Abesamis, Laguna 1. IDENTIFY THE TYPES OF

Abesamis, Beatrice A.                         Prelim                                 November 30, 2017

EC42FB1/ECE 003                      Homework No. 1                       Engr. Laguna

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1. IDENTIFY THE TYPES OF DIODES.

Fig. 1.   
Diode

A diode is a two-terminal device, having
two active electrodes, between which it allows the transfer of current in one
direction only. Technically, diodes are used for the purpose of rectifying
waveforms. They can also be used in circuits where ‘one way’ effect of diode is
needed.

Usually, diodes are made from
semiconductors as silicon. Diodes carries electric currents in one direction,
but, the manner in which they do so can vary. The different types of diodes are
presented below.

a.      
Light Emitting Diode

 

Fig. 2.   
Light Emitting Diode

. It is a semiconductor device that emits
visible light when an electric current pass through it.

b.      
Avalanche Diode

Fig. 3.   
Avalanche Diode

This operates in the reverse bias, and
used avalanche effect for its operation. When the voltage drop is constant and
is independent of current, the avalanche breakdown occurs across the entire PN
junction. Basically, this is used for photo-detection, wherein high levels of
sensitivity can be acquired by the avalanche process.

c.       
 Laser
Diode

Fig. 4.   
Laser Diode

The term LASER stands for Light Amplification
by Stimulated Emission of Radiation. Laser light is monochromatic, which means
that it consists of only a single color. Laser light is also called coherent
light which means it is only a single wavelength.

d.      
Schottky Diode

 

Fig. 5.   
Schottky Diode

This is also known as hot-carrier diodes.
These are high-current diodes used primarily in high-frequency and
fast-switching applications. The term hot-carrier is derived from the higher
energy level of electrons in the n region compared to those in the metal
region.

e.       
Zener Diode

Fig. 6.   
Zener Diode

It is a silicon p-n junction device that
is designed for operation in the reverse-breakdown region. The breakdown
voltage of a Zener diode is set by carefully controlling the doping level
during manufacture.

f.       
Photodiode

Fig. 7.   
Photodiode

It is a is a device that operates in
reverse bias and has a small transparent window that allows light to strike the
p-n junction.

g.      
Varicap/Varactor Diode

Fig. 8.   
Varactor

Varactor, short for variable
capacitance diode and also known as varicap, variable reactance diode, or
tuning diode. It is a diode that always operates in reverse bias and is doped
to maximize the inherent capacitance of the depletion region.

h.      
Rectifier Diode

Fig. 9.   
Rectifier Diode

      In power supplies, this is used to rectify
alternating power inputs. This can rectify current levels that range from an
amp upwards.

Diodes are used widely in the electronics industry, right from
electronics design to production, to repair. Besides the above-mentioned types
of diodes, the other diodes are PIN diode (It consists of heavily doped p and n
regions separated by an intrinsic region. The pin diode acts like a nearly
constant capacitance when reverse-biased and acts like a current-controlled
variable resistance when forward-biased.), point contact diode, signal diode,
step recovery diode (It uses graded doping where the doping level of the
semi-conductive materials are reduced as the p-n junction is approached. This
diode is used in very high frequency (VHF) and fast-switching applications.),
tunnel diode (exhibits a special characteristic known as the negative
resistance), IR LEDs, quantum dot display (These dots are semiconductor
nanocrystals which can produce pure monochromatic red, green, and blue light
and are made from semiconductor material such as silicon, germanium, cadmium
sulfide, cadmium selenide, and indium phosphide.), OLED (It is short for
Organic Light Emitting Diode. It is a flat light emitting technology, made by
placing a series of organic thin films between two conductors. When electrical
current is applied, a bright light is emitted.) transient voltage suppression
diodes, constant current diode, current regulator diode (It is often referred
to as a constant-current diode. Unlike the Zener diode, this diode maintains a
constant current.)  Shockley diode (It is
also known as “pnpn” diode. It is a four-layer semiconductor diode which is
equivalent to a thyristor with a disconnected gate), super barrier diode,
vacuum diode, peltier diode, and gold doped diodes. The type of diode to
transfer electric current depends on the type and amount of transmission, as
well as on specific applications.

 

2. DISCUSS, ILLUSTRATE AND DERIVE THE RELATED
EQUATIONS OF VARIOUS RECTIFIER CIRCUITS.

a.      
Half-Wave Rectifier

Fig. 10.
Half-Wave Rectification

      A half-wave rectifier is made
by connecting a diode to an AC source and to a load resistor, RL.
When the sinusoidal input voltage (Vin) is positive, the diode
becomes forward-biased and conducts current through the load resistor. Then,
the current produces an output voltage across the load RL, which has
the same shape as the positive half-cycle of the input voltage. Fig. 11 shows
this operation.

Fig. 11.
Positive Alternation of Half-Wave Rectification

On the other hand, when the input
voltage goes negative during the second half of its cycle, the diode becomes
reverse-biased. There is no current, so the voltage across the load resistor is
0 V, as shown in Fig. 12.

Fig. 12.
Negative Alternation of Half-Wave Rectification

The net result is that only the
positive half-cycles of the ac input voltage shows across the load. It is a
pulsating dc voltage since the output does not change polarity with a frequency
of 60 Hz, illustrated in Fig. 13.

Fig. 13.
Half-Wave Output Voltage

The value you would measure on a dc
voltmeter is the average value of the half-wave rectified output voltage.
Mathematically, it is determined by solving for the area under the curve over a
full cycle and then dividing by the number of radians in a full cycle (by 2?). Fig.
14 shows this.

Fig. 14.
Average Value of the Half-Wave Rectified Signal

The result of this is expressed in (1),
in which Vp is the peak value of the voltage. This equation shows
that VAVG is approximately 31.8% of Vp for a half-wave
rectified voltage.

           
                       (1)

In the previous presentation, the
diode was considered ideal so the output voltage Vout is equal to
the input voltage Vin. However, when the practical diode model is
used with the barrier potential of 0.7 V taken into account, this results in a
half-wave output with a peak value that is 0.7 V less than the peak value of
the input, as shown in Fig. 15. The reason for this is that during the positive
half-cycle, the input voltage must overcome the barrier potential before the
diode becomes forward-biased.

Fig. 15.
Effect of Barrier Potential on the Half-Wave Rectified Output
Voltage

Hence, the expression for the peak
output voltage is:

                        (2)

The peak inverse voltage (PIV) equals
the peak value of the input voltage, and the diode must be capable of
withstanding this amount of repetitive reverse voltage. For the diode in Fig. 16,
the maximum value of reverse voltage, designated as PIV, occurs at the peak of
each negative alternation of the input voltage when the diode is
reverse-biased. A diode should be rated at least 20% higher than the PIV.

Fig. 16.
Peak Inverse Voltage

Hence, the PIV for half-wave
rectification is:

                                   (3)

A transformer is often used to couple
the ac input voltage from the source to the rectifier, as shown in Fig. 7.
Transformer coupling provides two advantages. First, it allows the source
voltage to be stepped down as needed. Second, the ac source is electrically
isolated from the rectifier, thus preventing a shock hazard in the secondary
circuit.

Fig. 17.
Half-Wave Rectifier with Transformer-Coupled Input Voltage

The amount that the voltage is stepped
down is determined by the turns ratio of the transformer. According to IEEE,
turns ratio is the number of turns in the secondary (Nsec) divided
by the number of turns in the primary (Npri). A transformer with a
turns ratio less than 1 is a step-down type and one with a turns ratio greater
than 1 is a step-up type. It is common practice to show the numerical ratio
directly above the windings to show the turn ratio.

The secondary voltage of a transformer
equals the turns ratio, n, times the primary voltage:

                                   (4)

If n > 1,
the secondary voltage is greater than the primary voltage. If n

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