ASSIGNMENT TERM 4
CERTIII IN ELECTROTECNOLOGHY
SUBMITTED BY
VARUN SHARMA
CO5571
Clippers and its fault
A clipper is used to eliminate a portion of an input signal. There are two types of clippers:
series clippers and shunt clippers. The series clipper has a diode that is in series with
the load. The shunt clipper has a diode that is in parallel with the load.
The series clipper is a same as half wave rectifier. When the diode in the series clipper is
conducting, the load waveform resembles the input waveform. When the diode is not
conducting, the output is approximately 0 V .The direction of the diode shows the polarity
of the output waveform. If the diode symbol points toward the source, the circuit is a
positive series clipper, meaning that it clips the positive alternation of the input. If the
diode symbol points toward the load, the circuit is a negative series clipper, meaning that
it clips the negative alternation of the input.
faults
The faults that normally occur in series clipper are same as those of same as of halfwave rectifier. The common faults are shown below—
Faults
Rs open
Symptom
Since the source is isolated from the load, the load voltage and current
both zero
D1 open
Clipping action is lost, so the o/p waveform is nearly identical in shape to
the input waveform.
D1 shorted The symptoms are the same as those for Rs open because the full applied
voltage is droped across Rs.thed shorted diode shorts out the load.
Clampers (DC Restorers)
A clamper is designed to shift a waveform above or below a dc reference voltage without
altering the shape of the waveform. This results in a change in the dc average of the
waveform. (The clamper has changed the dc average of the input waveform from 0 V to
+5 V without altering its shape.)
There are two basic types of clampers:


A positive clamper shifts its input waveform in a positive direction, so that it lies
above a dc reference voltage. For example, the positive clamper in shifts the
input waveform so that it lies above 0 V (the dc reference voltage).
A negative clamper shifts its input waveform in a negative direction, so that it lies
below a dc reference voltage.
Both types of clampers, along with their input and output waveforms. The direction of the
diode determines whether the circuit is a positive or negative clamper.
Clamper faults
The common faults for clamper are given below---
Faults
C1 open
C1
shorted
C1 leaky
D1open
D1
shorted
Symptom
The voltage source is isolated from the rest of the circuit, so load voltage
and current drop to zero
The circuit resembles a shut clipper without a current –limiting resistor.
Either the diode will be destroyed from excessive current or the signal
source will be shorted to ground and damaged
The capacitor attempts to charge but is not able to hold the charge for any
period of time.this causes the dc reference of the output signal to change
constantly. The waveform itself is also extremely distorted.
All clamping action is lost and the output waveform is centred around 0v.
The voltage source is shorted to ground by the short RC time constant of the
capacitor and the shorted diode. The output from the voltage source is
loaded down and the voltage source itself may be damaged.
Voltage multiplier
A voltage multiplier provides a dc output voltage that is multiple of its peak input voltage.
For example, a voltage doubler provides a dc output voltage that is twice its peak input
voltage. When voltage multiplier provides an output voltage that is much greater than the
peak input voltage, they are not power generators. When a voltage multiplier increases a
peak input voltage by a given factor, the peak input current is decreased by
approximately the same factor. Thus, the actual output power from a voltage multiplier is
never greater then the input power.
Because the output current capability of a voltage multiplier is reduced evry time the
voltage is increased, the device normally ends up with very low current capability. As a
result, voltage multipliers are usually used in high- voltage, low-current applications. One
typical application is to supply the high-voltage, low current input required to operate the
cathode ray tube in the television.
There are few types of voltage multiplier
A Full-Wave Voltage Doubler
A voltage Tripler
A voltage quadrater
VOLTAGE MULTIPLIER FAULT
Faults
C2 faulty
C1 faulty
C3
not
charging
Symptoms
C4 close to Vs
C2 not equal to 2Vs
C4 close to 2 Vs
Questions
Q1
There are two types of faults in zener clamper.
In case 1 if D1 is shorts the clamper acts as an unbiased clamper .and in case 2
if D2 is shorts, zener turns on during both alternations of input waveform.
Q2
We should replace both the diodes if either diode fails because there is a strong
possibility for failing diodes to destroy any series diodes in the process. So the
best way to do whenever you find a faulty diode is to replace any diodes in
series with the component.
Q3
To trouble shoot a voltage multiplier we should start by reading the voltage
across the output filter capacitor. The voltage across the filter capacitor may
help to isolate the area where the fault is located. Also when working with
voltage multiplier, you should be extremely careful because of high voltage that
may be present within the circuit. All capacitors should be discharged before
any component is removed from the circuit. When replacing capacitors in
voltage multiplier, the replacement capacitors must have the same or higher
voltage ratings as the originals.
Using a lower rated capacitor can result in circuit damage or injury to you.
 Varactor diode
Varactor diode is diode with variable capacitance. that is a function of the voltage
impressed on its terminals. Varactors are principally used as a voltage-controlled
capacitor, They are commonly used in parametric amplifiers, parametric
oscillators and voltage-controlled oscillators as part of phase-locked loops and
frequency synthesizers.
Operation
Varactors are operated reverse-biased so no current flows, but since the
thickness of the depletion zone varies with the applied bias voltage, the capacitance of
the diode can be made to vary. Generally, the depletion region thickness is proportional
to the square root of the applied voltage; and capacitance is inversely proportional to the
depletion region thickness. Thus, the capacitance is inversely proportional to the square
root of applied voltage.
All diodes exhibit this phenomenon to some degree, but specially made varactor diodes
exploit the effect to boost the capacitance and variability range achieved - most diode
fabrication attempts to achieve the opposite.
Applications
Varactors are principally used as a voltage-controlled capacitor, rather than as rectifiers.
They are commonly used in parametric amplifiers, parametric oscillators and voltagecontrolled oscillators as part of phase-locked loops and frequency synthesizers.
 Constant current diode
Constant current diode is also called as current limiting diode, diode-connected
transistor. These are actually JFET with the gate shorted to the source, and it
functions like a two-terminal current limiter or current source (analog to voltage
limiting Zener diode). They allow a current through them to rise to a certain
value, and then level off at a specific value.
Applications
A series current regulator is circuit used to maintain a constant circuit or load
current.
 Tunnel diode
Tunnel diode is also known as Esaki diode. These are the type of semiconductor
diode which is capable of very fast operation, well into the microwave frequency
region, by using quantum mechanical effects. this type of semiconductor diode
which is capable of very fast operation, well into the microwave frequency region,
by using quantum mechanical effects.
OPERATION
Forward bias operation
Under normal forward bias operation, as voltage begins to increase, electrons at
first tunnel through the very narrow p-n junction barrier because filled electron states in
the conduction band on the n-side become aligned with empty valence band hole states
on the p-side of the pn junction. As voltage increases further these states become more
misaligned and the current drops — this is called negative resistance, because current
decreases with increasing voltage. As voltage increases yet further, the diode begins to
operate as a normal diode, where electrons travel by conduction across the pn junction,
and no longer by tunneling through the pn junction barrier. Thus the most important
operating region for a tunnel diode is the negative resistance region.
Reverse bias operation
When used in the reverse direction they are called back diodes and can act as fast
rectifiers with zero offset voltage and extreme linearity for power signals. (That is, they
have an accurate square law characteristic in the reverse direction.)
Under reverse bias filled states on the p-side become increasingly aligned with empty
states on the n-side and electrons now tunnel through the pn junction barrier in reverse
direction — this is the Zener effect that also occurs in zener diodes.
Application of tunnel diode
An tunnel diode oscillator is a circuit that is used to convert dc to an ac signal. the ac
signal created by tunnel diode oscillator could be used for many applications that require
a high-frequency sine wave.
 Schottky diode
The Schottky diode is a semiconductor diode with a low forward voltage drop and
a very fast switching action. Schottky diodes are constructed from a metal to
semiconductor contact. They have a lower forward voltage drop than p-n junction
diodes. Their forward voltage drop at forward currents of about 1 mA is in the
range 0.15 V to 0.45 V, which makes them useful in voltage clamping
applications and prevention of transistor saturation. These diodes can also be
used as low loss rectifires although their reverse leakage current is generally
higher than that of other diodes Schottky diodes are majority carrier devices and
so do not suffer from minority carrier storage problems that slow down many
other diodes so they have a faster “reverse recovery” than p-n junction diodes.
They also tend to have much lower junction capacitance than p-n diodes which
provides for high switching speeds and their use in high-speed circuitry and RF
devices such as switched-mode power supply, mixers and detectors.
Applications
Typical applications include discharge-protection for solar cells connected to lead-acid
batteries and in switched-mode power supplies; in both cases the low forward voltage
leads to increased efficiency. While standard silicon diodes have a forward voltage drop
of about 0.7 volts and germanium diodes 0.3 volts, Schottky diodes voltage drop at
forward biases of around 1 mA is in the range 0.15 V to 0.46 V,[citation needed] which makes
them useful in voltage clamping applications and prevention of transistor saturation. This
is due to the higher current density in the Schottky diode.
Schottky diodes can be used in power supply "OR"ing circuits in products that have both
an internal battery and a mains adaptor input, or similar. However, the high reverse
leakage current presents a problem in this case, as any high-impedance voltage sensing
circuit (e.g. monitoring the battery voltage or detecting whether a mains adaptor is
present) will see the voltage from the other power source through the diode leakage.
Commonly encountered Schottky diodes include the 1N5817 series 1 A rectifiers.
Schottky metal-semiconductor junctions are featured in the successors to the 7400 TTL
family of logic devices, the 74S, 74LS and 74ALS series, where they are employed as
clamps in parallel with the collector-base junctions of the bipolar transistors to prevent
their saturation, thereby greatly reducing their turn-off delays.
 Pin diode
A Pin diode is a diode with a wide, lightly doped 'near' intrinsic semiconductor
region between a p-type semiconductor and an n-type semiconductor regions.
The p-type and n-type regions are typically heavily doped because they are used
for ohmic contacts. The wide intrinsic region is in contrast to an ordinary PN
diode. The wide intrinsic region makes the PIN diode an inferior rectifier (the
normal function of a diode), but it makes the PIN diode suitable for attenuators,
fast switches, photodetectors, and high voltage power electronics applications
Operation
A PiN diode operates under what is known as high-level injection. In others words, the
intrinsic "i" region is flooded with charge carriers from the "p" and "n" regions. Its function
can be likened to filling up a water bucket with a hole on the side. Once the water
reaches the hole's level it will begin to pour out. Similarly, the diode will conduct current
once the flooded electrons and holes reach an equilibrium point, where the amount of
electrons are equal to the amount of holes in the intrinsic region. When the diode is
forward biased, the injected carrier concentration is typically several orders of
magnitudes higher than the intrinsic level carrier concentration
Applications
PIN diodes are useful as RF switches, attenuators, and photodetectors.
RF and Microwave Switches
Under zero or reverse bias, a PIN diode has a low capacitance. The low capacitance will
not pass much of an RF signal. Under a forward bias of 1 mA, a typical PIN diode will
have an RF resistance of about 1 ohm, making it a good RF conductor. The PIN diode
makes a good RF switch.
Although RF relays can be used as switches, they switch very slowly (on the order of 10
milliseconds). A PIN diode switch can switch much more quickly (e.g., 1 microsecond).
The capacitance of an off discrete PIN diode might be 1pF. At 160MHz, the reactance of
1pF is 1000 ohms. In a 50 ohm system, this can allow excessive RF through. In
applications that need high isolation, switches are cascaded to improve the isolation.
PIN diode switches might be used for signal selection, but they are also used for
component selection. Some low phase noise oscillators use PIN diodes to range switch
inductors.
RF and Microwave Variable Attenuators
By changing the bias current through a PIN diode, it's possible to quickly change the RF
resistance.
At high frequencies, the PIN diode appears as a resistor whose resistance is an inverse
function of its forward current. Consequently, PIN diode can be used in some variable
attenuator designs. Consequently, PIN diodes are used for amplitude modulators or
output leveling circuits.
PIN diodes might be used, for example, as the bridge and shunt resistors in a bridged-T
attenuator.
 Step recovery diode
Step recovery diode (SRD) is a semiconductor junction diode having the ability to
recover extremely quickly from a strong forward conduction state to a cut off
state. The SRD is basically employed to generate extremely fast voltage pulses
when switched quickly from full conduction to cut-off. It is also called Snap-off
diode or charge-storage diode or memory varactor, and has a variety of uses in
microwave electronics as pulse generator or parametric amplifier.i
Operation
The step rescovery diode is an ultrafast diode.like the PIN diode,the steprecovery diode’s characteristics are due to the unusal method of doping used.in
the case of the step-recovery diode,the p- and n- type materials are droped
much more heavily at the ends of the component than they are at the junction.
Applications
the unusual droping of the step-recovery diode affects the time required for the
device to switch from off to on,and vice versa. The typical switching time for a steprecovery diode is in the low-picosecond range. this makes them ideal for switching
applications in the VHF frequency range and above.
Q.2 What is the relationship between the amount of reverse bias voltage apply to
varactor and its capacitance?
Ans- Varactors are operated reverse-biased so no current flows, but since the thickness
of the depletion zone varies with the applied bias voltage, the capacitance of the diode
can be made to vary. Generally, the depletion region thickness is proportional to the
square root of the applied voltage; and capacitance is inversely proportional to the
depletion region thickness. Thus, the capacitance is inversely proportional to the square
root of applied voltage.
Q.3 What is diode capacitance temperature coefficient rating, what is unit of
measurement?
Ans- The diode capacitance temperature coefficient rating tells you how much the
component‘s capacitance changes for each 1 degree rise in temperature above 25
degree. The TCc rating for Mv209 series varactor is 300ppm/degree C.This means that
the varactor capacitance increases by 300 parts per millions for each 1 degree C rise in
temperature above 25 degree C.
The amount by which varactor capacitance changes when temperature changes.
The unit of diode capacitance coefficient rating is farad.
Q.4 what is the Q of the capacitor?
Ans. The ratio of the energy stored in capacitor to the energy lost through leakage
current. Higher the Q of a capacitor, better the quality of a component.
Q5. What is a Transient voltage suppression diode?
Ans- A transient voltage suppression (TVS) diode is an electronic component
used to protect sensitive electronics from voltage spikes induced on connected
wires. It is also commonly referred to as a transorb. The device operates by
shunting excess current when the induced voltage exceeds the avalanche
breakdown potential. It is a clamping device, suppressing all overvoltage above
its breakdown voltage. Like all clamping devices, it automatically resets when
the overvoltage goes away, but absorbs much more of the transient energy
internally than a similarly rated crowbar device.
Q.6 what is surge? How are ac power line surge commonly generated?
Ans- Surge is an abrupt, high voltage or current condition that lasts for brief time, usually
in the microsecond or millisecond range. Left unchecked, a surge in the ac power lines
can cause serious damage to power supply of any electronic system.
Surges are cause by various conditions. Most often, they are generated by electric
motors, air conditioning and heating units, arcing switches, and lightning.
Q.7 what is meant by the term negative resistance?
Ans. Negative resistance- a term used to describe any device with current and voltage
values data inversely related. The region of operation between the peek and valley
voltage is referred to as the negative resistance.