Basic Electrical Technology-2

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SWITCHES

A switch is an electrical component which can break an electrical circuit, interrupting the current or diverting it from one conductor to another.

Type of Switch

Circuit Symbol

Example

ON-OFF
Single Pole, Single Throw = SPST

A simple on-off switch. This type can be used to switch the power supply to a circuit.

When used with mains electricity this type of switch must be in the live wire, but it is better to use a DPST switch to isolate both live and neutral.

SPST toggle switch

(ON)-OFF
Push-to-make = SPST Momentary

A push-to-make switch returns to its normally open (off) position when you release the button, this is shown by the brackets around ON. This is the standard doorbell switch.

 
Push-to-make switch

ON-(OFF)
Push-to-break = SPST Momentary

A push-to-break switch returns to its normally closed (on) position when you release the button.

 
Push-to-break switch

ON-ON
Single Pole, Double Throw = SPDT

This switch can be on in both positions, switching on a separate device in each case. It is often called a changeover switch. For example, a SPDT switch can be used to switch on a red lamp in one position and a green lamp in the other position.

A SPDT toggle switch may be used as a simple on-off switch by connecting to COM and one of the A or B terminals shown in the diagram. A and B are interchangeable so switches are usually not labelled.

ON-OFF-ON
SPDT Centre Off
A special version of the standard SPDT switch. It has a third switching position in the centre which is off. Momentary (ON)-OFF-(ON) versions are also available where the switch returns to the central off position when released.

 
SPDT toggle switch

 
SPDT slide switch
(PCB mounting)

 
SPDT rocker switch

Dual ON-OFF
Double Pole, Single Throw = DPST

A pair of on-off switches which operate together (shown by the dotted line in the circuit symbol).

A DPST switch is often used to switch mains electricity because it can isolate both the live and neutral connections.

 
DPST rocker switch

Switches

 Component 

 Circuit Symbol 

Function of Component

Push Switch
(push-to-make)

A push switch allows current to flow only when the button is pressed. This is the switch used to operate a doorbell.

Push-to-Break Switch

This type of push switch is normally closed (on), it is open (off) only when the button is pressed.

On-Off Switch
(SPST)

SPST = Single Pole, Single Throw.
An on-off switch allows current to flow only when it is in the closed (on) position.

2-way Switch
(SPDT)

SPDT = Single Pole, Double Throw.
A 2-way changeover switch directs the flow of current to one of two routes according to its position. Some SPDT switches have a central off position and are described as ‘on-off-on’.

Dual On-Off Switch
(DPST)

DPST = Double Pole, Single Throw.
A dual on-off switch which is often used to switch mains electricity because it can isolate both the live and neutral connections.

Reversing Switch
(DPDT)

DPDT = Double Pole, Double Throw.
This switch can be wired up as a reversing switch for a motor. Some DPDT switches have a central off position.

Relay

An electrically operated switch, for example a 9V battery circuit connected to the coil can switch a 230V AC mains circuit.
NO = Normally Open, COM = Common, NC = Normally  Closed.

DIODES

Function

Diodes allow electricity to flow in only one direction. The arrow of the circuit symbol shows the direction in which the current can flow. Diodes are the electrical version of a valve and early diodes were actually called valves.

Forward Voltage Drop

Electricity uses up a little energy pushing its way through the diode, rather like a person pushing through a door with a spring. This means that there is a small voltage across a conducting diode, it is called the forward voltage drop and is about 0.7V for all normal diodes which are made from silicon. The forward voltage drop of a diode is almost constant whatever the current passing through the diode so they have a very steep characteristic (current-voltage graph).

Reverse Voltage

When a reverse voltage is applied a perfect diode does not conduct, but all real diodes leak a very tiny current of a few µA or less. This can be ignored in most circuits because it will be very much smaller than the current flowing in the forward direction. However, all diodes have a maximum reverse voltage (usually 50V or more) and if this is exceeded the diode will fail and pass a large current in the reverse direction, this is called breakdown.

VI Characteristics

Rating Of  Diodes

S.No

Diode Types

Maximum Current

Maximum Reverse Voltage

1

1N4001

1A

50V

2

1N4002

1A

100V

3

1N4007

1A

1000V

4

1N5401

3A

100V

5

1N5408

3A

1000V

Various Types Of Diodes

LED (Light Emitting Diode)

Function

LEDs emit light when an electric current passes through them.

Connection & Soldering

LEDs must be connected the correct way round, the diagram may be labelled a or + for anode and k or – for cathode (yes, it really is k, not c, for cathode!). The cathode is the short lead and there may be a slight flat on the body of round LEDs. If you can see inside the LED the cathode is the larger electrode (but this is not an official identification method).

LEDs can be damaged by heat when soldering, but the risk is small unless you are very slow. No special precautions are needed for soldering most LEDs.

Testing Of  LED

Never connect an LED directly to a battery or power supply!

It will be destroyed almost instantly because too much current will pass through and burn it out. LEDs must have a resistor in series to limit the current to a safe value, for quick testing purposes a 1k resistor is suitable for most LEDs if your supply voltage is 12V or less. Remember to connect the LED the correct way round!

VI Characteristics

RECTIFIERS

A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which is in only one direction, a process known as rectification.

Rectifiers have many uses including as components of power supplies and as detectors of radio signals. Rectifiers may be made of solid statediodes, vacuum tube diodes, mercury arc valves, and other components.

Diode Vs Rectifier

    Diode                                                               Rectifier

When only one diode is used to rectify AC (by blocking the negative or positive portion of the waveform), the difference between the term diode and the term rectifier is merely one of usage, i.e., the term rectifier describes a diode that is being used to convert AC to DC.

Almost all rectifiers comprise a number of diodes in a specific arrangement for more efficiently converting AC to DC than is possible with only one diode. Before the development of silicon semiconductor rectifiers, vacuum tube diodes and copper(I) oxide or selenium rectifier stacks were used.

Half-wave rectification

In half wave rectification, either the positive or negative half of the AC wave is passed, while the other half is blocked. Because only one half of the input waveform reaches the output, it is very inefficient if used for power transfer. Half-wave rectification can be achieved with a single diode in a one-phase supply, or with three diodes in a three-phase supply.

    Half-wave rectifier using a transformer and single diode

            The output DC voltage of a half wave rectifier can be calculated with the following two ideal equations:

Full-wave rectification

A full-wave rectifier converts the whole of the input waveform to one of constant polarity (positive or negative) at its output. Full-wave rectification converts both polarities of the input waveform to DC (direct current), and is more efficient. However, in a circuit with a non-center tappedtransformer, four diodes are required instead of the one needed for half-wave rectification. (See semiconductors, diode). Four diodes arranged this way are called a diode bridge or bridge rectifier:

Graetz bridge rectifier: a full-wave rectifier using 4 diodes

For single-phase AC, if the transformer is center-tapped, then two diodes back-to-back (i.e. anodes-to-anode or cathode-to-cathode) can form a full-wave rectifier. Twice as many windings are required on the transformer secondary to obtain the same output voltage compared to the bridge rectifier above.

Full-wave rectifier using a transformer and 2 diodes

Voltage-doubling rectifier

       Cockcroft Walton Voltage multiplier

The simple half wave rectifier can be built in two versions with the diode pointing in opposite directions, one version connects the negative terminal of the output direct to the AC supply and the other connects the positive terminal of the output direct to the AC supply. By combining both of these with separate output smoothing it is possible to get an output voltage of nearly double the peak AC input voltage. This also provides a tap in the middle, which allows use of such a circuit as a split rail supply.

Cascaded stages of diodes and capacitors can be added to make a voltage multiplier (Cockroft-Walton circuit). These circuits can provide a potential several times that of the peak value of the input AC, although limited in current output and regulation. Voltage multipliers are used to provide the high voltage for a CRT in a television receiver, or for powering high-voltage tubes such as image intensifiers or photo multipliers.

Applications

The primary application of rectifiers is to derive DC power from an AC supply. Virtually all electronic devices require DC, so rectifiers find uses inside the power supplies of virtually all electronic equipment.

Converting DC power from one voltage to another is much more complicated. One method of DC-to-DC conversion first converts power to AC (using a device called an inverter), then use a transformer to change the voltage, and finally rectifies power back to DC.

Rectifiers also find a use in detection of amplitude modulated radio signals. The signal may be amplified before detection, but if un-amplified, a very low voltage drop diode must be used. When using a rectifier for demodulation the capacitor and load resistance must be carefully matched. Too low a capacitance will result in the high frequency carrier passing to the output and too high will result in the capacitor just charging and staying charged.

Rectifiers are also used to supply polarised voltage for welding. In such circuits control of the output current is required and this is sometimes achieved by replacing some of the diodes in bridge rectifier with thyristors, whose voltage output can be regulated by means of phase fired controllers.

Thyristors are used in various classes of railwayrolling stock systems so that fine control of the traction motors can be achieved. Gate turn-off thyristors are used to produce alternating current from a DC supply, for example on the Eurostar Trains to power the three-phase traction motors.

TRANSISTOR

Transistors amplify current, for example they can be used to amplify the small output current from a logic IC so that it can operate a lamp, relay or other high current device. In many circuits a resistor is used to convert the changing current to a changing voltage, so the transistor is being used to amplify voltage.

A transistor may be used as a switch (either fully on with maximum current, or fully off with no current) and as an amplifier (always partly on).

The amount of current amplification is called the current gain, symbol hFE.

Basic Configurations

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. If you are new to electronics it is best to start by learning how to use NPN transistors.

The leads are labelled base (B), collector (C) and emitter (E).

These terms refer to the internal operation of a transistor but they are not much help in understanding how a transistor is used, so just treat them as labels!

A Darlington pair is two transistors connected together to give a very high current gain.

In addition to standard (bipolar junction) transistors, there are field-effect transistors which are usually referred to as FETs. They have different circuit symbols and properties and they are not (yet) covered by this page.

Transistor leads for some common case styles are shown below:

Working Of Transistor

Transistor Pinouts

NPN transistors

Code

Structure

Case
style

IC
max.

VCE
max.

hFE
min.

Ptot
max.

Category
(typical use)

Possible
substitutes

BC107

NPN

TO18

100mA

45V

110

300mW

Audio, low power

BC182 BC547

BC108

NPN

TO18

100mA

20V

110

300mW

General purpose, low power

BC108C BC183 BC548

BC108C

NPN

TO18

100mA

20V

420

600mW

General purpose, low power

BC109

NPN

TO18

200mA

20V

200

300mW

Audio (low noise), low power

BC184 BC549

BC182

NPN

TO92C

100mA

50V

100

350mW

General purpose, low power

BC107 BC182L

BC182L

NPN

TO92A

100mA

50V

100

350mW

General purpose, low power

BC107 BC182

BC547B

NPN

TO92C

100mA

45V

200

500mW

Audio, low power

BC107B

BC548B

NPN

TO92C

100mA

30V

220

500mW

General purpose, low power

BC108B

BC549B

NPN

TO92C

100mA

30V

240

625mW

Audio (low noise), low power

BC109

2N3053

NPN

TO39

700mA

40V

50

500mW

General purpose, low power

BFY51

BFY51

NPN

TO39

1A

30V

40

800mW

General purpose, medium power

BC639

BC639

NPN

TO92A

1A

80V

40

800mW

General purpose, medium power

BFY51

TIP29A

NPN

TO220

1A

60V

40

30W

General purpose, high power

TIP31A

NPN

TO220

3A

60V

10

40W

General purpose, high power

TIP31C TIP41A

TIP31C

NPN

TO220

3A

100V

10

40W

General purpose, high power

TIP31A TIP41A

TIP41A

NPN

TO220

6A

60V

15

65W

General purpose, high power

2N3055

NPN

TO3

15A

60V

20

117W

General purpose, high power

PNP transistors

Code

Structure

Case
style

IC
max.

VCE
max.

hFE
min.

Ptot
max.

Category
(typical use)

Possible
substitutes

BC177

PNP

TO18

100mA

45V

125

300mW

Audio, low power

BC477

BC178

PNP

TO18

200mA

25V

120

600mW

General purpose, low power

BC478

BC179

PNP

TO18

200mA

20V

180

600mW

Audio (low noise), low power

BC477

PNP

TO18

150mA

80V

125

360mW

Audio, low power

BC177

BC478

PNP

TO18

150mA

40V

125

360mW

General purpose, low power

BC178

TIP32A

PNP

TO220

3A

60V

25

40W

General purpose, high power

TIP32C

TIP32C

PNP

TO220

3A

100V

10

40W

General purpose, high power

TIP32A

Heat sinks

Waste heat is produced in transistors due to the current flowing through them. Heat sinks are needed for power transistors because they pass large currents. If you find that a transistor is becoming too hot to touch it certainly needs a heat sink! The heat sink helps to dissipate (remove) the heat by transferring it to the surrounding air.

FET’s

FET ( Field Effect Transistor)

The Field Effect Transistor is a unipolar device that has very similar properties to those of the Bipolar Transistor ie, high efficiency, instant operation, robust and cheap, and they can be used in most circuit applications that use the equivalent Bipolar Junction Transistors, (BJT). They can be made much smaller than an equivalent BJT transistor and along with their low power consumption and dissipation make them ideal for use in integrated circuits such as the CMOS range of chips.

Bipolar Transistor

Field Effect Transistor

Emitter – (E)

Source – (S)

Base – (B)

Gate – (G)

Collector – (C)

Drain – (D)

Function

A Field Effect Transistor is a solid-state device in which current is controlled between source and drain terminals by voltage applied to a non-conducting gate terminal (in contrast, bipolar transistors are current-controlled); a transistor in which output current is controlled by a variable electric field.

Terminal Details

All FETs have a gate, drain, and source terminal that correspond roughly to the base, collector, and emitter of BJTs.

The names of the terminals refer to their functions. The gate terminal may be thought of as controlling the opening and closing of a physical gate. This gate permits electrons to flow through or blocks their passage by creating or eliminating a channel between the source and drain. Electrons flow from the source terminal towards the drain terminal if influenced by an applied voltage. The body simply refers to the bulk of the semiconductor in which the gate, source and drain lie.

Making Of  FET

The FET can be constructed from a number of semiconductors, silicon being by far the most common. Most FETs are made with conventional bulk semiconductor processing techniques, using the single crystal semiconductorwafer as the active region, or channel.

Among the more unusual body materials are amorphous silicon, polycrystalline silicon or other amorphous semiconductors in thin-film transistors or organic field effect transistors that are based on organic semiconductors and often apply organic gate insulators and electrodes.

Types Of  FET’s

The channel of a FET is doped to produce either an N-type semiconductor or a P-type semiconductor. The drain and source may be doped of opposite type to the channel, in the case of depletion mode FETs, or doped of similar type to the channel as in enhancement mode FETs. Field-effect transistors are also distinguished by the method of insulation between channel and gate.

 Types of FETs are:

  • The DEPFET is a FET formed in a fully-depleted substrate and acts as a sensor, amplifier and memory node at the same time. It can be used as an image (photon) sensor.

  • The DGMOSFET is a MOSFET with dual gates.

  • The DNAFET is a specialized FET that acts as a biosensor, by using a gate made of single-strand DNA molecules to detect matching DNA strands.

  • The FREDFET (Fast Reverse or Fast Recovery Epitaxial Diode FET) is a specialized FET designed to provide a very fast recovery (turn-off) of the body diode.

  • The HEMT (High Electron Mobility Transistor), also called an HFET (heterostructure FET), can be made using bandgap engineering in a ternary semiconductor such as AlGaAs. The fully depleted wide-band-gap material forms the isolation between gate and body.

  • The IGBT (Insulated-Gate Bipolar Transistor) is a device for power control. It has a structure akin to a MOSFET coupled with a bipolar-like main conduction channel. These are commonly used for the 200-3000 V drain-to-source voltage range of operation. Power MOSFETs are still the device of choice for drain-to-source voltages of 1 to 200 V.

  • The ISFET is an Ion-Sensitive Field Effect Transistor used to measure ion concentrations in a solution; when the ion concentration (such as H+, see pH electrode) changes, the current through the transistor will change accordingly.

  • The JFET (Junction Field-Effect Transistor) uses a reverse biased p-n junction to separate the gate from the body.

  • The MESFET (Metal–Semiconductor Field-Effect Transistor) substitutes the p-n junction of the JFET with a Schottky barrier; used in GaAs and other III-V semiconductor materials.

  • The MODFET (Modulation-Doped Field Effect Transistor) uses a quantum well structure formed by graded doping of the active region.

  • The MOSFET (Metal–Oxide–Semiconductor Field-Effect Transistor) utilizes an insulator (typically SiO2) between the gate and the body.

  • The NOMFET is a Nanoparticle Organic Memory Field-Effect Transistor.[1]
  • The OFET is an Organic Field-Effect Transistor using an organic semiconductor in its channel.

Uses

IGBTs see application in switching internal combustion engine ignition coils, where fast switching and voltage blocking capabilities are important.

The most commonly used FET is the MOSFET. The CMOS (complementary-symmetry metal oxide semiconductor) process technology is the basis for modern digitalintegrated circuits. This process technology uses an arrangement where the (usually “enhancement-mode”) p-channel MOSFET and n-channel MOSFET are connected in series such that when one is on, the other is off.

The fragile insulating layer of the MOSFET between the gate and channel makes it vulnerable to electrostatic damage during handling. This is not usually a problem after the device has been installed in a properly designed circuit.

In FETs electrons can flow in either direction through the channel when operated in the linear mode, and the naming convention of drain terminal and source terminal is somewhat arbitrary, as the devices are typically (but not always) built symmetrically from source to drain. This makes FETs suitable for switching analog signals between paths (multiplexing). With this concept, one can construct a solid-state mixing board, for example.

JFET (Junction Field Effect Transistor)

The Junction field-effect transistor (JFET) is the simplest type of field effect transistor. It can be used as an electronically-controlled switch or as a voltage-controlled resistance. Electric charge flows through a semiconducting channel between “source” and “drain” terminals. By applying a bias voltage to a “gate” terminal, the channel is “pinched”, so that the electric current is impeded or switched off completely.

In a JFET device, the gate voltage is applied to the channel across a P-N junction, in contrast to its application across an insulator in a conventional MOSFET. JFETs are of both P-channel and N-channel types.

Structure

                        n-Channel JFET                                               p-Channel JFET

The JFET is a long channel of semiconductor material, doped to contain an abundance of positive charge carriers (p-type), or of negative carriers (n-type). Contacts at each end form the source(S) and drain(D). The gate(G) (control) terminal has doping opposite to that of the channel, which surrounds it, so that there is a P-N junction at the interface. Terminals to connect with the outside are usually made ohmic.

Function

JFET operation is like that of a garden hose.

The flow of water through a hose can be controlled by squeezing it to reduce the cross section; the flow of electric charge through a JFET is controlled by constricting the current-carrying channel.

 The current depends also on the electric field between source and drain (analogous to the difference in pressure on either end of the hose).

Drain Characteristics

Applications

The Junction Field Effect Transistor (JFET) exhibits characteristics which often make it more suited to a particular application than the bipolar transistor.

Some of these applications are:

      High Input Impedance Amplifier

      Low-Noise Amplifier

      Differential Amplifier

      Constant Current Source

      Analog Switch or Gate

      Voltage Controlled Resistor

MOSFET (Metal Oxide Semiconductor Field Effect Transistor)

The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a device used for amplifying or switching electronic signals.

Basic Principle

In MOSFETs, a voltage on the oxide-insulated gate electrode can induce a conducting channel between the two other contacts called source and drain.

Basic Configuration

The channel can be of n-type or p-type (see article on semiconductor devices), and is accordingly called an nMOSFET or a pMOSFET (also commonly nMOS, pMOS.

Operation

When a voltage is applied across a MOS structure, it modifies the distribution of charges in the semiconductor. If we consider a P-type semiconductor (with NA the density of acceptors, p the density of holes; p = NA in neutral bulk), a positive voltage, VGB, from gate to body (see figure) creates a depletion layer by forcing the positively charged holes away from the gate-insulator/semiconductor interface, leaving exposed a carrier-free region of immobile, negatively charged acceptor ions (see doping (semiconductor)).

Drain Characteristics

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