1.
3.1.1 Zener Breakdown -
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There are two types of breakdowns in zener diodes. o
Zener Breakdown
o
Avalanche Breakdown
Avalanche breakdown occurs at high voltages typically more than 5 V.
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Zener breakdown occurs at low voltages typically less than 5 V.
3.1.2 Breakdown Characteristics -
As the reverse voltage VR increases from 0V, the reverse current IR remains extremely small.
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The reverse current starts to increase rapidly when the VR reaches the knee point.
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The current at this point is called the zener knee
Figure 1 Zener diode symbol and V-I charateristics
current IZK. -
The breakdown effect starts at this point.
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After this, the current starts to increase as the zener impedance ZZ decreases.
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From the knee, the zener voltage V Z remains almost constant. Zener Regulation
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Keeping the voltage constant across its terminal is the main advantage of zener diode.
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The minimum zener current required maintaining voltage regulation is IZK.
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The maximum zener current allowed before it is damaged is IZM.
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Figure 2 Reverse characteristics of zener diode
The zener voltage VZ, specified in the datasheets is the voltage at the zener test current IZT.
Prepared By: Syed Muhammad Asad – Semester 102
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PHYS 162 - Chapter 3
Special-Purpose Diodes
3.1.3 Zener Diode Model or Equivalent Circuit -
Zener diodes are modeled in two approximations. o
Ideal Zener Diode
o
Practical Zener Diode
Ideal Zener Diode
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Ideal zener diode has constant voltage drop equal to the nominal zener voltage VZ.
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This drop is represented as a voltage source even though the zener diode does not produce any voltage.
Figure 3 Ideal zener diode
Practical Zener Diode
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The practical diode includes the zener impedance ZZ.
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Since the zener voltage is not constant, a change in zener current Δ , produces a small change in the zener voltage Δ .
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Then the zener impedance is given by
= -
Δ Δ
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It is best to avoid the zener diode to operate at the knee current because the impedance changes rapidly in this area.
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The impedance is usually very small and can be neglected in calculations.
Figure 4 Practical zener diode
NOTE: REFER EXAMPLE 3-2 PAGE 111
3.1.4 Temperature Coefficient -
Temperature coefficient ( ) describes the percent change in zener voltage for each degree Celsius change in temperature.
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The formula for calculating the change in zener voltage Δ is given by
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Δ = × × Δ A positive temperature coefficient means the zener vol tage will increase with increase in temperature or decrease with decrease in temperature.
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A negative temperature coefficient means the zener voltage will decrease with increase in temperature or increase with decrease in temperature.
3.1.5 Zener Power Dissipation and Derating -
Zener diode are specified to operate at the maximum DC power dissipation PD(max).
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This is given in datasheet of the diode.
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The power dissipated by a zener diode at any zener current IZ is given as = .
Power Derating
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The PD(max) of a zener diode is specified at a certain value, for example 50°C.
Prepared By: Syed Muhammad Asad – Semester 102
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PHYS 162 - Chapter 3
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Special-Purpose Diodes
Above this temperature, the maximum power dissipation is reduced according to a derating factor (DF).
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Derating factor (DF) is expressed in mW/°C.
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The maximum derated power can be expressed as = − × Δ. NOTE: REFER EXAMPLE 3-3 PAGE 111
3.1.6 Zener Diode Datasheet Information NOTE: You will find the datasheet of the zener diode at the end of the notes.
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The most popular zener diode series is 1N4728A-1N4764A. Absolute Maximum Ratings
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The maximum power dissipation, P D is given as 1W at 50°C.
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The power derate factor (DF) is given as 6.67mW/°C above 50°C.
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The maximum reverse current IZM is not given but can be found for any VZ as = Electrical Characteristics
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For each device, minimum, typical and maximum zener voltages VZ are given.
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The zener voltages are measured at zener test current IZ which are given in the datasheet.
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Maximum zener impedance, ZZ is given at IZ.
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Minimum zener impedance, ZZK, at the knee of the characteristics curve is given at IZK.
3-2 ZENER DIODE APPLICATIONS -
Zener diode can be used as a type of voltage regulator.
3.2.1 Zener Regulation with a Varying Input Voltage (No Load Condition) -
Zener diode regulators are not very efficient
so
they
are
limited
to
applications that require low current to the load. -
Figure 5 illustrates the concept of zener voltage regulation.
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The zener diode will regulate the output voltage provided > and
< < . -
As the input voltage V IN increases, IZ will increase and there will be very small change in the output voltage
+ Δ . -
Figure 5 Zener voltage regulation
As the input voltage VIN decreases, IZ will decrease and there will be very small change in the output voltage − Δ .
Prepared By: Syed Muhammad Asad – Semester 102
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PHYS 162 - Chapter 3
Special-Purpose Diodes
NOTE: REFER EXAMPLE 3-5 PAGE 116
3.2.2 Zener Regulation with a Variable Load -
The zener diode will maintain a constant voltage across the load resistor R L as long as zener current I Z in within IZK and IZM, i.e., < < .
3.2.3 From No Load to Full Load No Load
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When the output terminal of a zener diode are open it mean = ∞.
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All the current goes to the zener diode and the load current is zero. This is no load condition. Full Load
Figure 6 Zener regulation with variable load
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When a load resistance RL is connected, some current I L goes to RL.
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As the load resistance decreases, the load current I L increases and zener current I Z decreases.
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The zener diode will regulate the voltage until IZ is equal to IZK.
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At this point, load current I L is maximum and this is full load condition. NOTE: REFER EXAMPLE 3-6 & 3-7 PAGE 117-119
Figure 7 Basic zener diode limiter
Zener Limiting -
Zener diodes can be used in AC applications as AC voltage limiters.
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Circuit in Figure 7(a) will limit the forward voltage to VZ while in the negative cycle, the zener will act as a forward biased diode and limit the negative voltage to 0.7V.
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Circuit in Figure 7(b) will act in the opposite manner to the circuit in (a).
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Back to back zener diodes as in Figure 7(c) will limit both peaks to ± ± 0.7 .
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In the positive cycle, D1 acts like a forward biased diode while D 2 acts like zener diode so the peak will be limited to + 0.7V. In the negative cycle, D2 acts like a forward biased diode while D 1 acts like zener diode so the peak will be limited to − − 0.7V.
Prepared By: Syed Muhammad Asad – Semester 102
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PHYS 162 - Chapter 3
Special-Purpose Diodes
NOTE: REFER EXAMPLE 3-8 PAGE 121
3-3 THE VARACTOR DIODE -
The junction capacitance of diodes change with reverse bias.
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Diodes designed to be used as voltage controlled capacitors are called varactors.
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They always work in reverse bias.
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The depletion region acts as dielectric and p and n-types act like capacitor plates. Figure 8 Varactor
3.3.1 Basic Operation -
The capacitance of a material can be determined by plate area A, dielectric constant and plate separation d and is expressed as
=
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As the reverse bias increases, the depletion region becomes wide and increases the plate separation d . This decreases the capacitance.
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As the reverse bias decreases, the depletion region becomes narrow and decreases the plate separation d . This increases the capacitance.
3-4 OPTICAL DIODES -
Two types of optical diodes are discussed in this section. o
Light-Emitting Diode (LED) – These are light emitters.
o
Photodiodes – These are light detectors.
3.4.1 The Light-Emitting Diodes (LED) -
Figure 9 shows the symbol of an LED.
Figure 9 LED
Basic Operation
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We know that the free electrons in the n-type have high energy.
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When the LED is forward biased, electrons cross the pn junction and recombines with the holes in the p-type.
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When these high energy electrons recombine with holes, they release energy in the form of photons.
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The emission of these photons is called electroluminescence.
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The doping determines the wavelength of the emitted photons. LED Biasing
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The forward voltage VF across an LED is higher than silicon rectifier diodes (typically between 1.2V to 3.2V).
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Figure 10 Electroluminescence in LED
Reverse breakdown is lower than silicon rectifier diodes (3V to 10V).
Prepared By: Syed Muhammad Asad – Semester 102
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PHYS 162 - Chapter 3
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Special-Purpose Diodes
Figure 11(a) shows a typical LED circuit. The graph in Figure 11(b) shows that power of light output is directly proportional to the forward current IF. Applications of LED 1. Seven – Segment Display
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LED’s are widely used in various
Figure 11 LED operation
applications like indicator lights, read out displays etc. -
A very common application of LED’s is the seven-segment display shown in Figure 12.
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Each segment is an LED.
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By forward biasing selected combination of segments, any decimal digits can be formed.
Figure 12 7-segment display
2. Remote Controls
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Infrared LED’s are commonly used in remote controls.
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Infrared LED emits beam of invisible light.
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Each button corresponds to an electrical code which is converted to a light code and transmitted through the LED.
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The receiver recognizes the code and takes the required action.
Prepared By: Syed Muhammad Asad – Semester 102
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PHYS 162 - Chapter 3
Special-Purpose Diodes
Figure 13 Counting and control system
3. Industrial Application
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Infrared LED’s can be used in industry for various applications. Figure 13 shows a ball counting system.
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Each ball passing through the tube interrupts the IR beam emitted by the i nfrared LED.
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This is detected by the detector and the circuit counts each ball.
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When a set number of balls are counted, the flow of balls in stopped in order f or the next empty box to be set in the proper place.
3.4.2 High Intensity LED’s -
These LED’s produce high light output than ordinary LED’s. Applications 1. Traffic Lights
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Arrays of tiny LED’s form the red, yellow
and green lights in a traffic signal. -
Figure 14 LED array and circuit
LED arrays shown in Figure 14 have 3 advantages compared to ordinary bulbs. o
Brighter light
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Longer life (years vs. months)
o
Less energy consumption (about 90% less)
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The array is connected in series-parallel through limiting resistors.
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The lens in front of the LED (Figure 15) is used to properly direct light towards the viewer.
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Different color LED’s require different forward voltages to operate.
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Referring to Figure 16, red LED requires 2V, blue requires between 3V-4V, green requires between 2.5V-3V.
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PHYS 162 - Chapter 3
Special-Purpose Diodes
Figure 15 Lens directing LED light
Figure 16 V-I characteristics of visible light LED
NOTE: REFER EXAMPLE 3-11 PAGE 134 2. Display LED’s
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LED’s are widely used message boards and
large-screen television. -
Full-color displays use small groups of high intensity red, green and blue LED’s. This
group is called a pixel (Figure 17). -
A typical screen contains thousands of such pixels (Figure 18).
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Mixing of the primary colors red, green and blue (RGB) in various amount can produce any color (Figure 17).
Figure 17 RGB concept
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PHYS 162 - Chapter 3
Special-Purpose Diodes
Figure 18 Display formed by RGB pixels
3.4.3 The Photodiode -
A photodiode is a device that works in the reverse bias.
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It has small window that allows light to strike the pn junction.
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The reverse current increases when the pn junction is exposed to light (Figure 19).
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When there is no light on the pn junction, the reverse current is minimum and is called dark current.
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Increase in the amount of light produces an increase in the
Figure 19 Reverse bias operation of photodiode
reverse current .
3-5 OTHER TYPES OF DIODES -
There are other types of diodes used for special purpose and applications. They will be discussed in this section. Some of these are o
The Laser Diode
o
The Schottky Diode
o
The PIN Diode
o
The Tunnel Diode
o
Current Regulator Diode
3.5.1 The Laser Diode -
Laser stands for l ight amplification by stimulated emission
Figure 20 Laser diode symbol and construction
of r adiation.
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Laser light is monochromatic meaning it consists of single light.
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Construction The pn junction is formed by two layers of doped gallium arsenide (Figure 20).
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The length of the pn junction is related to the wavelength of the laser.
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There is highly reflective surface on one end of the pn junction and partially reflective on the other.
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PHYS 162 - Chapter 3
Special-Purpose Diodes
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Operation The laser diode is forward biased by external voltage source.
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As electrons move through the junction, they recombine with holes.
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This recombination releases photons.
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These photons strike atoms and release other photons.
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This process increases as the forward current is increased and more and more photons are r eleased due to avalanche effect.
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At the certain point, some of these photons strike the hi ghly reflective surface and move along the depletion region and pass through the partially reflective end of the junction (Figure 20c).
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Each photon produced is identical in wavelength, phase and frequency.
The Schottky Diode -
Schottky diodes (Figure 21) are high current diodes used in high frequency and fast switching applications.
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A schottky diode is formed by joining a doped n -type with a metal such as gold, silver of platinum.
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It means it has a metal-to-semiconductor junction rather pn junction.
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The forward voltage drop is 0.3V.
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There are only majority carriers with no reverse leakage current.
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The metal conductor has many conduction band electrons and n-type is also heavily doped.
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When forward biased, the n-type electrons move across to the metal region and rapidly loss energy.
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The process is very fast which makes schottky diodes ideal for fast switching application.
Figure 21 Schottky diode symbol and structure
The PIN Diode -
The pin diode consists of heavily doped p and n region separated by intrinsic (i) region (Figure 22a).
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In reverse bias, the pin diode acts like a constant capacitance (Figure 22b).
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When forward biased, it acts like a current-controlled variable resistance (Figure 22c).
The Tunnel Diode -
The tunnel diode (Figure 23) exhibit a characteristic known as negative resistance.
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This makes it useful in oscillator applications.
Figure 23 PIN diode Figure 22 Tunnel diode
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The p and n region of the tunnel diode are heavily doped.
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This means the depletion region is very narrow and results in large reverse current effective ly having no breakdown effect.
Prepared By: Syed Muhammad Asad – Semester 102
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PHYS 162 - Chapter 3
Special-Purpose Diodes
Current Regulator Diode -
The current regulator diode keeps a constant current rather than constant voltage as in the case of zener diode.
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The current regulator works in the forward bias and the forward current remains constant for forward voltage range from 1.5V to 6V.
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The constant forward current in called the regulator current I P.
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This device should never be operated in reve rse bias.
Prepared By: Syed Muhammad Asad – Semester 102
Figure 24 Current regulator diode
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