Different Instruments - 2018
Different Instruments - 2018
Different Instruments - 2018
Basic Electronics
B.Tech-I
Different Instruments
Topics
• Basic Introduction
• Cathode-RAY Oscilloscope(CRO)
• Function Generator,
• Digital Multi-meter (DMM).
Instrumentation
• Instrumentation is a branch of engineering that deals
with the measurement and control of different
parameters.
• Instrumentation is defined as "the art and science of
measurement and control".
• Measuring is used to monitor a process or operation.
• Collective term for measuring instruments used for
indicating, measuring and recording physical quantities
• Refer to a device or group of devices used for direct
reading
– Reading a thermometer
– Using sensors in complex industrial control systems.
Role and Importance of General purpose Test
Instruments
• MEASUREMENT AND CALIBRATION
– Measurement is vital in science, industry and
commerce. Measurement is also performed
extensively in our daily life. The following are some
examples:
– Measurements for health care: such as measuring
body temperature with a clinical thermometer,
checking blood pressure and many other tests
– Measurement for Household machines: home
security systems, refrigerators, microwave oven,
bread toasters, water level sensors etc.
– Measurement in Automobile Industry: rotational
speed of engine, vehicle linear speed, fluid levels, turn
signals, air pressure in tyres, etc.
– Measurement in Aircraft Systems: Navigation Systems,
Global Positioning Systems, weather radar etc.
– Measurement in Laboratory Instruments: To measure
electrical and chemical composition of a substance.
Example: Testing pollutants in water for drinking
– Other Measurements:
• Checking the time of day;
• Buying cloth for dresses;
• Purchase of vegetables and other groceries;
• Billing of power consumption through an energy meter
Measurement Parameters
• Pressure, Temperature, weight, length, levels
of liquid, density
• Voltage, current, frequency, resistivity,
inductance, capacitance,
• Viscosity, chemical composition, vibration etc
• Goal: To improve system productivity,
reliability, optimization and stability or to
control parameters.
Electronic Instruments
• Device for determining the values or
magnitude of any physical quantity which
works on some electronic principle.
• Electronic test equipment is used to create
signals and capture responses from electronic
devices under test (DUT)
– To ensure proper operation of DUT so that it is
proven and faults can be traced.
Types of Test Equipment
• Basic Equipment
– Voltmeter
– Ohmmeter
– Ammeter
– Multi-meter
• Instruments used for the stimulus of circuit under test
– Power supplies
– Signal generator or Function Generator
– Digital pattern generator
– Pulse generator.
• Instruments used to analyze the response of circuit under test
– Oscilloscope
– Frequency Counter
• Connectors
– Test probes
– BNC cables
– Power cables etc
General Purpose Test Instruments
• Oscilloscopes
• Function Generator
• Different Power Supply
• Digital Multi-meter
• Physicists
gun
produces a
n
ro
beam of
ct
electrons
Ele
Electron Gun Assembly
• The electron gun assembly consists of an indirectly
heated cathode, a control grid surrounding the cathode,
a focusing anode and an accelerating anode
• The sole function of the electron gun assembly is to
provide a focused electron beam which is accelerated
towards the phosphor screen.
• The cathode is a nickel cylinder coated with an oxide
coating and emits plenty of electrons, when heated. The
emitting surface of the cathode should be as small as
possible, theoretically a point.
• Rate of emission of electrons or say the intensity of
electron beam depends on the cathode current, which
can be controlled by the control grid in a manner similar
to a conventional vacuum tube.
• Control Grid: Biased negative with voltage (-1000
to 1500 volts) and its function is to control the
number of electrons generated by cathode.
– The number and energy of the electrons being emitted
by the cathode influence the intensity and focus aspects
of the electron beam on the phosphor coated screen
The electron beam after passing through the grid tends to
dilate (spread-out) as a solid cone and the constituent
electrons tend to lose velocity.
– For this it is necessary to have anode in the
arrangement which function is to accelerate the
electron beam in linear fashion with sharply focus
cathode rays
• Pre-accelerating anode: (Few 100V)
– Hollow cylinder with a hole in a diaphragm at its
center
– The diaphragm directs back all the electrons
striking it to the cathode.
– Although a narrow beam of electrons is achieved
after the First anode however, due to mutual
repulsion amongst the electrons the beam tends
to spread
• Focusing anode:
– Has two diaphragm with holes at its center.
– By means of an electric field a cylindrical shaped
converging electrical lens is generated and which
functions to shape the linearly accelerated beam of
electrons into a fine spot
– Electrostatic lenses are realized by applying suitable
potential to a pair of diaphragms wherein the electron
beam can be passed through the holes at their centers.
The focal length of the electrostatic lenses is varied by
varying the potential on the diaphragms
• Accelerating anode:
– Another level of amplified linear acceleration to the
electron beam and focuses it towards the screen
Deflection Plates
• Electron beam, after leaving the electron gun, passes
through the two pairs of deflection pates
• One pair of deflection plates is mounted vertically
and deflects the beam in horizontal or X-direction
and so called the horizontal or l-plates.
• The other pair is mounted horizontally and deflects
the beam in vertical or Y-direction and called the
vertical or Y-plates. These plates are to deflect the
beam according to the voltage applied across them
a a potential
potential difference
differenc across the x
e across plates
the y- deflects the
plates trace
deflects horizontally
the trace
verticall
y
Deflection Plates
Screen for CRT
• As we know that some crystalline materials, such a phosphor, have
property of emitting light when exposed to radiation. This is called
the fluorescence characteristic of the materials. These fluorescent
materials continue to emit light even after radiation exposure is cut
off. This is called the phosphorescence characteristic of the materials.
• The length of time during which phosphorescence occurs is called the
persistence of the phosphor. The end wall of the CRT, called the
screen, is coated with phosphor. When electron beam strikes the CRT
screen, a spot of light is produced on the screen. The phosphor
absorbs the kinetic energy of the bombarding electrons and emits,
energy at a lower frequency in a visual spectrum.
• Among the fluorescent materials used are zinc orthosilicate giving a
green trace very suitable for visual observations; calcium tungstate
giving blue and ultra-violet radiations very suitable for photography
and zinc sulphide with other materials giving a white light suitable for
TV.
light
produced on
the screen
by electron
beam
Fluorescent Screen
Glass Body and Case
• The whole assembly is protected in a conical highly evacuated
glass housing through suitable supports.
• The inner walls of CRT between neck and screen are usually
coated with a conducting material known as aquadag and
this coating is electrically connected to the accelerating
anode.
• The coating is provided in order to accelerate the electron
beam after passing between the deflecting plates and
to collect the electrons produced by secondary emission
when electron beam strikes the screen.
• Thus the coating prevents the formation of – ve charge on the
screen and state of equilibrium of screen is maintained.
• Horizontal and vertical marks are marked on the screen of the
CRT to provide user a correct measurement. These marks,
usually in rectangular form, are called graticule
Horizontal Deflection System(HDS)
DC
AC-DC
Linear
Regulator
Switched AC Power
mode Power
supply
Programmable
Uninterruptible
High Voltage
Types
• DC- DC power supply is one that supplies a constant DC
voltage to its load.
• AC to DC- DC power supplies use AC mains electricity as
an energy source.
• Linear- It uses linear voltage regulator whose function is
to convert a varying DC voltage to a constant, often
specific, lower DC voltage. In addition, they often
provide a current limiting function to protect the power
supply and load from overcurrent (excessive, potentially
destructive current).
• AC- AC power supply typically takes the voltage from a
wall outlet (mains supply) and lowers it to the desired
voltage. Some filtering may take place as well.
Variable AC Power Supply
• The different AC voltages are
generated by using a
transformer.
• The transformer may have
multiple windings or taps, in
which case the instrument uses
switches to select the different
voltage levels.
• Alternatively, a variable
transformer (adjustable
autotransformer) can be used to
continuously vary the voltages.
Some variable AC supplies are
included meters to monitor the
voltage, current, and/or power.
Linear Power Supply
• Linear power supplies gain their name from the fact
that they use linear, i.e. non-switching techniques to
regulate the voltage output from the power supply.
• The function of a linear voltage regulator is to
convert a varying DC/ac voltage to a constant, often
specific, lower DC voltage.
• In addition, they often provide a current limiting
function to protect the power supply and load from
overcurrent (excessive, potentially destructive
current).
Unregulated Linear Power Supply
• Unregulated power supplies contain a step-down
transformer, rectifier, filter capacitor, and a bleeder
resistor. This type of power supply, because of
simplicity, is the least costly and most reliable for low
power requirements.
• The main disadvantage is that the output voltage is not
constant. It will vary with the input voltage and the
load current, and the ripple is not suitable for
electronic applications. The ripple can be reduced by
changing the filter capacitor to an LC (inductor-
capacitor) filter, but the cost becomes more.
• Input Transformer: The input transformer is used to
convert the incoming line voltage down to the required
level of the power supply. It also isolates the output
circuit from the line supply. Here we are using a step-
down transformer.
• Rectifier: The rectifier used to convert the incoming
signal from an AC format into raw DC. (full-wave
rectifier)
• Filter Capacitor: The pulsated DC from the rectifier is
fed to the smoothing capacitor. It will remove the
unwanted ripples in the pulsated DC.
• Bleeder Resistor: Bleeder Resistor is also known as a
power supply drain resistor. It is connected across the
filter capacitors to drain their stored charge so that the
power system supply is not dangerous.
Regulated Linear Power Supply
• Regulated linear power supplies are same to the
unregulated linear power supply except that a 3-terminal
regulator is used in place of the bleeder resistor.
• The main aim of this supply is to provide the required
level of DC power to the load. The DC power supply uses
an AC supply as the input.
• Different applications require different levels of attributes
voltages, but nowadays the DC power supplies provide an
accurate output voltage.
• And this voltage is regulated by an electronic circuitry so
that it provides a constant output voltage over a wide
range of output loads
• Smoothing: Once rectified from an AC signal, the
DC needs to be smoothed to remove the varying
voltage level. Large value capacitors are generally
used for this purpose.
• Voltage Regulator: A linear regulator has an
active (BJT or MOSFET) pass device (series or
shunt) controlled by a high gain differential
amplifier (Op-Amp/Comparator). It compares the
output voltage with a precise reference voltage
and adjusts the pass device to maintain a
constant level output voltage.
Advantages and Disadvantages of Linear
Power Supplies
Advantages:
• Simplicity
• Reliability
• low noise levels
• low cost
Disadvantage:
• Linear regulators are ideal for many low-power applications.
• For higher power application disadvantages to linear power
supplies include size, high heat loss, and lower efficiency
levels
Switched Mode Power Supply (SMPS)
• A switch-mode power supply (switching-mode power
supply or SMPS) is an electronic power supply that uses
a switching regulator in order to control the conversion of
electrical power in a highly efficient manner.
• A (SMPS) is an electronic circuit that converts power
using switching devices that are turned on and off at high
frequencies, and storage components such as inductors or
capacitors to supply power when the switching device is
in its non-conduction state.
The SMPS has a rectifier, filter capacitor, series transistor,
regulator, transformer, but is more complicated than the
other power supplies that we have discussed.
D.C. to D.C. converters and D.C. to A.C. Converters belong to the category of Switched Mode
Power Supplies (SMPS). In a SMPS, the active device(transistor) that provides regulation is
always operated in cut-off or in saturation mode. The input D.C. Supply is chopped at a higher
frequency around 15 to 50 kHz using an active device like the BJT, power MOSFET or SCR and the
converter transformer. Here the size of the ferrite core reduces inversely with the frequency. The
lower limit is around 5 kHz for silent operation and an upper limit of 50 kHz to limit the losses in
the choke and in active switching elements. The transformed wave form is rectified and filtered.
A sample of the output voltage is used as the feedback signal for the drive circuit for the
switching transistor to achieve regulation.
• The oscillator in above figure allows the control element to be
switched ON and OFF. The control element usually consists of a
transistor switch, an inductor, and a diode. For each switch ON,
energy is pumped into the magnetic field associated with the
inductor which is a transformer winding in practice. This
energy is then released to the load at the desired voltage level.
• By varying the duty cycle or frequency of switching, we can
vary the stored energy in each cycle and thus control the
output voltage. Higher efficiency is obtained since only the
energy required is pumped to maintain the load current hence
no power dissipation.
Block diagram of SMPS (DC-DC Converter)
• Here, the primary power received from AC main is rectified
and filtered as high voltage DC. It is then switched at a huge
rate of speed approximately 15 kHz to 50 kHz and fed to the
primary side of the step-down transformer. The step-down
transformer is only a fraction of the size of a comparable 50
Hz unit thus reliving the size and weight problems. The
output at the secondary side of the transformer is rectified
and filtered. Then it is sent to the output of the power
supply. A sample of this output is sent back to the switch to
control the output voltage.
• SMPS rely on PWM to control the average value of the
output voltage. The average value of the repetitive pulse
waveform depends on the area under the waveform. As
load increases, output voltage tends to fall.
• Most switching power supplies regulate their output using
the method called Pulse – Width Modulation (PWM). The
power switch which feeds the primary of the step-down
transformer is driven by the PWM oscillator.
• When the duty cycle is at 50%, then the maximum amount
of energy will be passed through the step-down transformer.
As the duty cycle decreases the power transmitted is less
hence low power dissipation.
• The Pulse Width signal given to the switch is inversely
proportional to the output voltage. The width or the ON
time of the oscillator is controlled by the voltage feedback
from the secondary of the rectifier output and forms a
closed loop regulator. Since switching regulator is complex,
modern IC packages like Motorola MC 3420/3520 or Silicon
General SG 1524 can be used instead of discrete components
Advantages and Disadvantages of SMPS
• Advantage
– Lower weight
– Small size
– Higher efficiency
– Lower power dissipation
– Reduced costs
• Disadvantage
– Complexity of circuit
Uninterruptible Power Supply (UPS)
• UPS is a Backup power source that, in the case of
power failure or fluctuations or voltage level drop to
undesirable level, allows enough time for an orderly
shutdown of the system or for a standby generator to
start up. UPS consists usually of a bank of rechargeable
batteries and power sensing and conditioning circuitry.
• Need for UPS:
– Modern electronic gadgets and computer are highly
sensitive to the power fluctuation.
– Disruption of power leads to data loss /corruption/
malfunction
Programmable Power Supply
A programmable power supply is one that allows
remote control of its operation through an
analog input or digital interface such as RS232 or
GPIB. Controlled properties may include voltage,
current, and in the case of AC output power
supplies, frequency. They are used in a wide
variety of applications, including automated
equipment testing, crystal growth monitoring,
semiconductor fabrication, and x-ray generators.
High Voltage Power Supply
• High voltage power supply-A high voltage power supply
is one that outputs hundreds or thousands of volts.
• High voltage power supplies typically apply the bulk of
their input energy to a power inverter, which in turn
drives a voltage multiplier or a high turns ratio, high
voltage transformer, or both (usually a transformer
followed by a multiplier) to produce high voltage. The
high voltage is passed out of the power supply through
the special connector, and is also applied to a voltage
divider that converts it to a low voltage metering signal
compatible with low voltage circuitry.
END