Electronics for Mechanical Engineers

Electronics for Mechanical EngineersConductorsMaterials that allow the flow of electrons are called conductors. Some good conductors are:GoldSilverAluminiumCopperMost metals

ResistorsA resistor is a two-terminal electrical or electronic component that resists an electric current by producing a voltage drop between its terminals in accordance with Ohm's law.

Voltage & CurrentCurrent 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 or Voltage is the force that pushes the electrons thru the circuit.Voltage can be compared to pressure that pushes water thru a pipe.The basic unit of voltage is the volt.V IResistanceResistance 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-value resistorVariable resistor or potentiometerTypes Of ResistorsFixed ResistorsSome resistors are cylindrical, with the actual resistive 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 the cylinder at each end(axial lead). There are carbon film and metal film resistors

Types of ResistorsVariable ResistorsThe variable resistor is a resistor whose value can be adjusted by turning a shaft or sliding a control. They are also called potentiometers or rheostats and allow the resistance of the device to be altered by hand. The term rheostat is usually reserved for higher-powered devices, above about 1/2 watt. Adding ResistorsThe total resistance of tworesistors in series is thesum of their individualvalues.Rt = R1 + R2

The total resistance of two resistors in parallel is the product over the sum.

Rt = R1 x R2 R1 + R2Voltage and Current?It is possible to have voltage without current, as inthe case of a battery by itself. In order to have current, there must be an electrical path (circuit) from the positive to the negative terminal of the voltage source.EIR+-AOhms LawOhms 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.Inductors

Air CoreVariableIron CoreCapacitorsA capacitor is made by separating two conductive plates by an insulator or dielectric.

Capacitors store energy electrostatically.

Capacitors tend to block DC and pass AC.

TransistorsA transistor is a semiconductor device that uses a small amount of voltage or electrical current 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, and all other modern electronics.

Types of transistor

Transistor circuit symbols. There are two types of standard transistors, NPN and PNP, with different circuit symbols. 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).

TransistorsTransistors amplify signals using low voltages and currents.

This is a PNP transistor.

BaseEmitterCollectorIntegrated Circuits (IC's)

An IC (Integrated Circuit) combines several functions into one package.RelayRelay is an electrical switch that opens and closes 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.

Relay construction

An electric current through a conductor will produce a magnetic field at right angles to the 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 the current, the greater the strength of the magnetic field, all other factors being equal Working of a RelayIn the above schematic, the relay's coil is energized by the low-voltage (12 VDC) source, while the single-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 below 1 amp, while typical contact ratings for industrial relays are at least 10 amps

Relay PanelShown here are three small relays (about two inches in height, each), installed on a panel as part of an electrical control system

REVIEW:

A solenoid is a device that produces mechanical motion from the energization of an electromagnet coil. The movable portion of a solenoid is called an armature. A relay is a solenoid set up to actuate switch contacts when its coil is energized. Pull-in current is the minimum amount of coil current needed to actuate a solenoid or relay from its "normal" (de-energized) position. Drop-out current is the maximum coil current below which an energized relay will return to its "normal" state.

Contactors

When a relay is used to switch a large amount of electrical power through its contacts, it is designated 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 the most common industrial use for contactors is the control of electric motors.

REVIEW:

A contactor is 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 of overload 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.

Types of RelaysElectromechanical RelaysTime Delay RelaysSolid-State RelaysProgrammable Logic Relay Arrays

Time-delay relays

Some relays are constructed with a kind of "shock absorber" mechanism attached to the armature which prevents immediate, full motion when the coil is either energized or de-energized. This addition gives the relay the property of time-delay actuation. Time-delay relays can be constructed to delay armature motion on coil energization, de-energization, or both.

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 the event of an abnormal condition. These relays are referred to in the electrical power industry as protective relays

Solid-state relays

As versatile as electromechanical relays can be, they do suffer many limitations. They can be expensive to build, have a limited contact cycle life, take up a lot of room, and switch slowly, compared to modern semiconductor devices. These limitations 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 transistor output instead of mechanical contacts to switch the controlled power. The output device (SCR, TRIAC, or transistor) is optically-coupled to an LED light source inside the relay. The relay is turned on by energizing this LED, usually with low-voltage DC power.

REVIEW:

Large electric circuit breakers do not contain within themselves the necessary mechanisms to automatically trip (open) in the event of overcurrent conditions. They must be "told" to trip by external devices. Protective relays are devices built to automatically trigger the actuation coils of large electric circuit breakers under certain conditions.

Schematic Symbols You Need to KnowFixed-value iron-core inductorSingle-cell batteryNPN transistorFixed-value resistorVariable resistor or potentiometerFixed-value capacitorSingle-pole, double-throw switchDouble-pole, single-throw switchProgrammable Logic RelaysAn IntroductionWhat is a PLC? A PLC (i.e. Programmable Logic Controller) is a device that was invented to replace the necessary 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 a program, usually via software, that gives the desired results. PLC

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 Logic Controllers to take on more complex tasks and larger functions as the speed of the processor increased.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 work your way up the "ladder".LADDER LOGICLadder LogicPLC had to be maintainable by technicians and electrical personnel. To support this the programming language of Ladder Logic was developed. Ladder Logic is based on the relay and contact symbols technicians were used to through wiring diagrams of electrical control panels.