Electronics for Mechanical Engineers

8/9/2019 Electronics for Mechanical Engineers

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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.

Electronics for Mechanical Engineers

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