electronics for mechanical engineers
TRANSCRIPT
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Electronics for MechanicalEngineers
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Conductors
Materials that allow the flow of electrons are called
conductors. Some good conductors are:
Gold
Silver
Aluminium
Copper
Most metals
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Resistors
A resistoris a two-terminal electrical or
electronic component that resists an electriccurrent by producing a voltage drop between
its terminals in accordance with Ohm's law.
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Voltage & Current
Current is the flow of electrons in an electric circuit.
Current can be compared to the flow of water in a pipe.
The basic unit of current is the ampere.
Electromotive Force orVoltage is the force that pushes the
electrons thru the circuit.
Voltage can be compared to pressure that pushes waterthru a pipe.
The basic unit of voltage is the volt.
V I
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Resistance
Resistance opposes the flow
of electrons in a material.
Resistance limits the current
that can flow thru a circuit.
Resistance can be compared
to a restriction in a water pipe.
The ohm is the basic unit of
resistance. Fixed-valueresistor
Variable resistororpotentiometer
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Types OfResistors
Fixed Resistors
Some resistors are cylindrical, with the actualresistive material in the center (composition
resistors, now obsolete) or on the surface of
the cylinder (film) resistors, and a conducting
metal lead projecting along the axis of thecylinder at each end(axial lead). There are
carbon film and metal film resistors
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Types ofResistors
Variable Resistors
The variable resistor is a resistor whosevalue can be adjusted by turning a shaft orsliding a control. They are also calledpotentiometers orrheostats and allow the
resistance of the device to be altered byhand. The term rheostatis usually reservedfor higher-powered devices, above about 1/2watt.
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Adding Resistors
The total resistance of two
resistors in series is thesum of their individualvalues.R
t= R1 + R2
The total resistance of tworesistors in parallel is theproduct over the sum.
Rt= R1 x R2R1 + R2
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Voltage and Current?
It is possible to have voltage without current, as inthe case of a battery by itself. In order to havecurrent, there must be an electrical path (circuit) fromthe positive to the negative terminal of the voltage
source.
E I R
+
- A
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Ohms Law
Ohms Law is a formula that shows the
relationship between Voltage (E), Current(I), and Resistance (R).
E = I x R
Ohms Law is applicable to all electrical
circuits.
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Inductors
Air Core Variable Iron Core
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Capacitors
A capacitor is made by
separating two conductiveplates by an insulator ordielectric.
Capacitors store energy
electrostatically.
Capacitors tend to block DC
and pass AC.
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Transistors
A transistoris a semiconductor devicesemiconductor device that
uses a small amount of voltage or electricalcurrent to control a larger change in voltage
or current. The transistor is the fundamental
building block of the circuitry that governs the
operation of computers, cellular phones, andall other modern electronics.
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Types of transistor
Transistor circuit symbols. There are two types of
standard transistors,NPN
andPNP
, with different circuitsymbols. The letters refer to the layers of semiconductor
material used to make the transistor. Most transistors used
today are NPN because this is the easiest type to make
from silicon.
. The leads are labeled base (B), collector (C) and emitter (E).
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Transistors
Transistors amplify signalsusing low voltages andcurrents.
This is a PNP transistor.Base
Emitter
Collector
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Integrated Circuits (IC's)
An IC (Integrated Circuit)
combines several functions intoone package.
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Relay
Relay is an electrical switch that opens andcloses under control of another electrical
circuit. In the original form, the switch is
operated by an electromagnet to open or
close one or many sets of contacts.
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Relay construction
An electric current through a conductor will
produce a magnetic field at right angles tothe direction of electron flow. If that
conductor is wrapped into a coil shape, the
magnetic field produced will be oriented
along the length of the coil. The greater thecurrent, the greater the strength of the
magnetic field, all other factors being equal
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Working of a Relay
In the above schematic, the relay's coil is energized
by the low-voltage (12VD
C) source, while thesingle-pole, single-throw (SPST) contact interrupts
the high-voltage (480 VAC) circuit. It is quite likely
that the current required to energize the relay coil will
be hundreds of times less than the current rating of
the contact. Typical relay coil currents are well below1 amp, while typical contact ratings for industrial
relays are at least 10 amps
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Relay Panel
Shown here are three small relays (abouttwo inches in height, each), installed on a
panel as part of an electrical control system
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REVIEW:
A solenoidis a device that produces mechanical motion fromthe energization of an electromagnet coil. The movable portion
of a solenoid is called an armature. A relayis a solenoid set up to actuate switch contacts when its
coil is energized.
Pull-in current is the minimum amount of coil current needed toactuate a solenoid or relay from its "normal" (de-energized)position.
Drop-outcurrent is the maximum coil current below which anenergized relay will return to its "normal" state.
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Contactors
When a relay is used to switch a large amount of
electrical power through its contacts, it isdesignated by a special name: contactor.
Contactors typically have multiple contacts, and
those contacts are usually (but not always)
normally-open, so that power to the load is shut
off when the coil is de-energized. Perhaps themost common industrial use for contactors is the
control of electric motors.
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REVIEW:
A contactoris a large relay, usually used to switch
current to an electric motor or other high-power load.
Large electric motors can be protected from over
current damage through the use ofoverload heaters
and overload contacts. If the series-connected
heaters get too hot from excessive current, the
normally-closed overload contact will open, de-energizing the contactor sending power to the motor.
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Types ofRelays
Electromechanical Relays
Time Delay Relays Solid-State Relays
Programmable Logic Relay Arrays
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Time-delay relays
Some relays are constructed with a kind of
"shock absorber" mechanism attached to thearmature which prevents immediate, fullmotion when the coil is either energized orde-energized. This addition gives the relaythe property oftime-delayactuation. Time-
delay relays can be constructed to delayarmature motion on coil energization, de-energization, or both.
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Protective relays
A special type of relay is one which
monitors the current, voltage, frequency,or any other type of electric power
measurement either from a generating
source or to a load for the purpose of
triggering a circuit breaker to open in theevent of an abnormal condition. These
relays are referred to in the electrical
power industry as protectiverelays
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Solid-state relays
As versatile as electromechanical relays can be, they do suffermany limitations. They can be expensive to build, have a
limited contact cycle life, take up a lot of room, and switchslowly, compared to modern semiconductor devices. Theselimitations are especially true for large power contactor relays.To address these limitations, many relay manufacturers offer"solid-state" relays, which use an SCR, TRIAC, or transistoroutput instead of mechanical contacts to switch the controlled
power. The output device (SCR
, TR
IAC, or transistor) isoptically-coupled to an LED light source inside the relay. Therelay is turned on by energizing this LED, usually with low-voltage DC power.
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REVIEW:
Large electric circuit breakers do not contain
within themselves the necessarymechanisms to automatically trip (open) inthe event of overcurrent conditions. Theymust be "told" to trip by external devices.
Protective relays are devices built toautomatically trigger the actuation coils oflarge electric circuit breakers under certainconditions.
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Schematic Symbols You Need toKnow
Fixed-valueiron-coreinductor
Single-cellbattery
NPNtransistor
Fixed-valueresistor
Variable resistororpotentiometer
Fixed-valuecapacitor
Single-pole,double-throwswitch
Double-pole,single-throwswitch
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Programmable Logic Relays
An Introduction
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What is a PLC?
A PLC (i.e. Programmable Logic Controller)
is a device that was invented to replace thenecessary sequential relay circuits for
machine control. The PLC works by looking
at its inputs and depending upon their state,
turning on/off its outputs. The user enters aprogram, usually via software, that gives the
desired results.
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PLC
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How They are Developed ?
Programmable Logic Controllers were developed to
provide a replacement for large relay based control
panels. These systems were inflexible requiring
major rewiring or replacement whenever the control
sequence was to be changed.
The development of the micro processor from the
mid 1970's have allowed Programmable LogicControllers to take on more complex tasks and larger
functions as the speed of the processor increased.
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How a PLC works?
Modern PLCs are quite complex with many possible
functions all programmed in a step/ladder program.
A typical manual will comprize many pages. Most
manufactures have their manuals downloadable at
their respective web pages and are to the fresh user
a heck of a hand full to follow. 50 to 150 pages is not
uncommon. So as you said, start simple and workyour way up the "ladder".
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LADDER LOGIC
Ladder Logic
PLC had to be maintainable by techniciansand electrical personnel. To support this the
programming language of Ladder Logic was
developed. Ladder Logic is based on the
relay and contact symbols technicians wereused to through wiring diagrams of electrical
control panels.