Contents1. CERTIFICATE............................................................................2

2. ACKNOWLEDGEMENT…………………………… ...............…..3

3. FOREWORD................………………….....................................4

4. ABSTRACT AND KEY WORDS...........................………..........5

5. REFERANCES………………………………..…………..60

What is a Robotic Arm?The first robotic arm was developed in the 1950s by a scientist named George Devol, Jr., before which robotics were largely the products of science fiction and the imagination. The development of robotics was slow for a while, with many of the most useful applications being involved with space exploration. The use of robots to aid in industrialization weren’t fully realized until the 1980s, when robotic arms began to be integrated in automobile and other manufacturing assembly lines.

While working in a fashion similar to the human arm, robot arms can still have a much wider range of motion since their design can be purely up to the imagination of their creator. The joint that connects the segments of a robotic arm, for example, can rotate as well as moving like a hinge. The end of the robotic arm designed to actually do the work that it was designed for is known as the end effectors, and can be designed for practically any task, for example gripping like a hand, painting, tightening screws and more. These robots can be fixed in one place, for example along an assembly line, or they can be mobile so they can be transported to do a variety of tasks in different places.

Autonomous robotic arms are designed to be programmed and then left alone to repeat their tasks independent of human control. Conversely, a robotic arm can also be designed to be operated and controlled by a human being. A situation where human-controlled robotic arms are essential is in space exploration, where robotic arms can be used to manipulate a heavy payload or do other work in space that would be difficult or even impossible for an astronaut to do.

Conveyor belt

A belt conveyor consists of two or more pulleys, with a continuous loop of material - the conveyor belt - that rotates about them. One or both of the pulleys are powered, moving the belt and the material on the belt forward. The powered pulley is called the drive pulley while the unpowered pulley is called the idler. There are two main industrial classes of belt conveyors; Those in general material handling such as those moving boxes along inside a factory and bulk material handling such as those used to transport industrial and agricultural materials, such as grain, coal, ores, etc. generally in outdoor locations. Generally companies providing general material handling type belt conveyors do not provide the conveyors for bulk material handling. In addition there are a number of commercial applications of belt conveyors such as those in grocery stores.

The belt consists of one or more layers of material they can be made out of rubber. Many belts in general material handling have two layers. An under layer of material to provide linear strength and shape called a carcass and an over layer called the cover. The carcass is often a cotton or plastic web or mesh. The cover is often various rubber or plastic compounds specified by use of the belt. Covers can be made from more exotic materials for unusual applications such as silicone for heat or gum rubber when traction is essential.

Material flowing over the belt may be weighed in transit using a beltweigher. Belts with regularly spaced partitions, known as elevator belts, are used for transporting loose materials up steep inclines. Belt Conveyors are used in self-unloading bulk freighters and in live bottom trucks. Conveyor technology is also used in conveyor transport such as moving sidewalks or escalators, as well as on many manufacturing assembly lines. Stores often have conveyor belts at the check-out counter to move shopping items. Ski areas also use conveyor belts to transport skiers up the hill. A wide variety of related conveying machines are available, different as regards principle of operation, means and direction of conveyance,

including screw conveyors, vibrating conveyors, pneumatic conveyors, the moving floor system, which uses reciprocating slats to move cargo, and roller conveyor system, which uses a series of powered rollers to convey boxes or pallets.

ABOUT PROJECTS

In our project we are constructing a model of automated material handling robot. This robot counts pass product for robotic arm.

We are using one conveyor built in our project, which is rotate, by one dc gear motor.

Next we are using two pair of IR sensor for counting objects and second sensor stop conveyor built when object reach pickup position of robotic arm.

WORKING

A. MOTOR USED IN ARM CONSTRUCTION

Step-1

WE ARE USING THREE DC GEAR MOTORS FOR OUR ROBOTIC ARM, WHICH IS CONTROL BY SIMPLE PROGRAM CIRCUIT.

WE USE BUHLER DC GEARHEAD MOTOR FOR GRIPING.

MOTOR DETAIL: COMPACT, SMOOTH-RUNNING BUHLER GEARHEAD MOTOR. OPERATES FROM 3 - 24 VDC. NO-LOAD RATING: 140 RPM @ 18 VDC / 140 MA. BODY: 1.91" X 1.59" X 1.14." 3MM DIAMETER SHAFT IS 0.4" LONG. 8" WIRE LEADS. THREE THREADED MOUNTING HOLES ON FACE OF MOTOR.

WE USE 12VDC 10RPM CROUZET MOTOR W/GEARBOX FOR RIGHT/LEFT AND UP/DOWN MOVEMENT.

Motor detail: Powerful gear motor. 10 RPM @ 12 Vdc / 90 mA (no-load). Operates on 4-15 Vdc. 3.6" x 2.36" x 2.24" overall dimensions. Crouzet

motor and final drive shaft both extend from same side of plastic gearbox. 5/16" diameter flatted shaft is 0.9" long. 8" pigtail leads.

WE USE CUTOFF SWITCHES FOR SET MOTOR MOVEMENT.

B. OBJECT MONITORING SYSTEMIDENTIFICATION OF RIGHT PRODUCT.IN THIS SYSTEM WE ARE SENSING THE HIGHT OF THE PRODUCT BY USING IR SENSOR CIRCUIT.

CIRCUIT WORKINGIN OUR CIRCUIT WE HAVE THREE PAIR IR SENSER. TWO IS FIX ON THE STARTING OF CONVEYOR AND ONE IS FIXED IN FRONT OF REJECTION COUNTER.

HOW SYSTEM WORKSWHEN TWO LIGHT FALL ON THE OBJECT, REJECTION COUNTER REJECTS IT, AS SHOWN IN THE CASE-1 DIAGRAM BELOW.

IN CASE 2 OK OBJECT PASS THROUGH REJECTION COUNTER.

CASE 3: WHEN OUR OJECT IS BELOW THE HEIGHT IT SENSE BY THE REJECTION COUNTER SENSOR AND REJECT IT AS A UNDER HEIGHT OBJECT.

C .IR CONVEYOR MOVEMENT CONTROL SYSTEM

IR Sensor control conveyor: one of conveyor built system control by ir sensor, when ir sensor interrupts, they send pulse to circuit and circuit switch off conveyor built dc gear motor.

Dc motor used:

DC GEAR MOTOR: Brand HOSIDEN motors (Japan) R.P.M: 75-100 VOLT: 12-18V. DC

CONVEYOR BULT CIRCUIT

D. OBJECT COUNTER CIRCUIT

ROBOTIC ARM CIRCUIT

COMPONENTS REQUIREMENTS1. ONE BUHLER DC GEARHEAD MOTOR2. TWO CROUZET MOTOR

3. ONE HOSIDEN MOTOR4. PLASTIC ROLLERS5. LASTIC (USED AS CONVEYOR)6. CUTOFF SWITCH7. IC LM 5678. REGULATOR9. RELAY10.OBJECT COUNTER11.RESISTANCE12.PROGRAMMING IC13.CAPACITOR14.TRANSISTOR15.DIODE16.TRANFORMER17.IR SENSOR18.WOODEN FRAM19.IRON GRIPPER

COMPONENTS DETAIL1. DC motors

One of the first electromagnetic rotary motors was invented by Michael Faraday in 1821 and consisted of a free-hanging wire dipping into a pool of mercury. A permanent magnet was placed in the middle of the pool of mercury. When a current was passed through the wire, the wire rotated around the magnet, showing that the current gave rise to a circular magnetic field around the wire. This motor is often demonstrated in school physics classes, but brine(salt water) is sometimes used in place of the toxic mercury. This is the simplest form of a class of electric motors called homopolar motors. A later refinement is the Barlow's Wheel.

Another early electric motor design used a reciprocating plunger inside a switched solenoid; conceptually it could be viewed as an electromagnetic version of a two stroke internal combustion engine.

The modern DC motor was invented by accident in 1873, when Zénobe Gramme connected a spinning dynamo to a second similar unit, driving it as a motor.

The classic DC motor has a rotating armature in the form of an electromagnet. A rotary switch called a commutator reverses the direction of the electric current twice every cycle, to flow through the armature so that the poles of the electromagnet push and pull against the permanent magnets on the outside of the motor. As the poles of the armature electromagnet pass the poles of the permanent magnets, the commutator reverses the polarity of the armature electromagnet. During that instant of switching polarity, inertia keeps the classical motor going in the proper direction. (See the diagrams below.)

A simple DC electric motor. When the coil is powered, a magnetic field is generated around the armature. The left side of the armature is pushed away from the left magnet and drawn toward the right, causing rotation.

The armature continues to rotate.

When the armature becomes horizontally aligned, the commutator reverses the direction of current through the coil, reversing the magnetic field. The process then repeats.

2. MICROCONTROLLER (AT89S52)

8051 microcontroller has 128 bytes of RAM, 4K bytes of on-chip ROM, two timers, one serial port, and four ports (each 8-bits wide) all on a single chip. The 8051 is an 8-bit processor i.e. the CPU can work on only 8 bits of data at a time. The fixed amount of on-chip ROM, RAM, and number of I/O ports in microcontroller makes them ideal for many applications in which cost and space are critical.

The AT89C51 is a low-power, high-performance CMOS 8-bit microcomputer with 4K bytes of Flash programmable and erasable read only memory (PEROM). The on-chip Flash allows the program memory to be reprogrammed in-system or by a conventional nonvolatile memory programmer. By combining a versatile 8-bit CPU with Flash on a monolithic chip, the Atmel AT89C51 is a powerful microcomputer, which provides a highly flexible and cost-effective solution to many embedded control applications.

FEATURES:

• Compatible with MCS-51™ Products

• 4K Bytes of In-System Reprogrammable Flash Memory– Endurance: 1,000 Write/Erase Cycles

• Fully Static Operation: 0 Hz to 24 MHz

• Three-level Program Memory Lock

• 128 x 8-bit Internal RAM

• 32 Programmable I/O Lines

• Two 16-bit Timer/Counters

• Six Interrupt Sources

• Programmable Serial Channel

• Low-power Idle and Power-down Modes

BLOCK DIAGRAM:

Interrupt control

Buscontrol

Serial port

ETC.

Osc

CPU

4 I/O Ports

On-chip RAM

On-chip ROM for program code

Timer 0

Timer 1

Counter Inputs

P0 P1 P2 P3 TXD RXD

ADDRESS/DATA

External Interrupt

s

PIN CONFIGURATION:

PIN DESCRIPTION:

1 40

2 39

3 38

4 37

5 36

6 35

7 34

8 339 32

10 31

11 30

12 29

13 28

14 27

15 26

16 25

17 24

18 23

19 2220 21

P1.0

P1.1

P1.2

P1.3

P1.4

P1.5

P1.6

P1.7

RST

(RXD) P3.0

(TXD) P3.1

(INT0) P3.2(INT1) P3.3

(T0) P3.4

(T1) P3.5

(WR) P3.6

(RD) P3.7XTAL2

XTAL1

GND

Vcc

P0.0 (AD0)

P0.1 (AD1)

P0.2 (AD2)

P0.3 (AD3)

P0.4 (AD4)

P0.5 (AD5)

P0.6 (AD6)

P0.7 (AD7)

EA/VPP

ALE/PROG

PSEN

P2.7 (A15)

P2.6 (A14)P2.5 (A13)

P2.4 (A12)

P2.3 (A11)

P2.2 (A10)

P2.1 (A9)

P2.0 (A8)

PIN DESCRIPSION:

VCC - Supply voltage.GND - Ground.Port 0 - Port 0 is an 8-bit open-drain bi-directional I/O port. As an output port, each pin can sink eight TTL inputs. When 1s are written to port 0 pins, the pins can be used as high-impedance inputs.

Port 0 may also be configured to be the multiplexed low-order address/data bus during accesses to external program and data memory. In this mode P0 has internal pull-ups.

Port 0 also receives the code bytes during Flash programming, and outputs the code bytes during program verification. External pull-ups are required during program verification.Port 1 - Port 1 is an 8-bit bi-directional I/O port with internal pull-ups.The Port 1 output buffers can sink/source four TTL inputs. When 1s are written to Port 1 pins they are pulled high by the internal pull-ups and can be used as inputs. As inputs, Port 1 pins that are externally being pulled low will source current (IIL) because of the internal pull-ups. Port 1 also receives the low-order address bytes during Flash programming and verification.Port 2 - Port 2 is an 8-bit bi-directional I/O port with internal pull-ups.The Port 2 output buffers can sink/source four TTL inputs.When 1s are written to Port 2 pins they are pulled high by the internal pull-ups and can be used as inputs. As inputs, Port 2 pins that are externally being pulled low will source current (IIL) because of the internal pull-ups. Port 2 emits the high-order address byte during fetches from external program memory and during accesses to external data memory that use 16-bit addresses (MOVX @ DPTR). In this application, it uses strong internal pull-ups when emitting 1s. During accesses to external data memory that use 8-bit addresses (MOVX @ RI), Port 2 emits the contents of the P2 Special Function Register.Port 2 also receives the high-order address bits and some control signals during Flash programming and verification.Port 3 - Port 3 is an 8-bit bi-directional I/O port with internal pullups.The Port 3 output buffers can sink/source four TTL inputs. When 1s are written to Port 3 pins they are pulled high by the internal pullups and can be used as inputs. As inputs, Port 3 pins that are externally being pulled low

will source current (IIL) because of the pullups. Port 3 also serves the functions of various special features of the AT89C51 as listed below:RST - Reset input. A high on this pin for two machine cycles while the oscillator is running resets the device.ALE/PROG - Address Latch Enable output pulse for latching the low byte of the address during accesses to external memory. This pin is also the program pulse input (PROG) during Flash programming.In normal operation ALE is emitted at a constant rate of 1/6 the oscillator frequency, and may be used for external timing or clocking purposes. Note, however, that one ALE pulse is skipped during each access to external Data Memory.

If desired, ALE operation can be disabled by setting bit 0 of SFR location 8EH. With the bit set, ALE is active only during a MOVX or MOVC instruction. Otherwise, the pin is weakly pulled high. Setting the ALE-disable bit has no effect if the microcontroller is in external execution mode.PSEN - Program Store Enable is the read strobe to external program memory. When the AT89C51 is executing code from external program memory, PSEN is activated twice each machine cycle, except that two PSEN activations are skipped during each access to external data memory.EA/ VPP - External Access Enable. EA must be strapped to GND in order to enable the device to fetch code from external program memory

locations starting at 0000H up to FFFFH. Note, however, that if lock bit 1 is programmed, EA will be internally latched on reset.EA should be strapped to VCC for internal program executions.

This pin also receives the 12-volt programming enable voltage (VPP) during Flash programming, for parts that require 12-volt VPP.XTAL1 - Input to the inverting oscillator amplifier and input to the internal clock operating circuit.

PORT PIN ALTERNATE FUNCTIONSP3.0 RXD (serial input port)P3.1 TXD (serial output port)P3.2 INT0 (external interrupt 0)

P3.3 INT1 (external interrupt 1)P3.4 T0 (timer 0 external input)P3.5 T1 (timer 1 external input)P3.6 WR (external data memory write strobe)P3.7 RD (external data memory read strobe)

XTAL2 - Output from the inverting oscillator amplifier.OSCILLATOR CHARACTERISTICS:XTAL1 and XTAL2 are the input and output, respectively, of an inverting amplifier, which can be configured for use as an on-chip oscillator. Either a quartz crystal or ceramic resonator may be used. To drive the device from an external clock source, XTAL2 should be left unconnected while XTAL1 is driven.There are no requirements on the duty cycle of the external clock signal, since the input to the internal clocking circuitry is through a divide-by-two flip-flop, but minimum and maximum voltage high and low time specifications must be observed.

Figure 1. Oscillator Connections

Note: C1, C2 = 30 pF +/- 10 pF for Crystals = 40 pF +/- 10 pF for Ceramic Resonators

THE 8051 REGISTERS:

The most widely used registers of the 8051 are A (accumulator), B, R0, R1, R2, R3, R4, R5, R6, R7, DPTR (data pointer), and PC (program counter). All of the above registers are 8-bits, except DPTR and the program counter.

XTAL1

XTAL2

C1

C2

GND

The 8 bots of a register are shown below from the MSB (most significant bit) D7 to the LSB (least significant bit) D0.

D7 D6 D5 D4 D3 D2 D1 D0

PROGRAM COUNTER:

The program counter points to the address of the next instruction to be executed. As the CPU fetches the opcode from the program ROM, the program counter is incremented to point to the next instruction. The PC is 16 bits wide i.e. it can access program addresses 0000 to FFFFH, a total of 64K bytes of code.

PSW (PROGRAM STATUS WORD) REGISTER

The PSW contains status bits that reflect the current state of the CPU and is also called flag register. The PSW contains the Carry bit, the Auxiliary Carry bit, the two register bank select bits, the overflow flag bit, a parity bit, and two user definable status flags.

CY AC F0 RS1 RS0 OV --- P

CY PSW.7 Carry flag.AC PSW.6 Auxiliary carry flag.--- PSW.5 Available to the user for general purpose.RS1 PSW.4 Register Bank selector bit 1.RS0 PSW.3 Register Bank selector bit 0.OV PSW.2 Overflow flag.--- PSW.1 User definable bit.P PSW.0 Parity flag.

RS1 RS0 Register Bank Address 0 0 0 00H – 07H 0 1 1 08H – 0FH 1 0 2 10H – 17H 1 1 3 18H – 1FH

CY, THE CARRY FLAG

This flag is set whenever there is a carry out from the D7 bit. This flag bit is affected after an 8-bit addition or subtraction. It can also be set to

1 or 0 directly by an instruction such as “SETB C” and “CLR C” where “SETB C” stands for “set bit carry” and “CLR C” for “clear carry”.

AC, THE AUXILIARY CARRY FLAG

If there is a carry from D3 to D4 during an ADD or SUB operation, this bit is set; otherwise, it is cleared. This flag is used by instructions that perform BCD (binary coded decimal) arithmetic.P, THE PARITY FLAG

The parity flag reflects the number of 1s in the A (accumulator) register only. If the A register contains an odd number of 1s, then P=1. Therefore, P=0 if A has an even number of 1s.

OV, THE OVERFLOW FLAG

This flag is set whenever the result of a signed number operation is too large, causing the high-order bit to overflow into the sign bit.

RAM MEMORY SPACE ALLOCATION IN THE 8051

There are 128 bytes of RAM in the 8051, which are assigned addresses 00 to 7FH. These 128 bytes are divided into three different groups:

1. A total of 32 bytes from locations 00 to 1H hex are set aside for register banks and the stack.

2. A total of 16 bytes from locations 20H to 2FH are set aside for bit-addressable read/write memory.

3. A total of 80 bytes from locations 30H to 7FH are used for read and write storage, or what is normally called a scratch pad. These 80 locations of RAM are widely used for the purpose of storing data and parameters by 8051 programmers.

7FScratch pad RAM

302F

Bit-Addressable RAM 201F Register Bank 318

17 Register Bank 2 10

0FRegister Bank 1 (stack)08

07Register Bank 0

00REGISTER BANKS IN THE 8051

The 32 bytes of RAM which is set aside for the register banks and stack is divided into 4 banks of registers in which each bank has 8 registers, R0 – R7. RAM locations from 0 to 7 are set aside for bank 0 of R0 – R7

where R0 is RAM location 0, R1 is RAM location 1, R2 is location 2, and so on, until memory location 7 which belongs to R7 of bank 0. The second bank of registers R0 – R7 starts at RAM location 08 and goes to location 0FH. The third bank of R0 – R7 starts at memory location 10H and goes to location 17H; and finally RAM locations 18H to 1FH are set aside for the fourth bank of R0 – R7. The following tables shows how the 32 bytes are allocated into 4 banks:

Bank 0 Bank 1 Bank 2 Bank 3

R7 7R6 6R5 5R4 4R3 3R2 2R1 1R0 0

R7 7R6 6R5 5R4 4R3 3R2 2R1 1R0 0

R7 7R6 6R5 5R4 4R3 3R2 2R1 1R0 0

R7 7R6 6R5 5R4 4R3 3R2 2R1 1R0 0

STACK IN THE 8051

The stack is a section of RAM used by the CPU to store information temporarily. This information could be data or an address. The CPU needs this storage area since there are only a limited number of registers. The register used to access the stack is called the SP (stack pointer) register. The stack pointer in the 8051 is only 8 bits wide i.e. it can take values of 00 to FFH. When the 8051 is powered up, the SP

register contains value 07 which implies that RAM location 08 is the first location being used for the stack by the 8051. The storing of a CPU register in the stack is called a PUSH, and loading the contents of the stack back into a CPU register is called a POP. In other words, a register is pushed onto the stack to save it and popped off the stack to retrieve it.

PUSHING ONTO THE STACK:

In the 8051 the stack pointer (SP) is pointing to the last used location of the stack. As data is pushed onto the stack, the stack pointer (SP) is incremented by one and the contents of the register are saved on the stack. To push the registers onto the stack, RAM addresses are used.

POPPING FROM THE STACK:

Popping the contents of the stack back into a given register is the opposite process of pushing. With every pop, the top byte of the stack is copied to the register specified by the instruction and the stack pointer is decremented once.

ADDRESSING MODES:

The addressing modes in the microcontroller instruction set are as follows:

1. DIRECT ADDRESSING

In direct addressing, the operand is specified by an 8-bit address field in the instruction. Only internal RAM and SFRs cab be directly accessed.

2. INDIRECT ADDRESSING

In indirect addressing, the instruction specifies a register that specifies a register that contains the address of the operand. Both internal and external RAM can be indirectly accessed.

The address register for 8-bit addresses can be either the stack pointer or R0 or R1 of the selected register bank. The address register for 16-bit addresses can be only the 16-bit data pointer register, DPTR.

3. REGISTER INSTRUCTIONS

The register banks, which contain registers R0 through R7, can be accessed by instructions whose opcodes carry a 3-bit register specification. Instructions that access the registers this way make efficient use of code, since this mode eliminates an address byte. When the instruction is executed, one of the eight registers in the selected bank is accessed. One of four banks is selected at execution time by the two bank select bits in the PSW.

4. REGISTER-SPECIFIC INSTRUCTIONS

Some instructions are specific to a certain register. For example, some instructions always operate on the Accumulator, so no address byte is

needed to point to it. In these cases, the opcode itself points to the correct register.

5. IMMEDIATE CONSTANTS

The value of a constant can follow the opcode in program memory. For example,

MOV A, #100

Loads the Accumulator with the decimal number 100. The same number could be specified in hex digits as 64H.

6. INDEXED ADDRESSING

Program memory can only be accessed via indexed addressing. This addressing mode is intended for reading look-up labels in program memory. A 16-bit base register (either DPTR or the Program Counter) points to the base of the table, and the accumulator is set up with the table entry number. The address of the table entry in program memory is formed by adding the accumulator data to the base pointer.

8051 INSTRUCTION SET MNEMONIC: The MNEMONIC column contains the 8051 Instruction Set Mnemonic and a brief description of the instruction's operation.

OPERATION:

The OPERATION column describes the 8051 Instruction Set in unambiguous symbology. Following are the definitions of the symbols used in this column. <n:m> Bits of a register inclusive. For example, PC<10:0> means bits

0 through 10 inclusive of the PC. Bit 0 is always the least significant bit.

+ Binary addition- Binary 2s complement subtraction/ Unsigned integer divisionX Unsigned integer multiplication~ Binary complement (1s complement)^ Logical Andv Inclusive Orv Exclusive Or> Greater than<> Not equal to = Equals

-> Is written into. For example, A + SOper -> A means the result of the binary addition between A and the Source

Operand is written into A. A The 8-bit Accumulator Register.AC The Auxiliary Carry Flag in the Program Status WordCF The Carry Flag in the Program Status WordDoperThe Destination Operand used in the instruction.

DPTR 16-bit Data Pointer

Interrupt Active Flag Internal Flag that holds off interrupts until the Flag is cleared.

Jump Relative to PC A Jump that can range between -128 bytes and

+127 bytes from the PC value of the next instruction.

Paddr A 16-bit Program Memory address PC The 8051 Program Counter. This 16-bit register

points to the byte in the Program Memory space that is fetched as part of the instruction stream.

PM (addr) Byte in Program Memory space pointed to by addr.

Remainder Integer remainder of unsigned integer division

Soper The Source Operand used in the instruction.

SP 8-bit Stack Pointer

STACK The Last In First Out data structure that is controlled by the 8-bit Stack Pointer (SP). Sixteen bit quantities are pushed on the stack low byte first.

HEX OPCODE: This column gives the machine language hexadecimal opcode for each 8051 instruction.

BYTE: This column gives the number of bytes in each 8051 instruction.

CYC: This column gives the number of cycles of each 8051 instruction. The time value of a cycle is defined as 12 divided by the oscillator frequency. For example, if running an 8051 family component at 12 MHz, each cycle takes 1 microsecond.

3. PHOTO-DIODESIf a conventional silicon diode is connected in the reverse-biased circuit of fig. 1, negligible current will flow through the diode and zero voltage will

develop across R1. If the diode casing is now carefully removed so that the diode's semiconductor junction is revealed, and the junction is them exposed to visible light in the same circuit, the diode current will rise, possibly to as

Fig. 1 Reverse-baised diode circuit.

high as 1 mA, producing a significant output across R1. Further investigation will show that the diode current (and thus the output voltage) is directly proportional to light intensity, and that the diode is therefore photosensitive.

In practice, all silicon junctions are photosensitive, and a photodiode can be regarded as a conventional diode housed in a case that lets external light reach its photosensitive semiconductor junction. Fig. 2 shows the standard photodiode symbol. In use, the photodiode is reverse biased and the output voltage is taken from across a series-connected load resistor. This resistor may be connected between the diode and ground, as in fig. 1, or between the diode and the positive supply line, as in fig. 3

Fig. 2 Photodiode symbol

Photodiode symbol

The human eye is sensitive to a range of light radiation, as shown in fig. 4. It has a peak spectral response to the colour green, which has a wave length of about 550 nm, but has a relatively low sensitivity to the colour violet (400 nm) at one end of the spectrum and to dark red (700 nm) at the other. Photodiodes also have spectral response characteristics, and these are determined by the chemistry used in the semiconductor junction material. Fig. 4 shows typical response curves of a general-purpose photodiode, and infrared (IR) photodiode.

Photodiodes have a far lower light-sensitivity than cadmium-sulphide LDRs, but give a far quicker response to changes in light level. Generally, LDRs are ideal for use in slow-acting direct-coupled light-level sensing applications, while photodiodes are ideal for use in fast-acting AC-coupled signalling applications. Typical photodiode applications include IR remote-control circuits, IR beam switches and alarm circuits, and photographic flash slave circuits, etc.

Fig 3 Photodiode circuit with D1-to-V + load

Fig. 4 Typical spectral response curves of (a) the human eye, (b) a general-purpose photodiode, and (c) an infra-red photodiode.

PHOTOTRANSISTORS

Fig. 5 shows the standard symbol of a phototransistor, which can be regarded as a conventional transistor housed in a case that enables its semiconductor junctions to be exposed to external light. The device is normally used with its base open circuit, in either of the configurations shown in fig. 6, and functions as follows.

Fig. 5 Phototransistor symbol.

In fig. 6(a), the base-collector junction of the transistor is effectively reverse biased and thus acts as a photodiode. The photo-generated currents of the base-collector junction feed directly into the base of the device, and the normal current-amplifying transistor action causes the output current to appear (in greatly amplified form) as collector current, and in fig. 6(a) R1 causes this current to generate an output voltage as shown.

In practice, the collector and emitter current of the transistor are virtually identical and, since the base is open circuit, the device is not subjected to significant negative feedback. Consequently, the alternative fig. 6(b) circuit, in which R1 is connected to Q1 emitter, gives a virtually identical performance to that of fig. 6(a).

Fig. 6 Alternative phototransistor configuration.

The sensitivity of a phototransistor is typically one hundred times greater than that of a photodiode, but is useful maximum operating frequency (a few hundred kilohertz) is proportionally lower than that of a photodiode by using only its base and collector terminals and ignoring the emitter, as shown in fig. 7.

Fig. 7 Phototransistor used as a photodiode

4. CAPACITORS

It is an electronic component whose function is to accumulate charges and then release it.

To understand the concept of capacitance, consider a pair of metal plates which all are placed near to each other without touching. If a battery is connected to these plates the positive pole to one and the negative pole to the other, electrons from the battery will be attracted from the plate connected to the positive terminal of the battery. If the battery is then disconnected, one plate will be left with an excess of electrons, the other with a shortage, and a potential or voltage difference will exists between them. These plates will be acting as capacitors. Capacitors are of two types: - (1) fixed type like ceramic, polyester, electrolytic capacitors-these names refer to the material they are made of aluminium foil. (2) Variable type like gang condenser in radio or trimmer. In fixed type capacitors, it has two leads and its value is written over its body and variable type has three leads. Unit of measurement of a capacitor is farad denoted by the symbol F. It is a very big unit of capacitance. Small unit capacitor are pico-farad denoted by pf (Ipf=1/1000,000,000,000 f) Above all, in case of electrolytic capacitors, it's two terminal are marked as (-) and (+) so check it while using capacitors in the circuit in right direction. Mistake can destroy the capacitor or entire circuit in operational.

5. DIODE

The simplest semiconductor device is made up of a sandwich of P-type semi conducting material, with contacts provided to connect the p-and n-type layers to an external circuit. This is a junction Diode. If the positive terminal of the battery is connected to the p-type material (cathode) and the negative terminal to the N-type material (Anode), a large current will flow. This is called forward current or forward biased.

If the connections are reversed, a very little current will flow. This is because under this condition, the p-type material will accept the electrons from the negative terminal of the battery and the N-type material will give up its free electrons to the battery, resulting in the state of electrical equilibrium since the N-type material has no more electrons. Thus there will be a small current to flow and the diode is called Reverse biased.

Thus the Diode allows direct current to pass only in one direction while blocking it in the other direction. Power diodes are used in concerting AC into DC. In this, current will flow freely during the first half cycle (forward biased) and practically not at all during the other half cycle (reverse biased). This makes the diode an effective rectifier, which convert ac into pulsating dc. Signal diodes are used in radio circuits for detection. Zener diodes are used in the circuit to control the voltage.

Some common diodes are:-

1. Zener diode.

2. Photo diode.

3. Light Emitting diode.

1. ZENER DIODE:-

A zener diode is specially designed junction diode, which can operate continuously without being damaged in the region of reverse break down voltage. One of the most important applications of zener diode is the design of constant voltage power supply. The zener diode is joined in reverse bias to d.c. through a resistance R of suitable value.

2. PHOTO DIODE:-

A photo diode is a junction diode made from photo- sensitive semiconductor or material. In such a diode, there is a provision to allow the light of suitable frequency to fall on the p-n junction. It is reverse biased, but the voltage applied is less than the break down voltage. As the intensity of incident light is increased, current goes on increasing till it becomes maximum. The maximum current is called saturation current.

3. LIGHT EMITTING DIODE (LED):-

When a junction diode is forward biased, energy is released at the junction diode is forward biased, energy is released at the junction due to recombination of electrons and holes. In case of silicon and germanium diodes, the energy released is in infrared region. In the junction diode made of gallium arsenate or indium phosphide, the energy is released in visible region. Such a junction diode is called a light emitting diode or LED.

6. POWER SUPPLY

In alternating current the electron flow is alternate, i.e. the electron flow increases to maximum in one direction, decreases back to zero. It then increases in the other direction and then decreases to zero again. Direct current flows in one direction only. Rectifier converts alternating current to flow in one direction only. When the anode of the diode is positive with respect to its cathode, it is forward biased, allowing current to flow. But when its anode is negative with respect to the cathode, it is reverse biased and does not allow current to flow. This unidirectional property of the diode is useful for rectification. A single diode arranged back-to-back might allow the electrons to flow during positive half cycles only and suppress the negative half cycles. Double diodes arranged back-to-back might act as full wave rectifiers as they may allow the electron flow during both positive and negative half cycles. Four diodes can be arranged to make a full wave bridge rectifier. Different types of filter circuits are used to smooth out the pulsations in amplitude of the output voltage from a rectifier. The property of capacitor to oppose any change in the voltage applied across them by storing energy in the electric field of the capacitor and of inductors to oppose any change in the current flowing through them by storing energy in the magnetic field of coil may be utilized. To remove pulsation of the direct current obtained from the rectifier, different types of combination of capacitor, inductors and resistors may be also be used to increase to action of filtering.

NEED OF POWER SUPPLY

Perhaps all of you are aware that a ‘power supply’ is a primary requirement for the ‘Test Bench’ of a home experimenter’s mini lab. A battery eliminator can eliminate or replace the batteries of solid-state electronic equipment and the equipment thus can be operated by 230v A.C. mains instead of the batteries or dry cells. Nowadays, the use of commercial battery eliminator or power supply unit has become increasingly popular as power source for household appliances like transreceivers, record player, cassette players, digital clock etc.

THEORY

U SE OF DIODES IN RECTIFIERS:

Electric energy is available in homes and industries in India, in the form of alternating voltage. The supply has a voltage of 220V (rms) at a frequency of 50 Hz. In the USA, it is 110V at 60 Hz. For the operation of most of the devices in electronic equipment, a dc voltage is needed. For instance, a transistor radio requires a dc supply for its operation. Usually, this supply is provided by dry cells. But sometime we use a battery eliminator in place of dry cells. The battery eliminator converts the ac voltage into dc voltage and thus eliminates the need for dry cells. Nowadays, almost all-electronic equipment includes a circuit that converts ac voltage of mains supply into dc voltage. This part of the equipment is called Power Supply. In general, at the input of the power supply, there is a power transformer. It is followed by a diode circuit called Rectifier. The output of the rectifier goes to a smoothing filter, and then to a voltage regulator circuit. The rectifier circuit is the heart of a power supply.

RECTIFICATION

Rectification is a process of rendering an alternating current or voltage into a unidirectional one. The component used for rectification is called ‘Rectifier’. A rectifier permits current to flow only during the positive half cycles of the applied AC voltage by eliminating the negative half cycles or alternations of the applied AC voltage. Thus pulsating DC is obtained. To obtain smooth DC power, additional filter circuits are required.

A diode can be used as rectifier. There are various types of diodes. But, semiconductor diodes are very popularly used as rectifiers. A semiconductor diode is a solid-state device consisting of two elements is being an electron emitter or cathode, the other an electron collector or anode. Since electrons in a semiconductor diode can flow in one direction only-from emitter to collector- the diode provides the unilateral conduction necessary for rectification. Out of the semiconductor diodes, copper oxide and selenium rectifier are also commonly used.

FULL WAVE RECTIFIERIt is possible to rectify both alternations of the input voltage by using

two diodes in the circuit arrangement. Assume 6.3 V rms (18 V p-p) is applied to the circuit. Assume further that two equal-valued series-connected resistors R are placed in parallel with the ac source. The 18 V p-p appears across the two resistors connected between points AC and CB, and point C is the electrical midpoint between A and B. Hence 9 V p-p appears across each resistor. At any moment during a cycle of vin, if point A is positive relative to C, point B is negative relative to C. When A is negative to C, point B is positive relative to C. The effective voltage in proper time phase which each diode "sees" is in Fig. The voltage applied to the anode of each diode is equal but opposite in polarity at any given instant.

When A is positive relative to C, the anode of D1 is positive with respect to its cathode. Hence D1 will conduct but D2 will not. During the second alternation, B is positive relative to C. The anode of D2 is therefore positive with respect to its cathode, and D2 conducts while D1 is cut off.

There is conduction then by either D1 or D2 during the entire input-voltage cycle.

Since the two diodes have a common-cathode load resistor RL, the output voltage across RL will result from the alternate conduction of D1 and D2. The output waveform vout across RL, therefore has no gaps as in the case of the half-wave rectifier.

The output of a full-wave rectifier is also pulsating direct current. In the diagram, the two equal resistors R across the input voltage are necessary to provide a voltage midpoint C for circuit connection and zero reference. Note that the load resistor RL is connected from the cathodes to this center reference point C.

An interesting fact about the output waveform vout is that its peak amplitude is not 9 V as in the case of the half-wave rectifier using the same

power source, but is less than 4½ V. The reason, of course, is that the peak positive voltage of A relative to C is 4½ V, not 9 V, and part of the 4½ V is lost across R.

Though the full wave rectifier fills in the conduction gaps, it delivers less than half the peak output voltage that results from half-wave rectification.

BRIDGE RECTIFIERA more widely used full-wave rectifier circuit is the bridge rectifier. It

requires four diodes instead of two, but avoids the need for a centre-tapped transformer. During the positive half-cycle of the secondary voltage, diodes D2 and D4 are conducting and diodes D1 and D3 are non-conducting. Therefore, current flows through the secondary winding, diode D2, load resistor RL and diode D4. During negative half-cycles of the secondary voltage, diodes D1 and D3 conduct, and the diodes D2 and D4 do not conduct. The current therefore flows through the secondary winding, diode D1, load resistor RL and diode D3. In both cases, the current passes through the load resistor in the same direction. Therefore, a fluctuating, unidirectional voltage is developed across the load.

FiltrationThe rectifier circuits we have discussed above deliver an output

voltage that always has the same polarity: but however, this output is not suitable as DC power supply for solid-state circuits. This is due to the pulsation or ripples of the output voltage. This should be removed out before the output voltage can be supplied to any circuit. This smoothing is done by incorporating filter networks. The filter network consists of inductors and capacitors. The inductors or choke coils are generally connected in series with the rectifier output and the load. The inductors oppose any change in the magnitude of a current flowing through them by storing up energy in a magnetic field. An inductor offers very low resistance for DC whereas; it offers very high resistance to AC. Thus, a series connected choke coil in a rectifier circuit helps to reduce the pulsations or ripples to a great extent in the output voltage. The fitter capacitors are usually connected in parallel with the rectifier output and the load. As, AC can pass through a capacitor but DC cannot, the ripples are thus limited and the output becomes smoothed. When the voltage across its plates tends to rise, it stores up energy back into voltage and current. Thus, the fluctuations in the output

voltage are reduced considerable. Filter network circuits may be of two types in general:

CHOKE INPUT FILTERIf a choke coil or an inductor is used as the ‘first- components’ in the

filter network, the filter is called ‘choke input filter’. The D.C. along with AC pulsation from the rectifier circuit at first passes through the choke (L). It opposes the AC pulsations but allows the DC to pass through it freely. Thus AC pulsations are largely reduced. The further ripples are by passed through the parallel capacitor C. But, however, a little nipple remains unaffected, which are considered negligible. This little ripple may be reduced by incorporating a series a choke input filters.

CAPACITOR INPUT FILTERIf a capacitor is placed before the inductors of a choke-input filter

network, the filter is called capacitor input filter. The D.C. along with AC ripples from the rectifier circuit starts charging the capacitor C. to about peak value. The AC ripples are then diminished slightly. Now the capacitor C, discharges through the inductor or choke coil, which opposes the AC ripples, except the DC. The second capacitor C by passes the further AC ripples. A small ripple is still present in the output of DC, which may be reduced by adding additional filter network in series.

7. RelaysA relay is an electrically operated switch. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and they are double throw (changeover) switches.

Relays allow one circuit to switch a second circuit which can be completely separate from the first. For example a low voltage battery circuit can use a relay to switch a 230V AC mains circuit. There is no electrical connection inside the relay between the two circuits, the link is magnetic and mechanical.

The coil of a relay passes a relatively large current, typically 30mA for a 12V relay, but it can be as much as 100mA for relays designed to operate from lower voltages. Most ICs (chips) cannot provide this current and a transistor is usually used to amplify the small IC current to the larger value required for the relay coil. The maximum output current for the popular 555 timer IC is 200mA so these devices can supply relay coils directly without amplification.

Relays are usuallly SPDT or DPDT but they can have many more sets of switch contacts, for example relays with 4 sets of changeover contacts are readily available. For further information about switch contacts and the terms used to describe them please see the page on switches.

Most relays are designed for PCB mounting but you can solder wires directly to the pins providing you take care to avoid melting the plastic case of the relay.

Circuit symbol for a relay

Relays

Photographs © Rapid Electronics

Relay showing coil and switch contacts

The supplier's catalogue should show you the relay's connections. The coil will be obvious and it may be connected either way round. Relay coils produce brief high voltage 'spikes' when they are switched off and this can destroy transistors and ICs in the circuit. To prevent damage you must connect a protection diode across the relay coil.

The animated picture shows a working relay with its coil and switch contacts. You can see a lever on the left being attracted by magnetism when the coil is switched on. This lever moves the switch contacts. There is one set of contacts (SPDT) in the foreground and another behind them, making the relay DPDT.

The relay's switch connections are usually labelled COM, NC and NO:

COM = Common, always connect to this, it is the moving part of the switch.

NC = Normally Closed, COM is connected to this when the relay coil is off.

NO = Normally Open, COM is connected to this when the relay coil is on.

Connect to COM and NO if you want the switched circuit to be on when the relay coil is on.

Connect to COM and NC if you want the switched circuit to be on when the relay coil is off.

Choosing a relayYou need to consider several features when choosing a relay:

1. Physical size and pin arrangement If you are choosing a relay for an existing PCB you will need to ensure that its dimensions and pin arrangement are suitable. You should find this information in the supplier's catalogue.

2. Coil voltage The relay's coil voltage rating and resistance must suit the circuit powering the relay coil. Many relays have a coil rated for a 12V supply but 5V and 24V relays are also readily available. Some relays operate perfectly well with a supply voltage which is a little lower than their rated value.

3. Coil resistance The circuit must be able to supply the current required by the relay coil. You can use Ohm's law to calculate the current:

Relay coil current   =     supply voltage     coil resistance

4. For example: A 12V supply relay with a coil resistance of 400 passes a current of 30mA. This is OK for a 555 timer IC (maximum output current 200mA), but it is too much for most ICs and they will require a transistor to amplify the current.

5. Switch ratings (voltage and current) The relay's switch contacts must be suitable for the circuit they are to control. You will need to check the voltage and current ratings. Note that the voltage rating is usually higher for AC, for example: "5A at 24V DC or 125V AC".

6. Switch contact arrangement (SPDT, DPDT etc) Most relays are SPDT or DPDT which are often described as "single pole changeover" (SPCO) or "double pole changeover" (DPCO). For further information please see the page on switches.

Protection diodes for relaysTransistors and ICs (chips) must be protected from the brief high voltage 'spike' produced when the relay coil is switched off. The diagram shows how a signal diode (eg 1N4007) is connected across the relay coil to provide this protection. Note that the diode is connected 'backwards' so that it will normally not conduct. Conduction only occurs when the relay coil is switched off, at this moment current tries to continue flowing through the coil and it is harmlessly diverted through the diode. Without the diode no current could flow and the coil would produce a damaging high voltage 'spike' in its attempt to keep the current flowing.

Reed relaysReed relays consist of a coil surrounding a reed switch. Reed switches are normally operated with a magnet, but in a reed relay current flows through the coil to create a magnetic field and close the reed switch.

Reed relays generally have higher coil resistances than standard relays (1000 for example) and a wide range of supply voltages (9-20V for example).

They are capable of switching much more rapidly than standard relays, up to several hundred times per second; but they can only switch low currents (500mA maximum for example).

The reed relay shown in the photograph will plug into a standard 14-pin DIL socket ('chip holder').

For further information about reed switches please see the page on switches.

Relays and transistors comparedLike relays, transistors can be used as an electrically operated switch. For switching small DC currents (< 1A) at low voltage they are usually a better choice than a relay. However transistors cannot switch AC or high voltages (such as mains electricity) and they are not usually a good choice for switching large currents (> 5A). In these cases a relay will be needed, but

Reed Relay

Photograph © Rapid Electronics

note that a low power transistor may still be needed to switch the current for the relay's coil! The main advantages and disadvantages of relays are listed below:

8. TRANSISTOR

The name is transistor derived from ‘transfer resistors’ indicating a solid state Semiconductor device. In addition to conductor and insulators, there is a third class of material that exhibits proportion of both. Under some conditions, it acts as an insulator, and under other conditions it’s a conductor. This phenomenon is called Semi-conducting and allows a variable control over electron flow. So, the transistor is semi conductor device used in electronics for amplitude. Transistor has three terminals, one is the collector, one is the base and other is the emitter, (each lead must be connected in the circuit correctly and only then the transistor will function). Electrons are emitted via one terminal and collected on another terminal, while the third terminal acts as a control element. Each transistor has a number marked on its body. Every number has its own specifications.

There are mainly two types of transistor (i) NPN & (ii) PNP

NPN Transistors:

When a positive voltage is applied to the base, the transistor begins to conduct by allowing current to flow through the collector to emitter circuit. The relatively small current flowing through the base circuit causes a much greater current to pass through the emitter / collector circuit. The phenomenon is called current gain and it is measure in beta. PNP Transistor:

It also does exactly same thing as above except that it has a negative voltage on its collector and a positive voltage on its emitter.

Transistor is a combination of semi-conductor elements allowing a controlled current flow. Germanium and Silicon is the two semi-conductor elements used for making it. There are two types of transistors such as

POINT CONTACT and JUNCTION TRANSISTORS. Point contact construction is defective so is now out of use. Junction triode transistors are in many respects analogous to triode electron tube.

A junction transistor can function as an amplifier or oscillator as can a triode tube, but has the additional advantage of long life, small size, ruggedness and absence of cathode heating power.

Junction transistors are of two types which can be obtained while manufacturing.

The two types are: -

1) PNP TYPE:This is formed by joining a layer of P type of germanium to an N-P Junction

2) NPN TYPE:This is formed by joining a layer of N type germanium to a P-N Junction.

P N P

N P N

Both types are shown in figure, with their symbols for representation. The centre section is called the base, one of the outside sections-the emitter and the other outside section-the collector. The direction of the arrowhead gives the direction of the conventional current with the forward bias on the emitter. The conventional flow is opposite in direction to the electron flow.

OPERATION OF PNP TRANSISTOR:-

A PNP transistor is made by sand witching two PN germanium or silicon diodes, placed back to back. The centre of N-type portion is extremely thin in comparison to P region. The P region of the left is connected to the positive terminal and N-region to the negative terminal i.e. PN is biased in the forward direction while P region of right is biased negatively i.e. in the reverse direction as shown in Fig. The P region in the forward biased circuit is called the emitter and P region on the right, biased negatively is called collector. The centre is called base.

The majority carriers (holes) of P region (known as emitter) move to N region as they are repelled by the positive terminal of battery while the electrons of N region are attracted by the positive terminal. The holes overcome the barrier and cross the emitter junction into N region. As the width of base region is extremely thin, two to five percent of holes recombine with the free electrons of N-region which result in a small base current while the remaining holes (95% to 98%) reach the collector junction. The collector is biased negatively and the negative collector voltage aids in sweeping the hole into collector region.

As the P region at the right is biased negatively, a very small current should flow but the following facts are observed:-

1) A substantial current flows through it when the emitter junction is biased in a forward direction.

2) The current flowing across the collector is slightly less than that of the emitter, and

3) The collector current is a function of emitter current i.e. with thedecrease or increase in the emitter current a corresponding change in the collector current is observed.

The facts can be explained as follows:-

1. As already discussed that 2 to 5% of the holes are lost in recombination with the electron n base region, which result in a

small base current and hence the collector current is slightly less than the emitter current.

2. The collector current increases as the holes reaching the collector junction are attracted by negative potential applied to the collector.

3. When the emitter current increases, most holes are injected into the base region, which is attracted by the negative potential of the collector and hence results in increasing the collector current. In this way emitter is analogous to the control of plate current by small grid voltage in a vacuum triode.

Hence we can say that when the emitter is forward biased and collector is negatively biased, a substantial current flows in both the circuits. Since a small emitter voltage of about 0.1 to 0.5 volts permits the flow of an appreciable emitter current the input power is very small. The collector voltage can be as high as 45 volts.

10. SEGMENT DISPLAYS

The 7 segment display is used as a numerical indicator on many types of test equipment.

It is an assembly of light emitting diodes which can be powered individually. They most commonly emit red light. They are arranged and labelled as shown in the diagram.

Powering all the segments will display the number 8.

Powering a,b,c d and g will display the number 3. Numbers 0 to 9 can be displayed. The d.p represents a decimal point.

The one shown is a common anode display since all anodes are joined together and go to the positive supply. The cathodes are connected individually to zero volts. Resistors must be placed in series with each diode to limit the current through each diode to a safe value.

Early wrist watches used this type of display but they used so much current that the display was normally switched off. To see the time you had to push a button.

Common cathode displays where all the cathodes are joined are also available.

Liquid crystal displays do a similar job and consume much less power.

11 .LM567Tone Decoder

Features • 20 to 1 frequency range with an external resistor• Logic compatible output with 100 mA current sinking capability• Bandwidth adjustable from 0 to 14%• High rejection of out of band signals and noise• Immunity to false signals• Highly stable center frequency• Center frequency adjustable from 0.01 Hz to 500 kHz

Description

The LM567 and LM567C are general purpose tone decoders designed to provide a saturated transistor switch to ground when an input signal is present within the passband. The circuit consists of an I and Q detector driven by a voltage controlled oscillator which determines the center frequency of the decoder. External components are used to independently set center frequency, bandwidth and output delay.

Applications

• Touch tone decoding• Precision oscillator• Frequency monitoring and control• Wide band FSK demodulation• Ultrasonic controls• Carrier current remote controls• Communications paging decoders

12 TransformerA transformer is an electrical device that transfers energy from one circuit to another by magnetic coupling with no moving parts. A transformer comprises two or more coupled windings, or a single tapped winding and, in most cases, a magnetic core to concentrate magnetic flux. A changing current in one winding creates a time-varying magnetic flux in the core,

which induces a voltage in the other windings. Michael Faraday built the first transformer, although he used it only to demonstrate the principle of electromagnetic induction and did not foresee the use to which it would eventually be put.

A historical Stanley transformer.

Lucien Gaulard and John Dixon Gibbs, who first exhibited a device called a 'secondary generator' in London in 1881 and then sold the idea to American company Westinghouse. This may have been the first practical power transformer. They also exhibited the invention in Turin in 1884, where it was adopted for an electric lighting system. Their early devices used an open iron core, which was soon abandoned in favour of a more efficient circular core with a closed magnetic path.

William Stanley, an engineer for Westinghouse, who built the first practical device in 1885 after George Westinghouse bought Gaulard and Gibbs' patents. The core was made from interlocking E-shaped iron plates. This design was first used commercially in 1886.

Hungarian engineers Károly Zipernowsky, Ottó Bláthy and Miksa Déri at the Ganz company in Budapest in 1885, who created the efficient "ZBD" model based on the design by Gaulard and Gibbs.

Nikola Tesla in 1891 invented the Tesla coil, which is a high-voltage, air-core, dual-tuned resonant transformer for generating very high voltages at high frequency.

OverviewThe transformer is one of the simplest of electrical devices, yet transformer designs and materials continue to be improved. Transformers are essential for high voltage power transmission, which makes long distance transmission economically practical. This advantage was the principal factor in the selection of alternating current power transmission in the "War of Currents" in the late 1880s.

Audio frequency transformers (at the time called repeating coils) were used by the earliest experimenters in the development of the telephone. While some electronics applications of the transformer have been made obsolete by new technologies, transformers are still found in many electronic devices.

Transformers come in a range of sizes from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge gigawatt units used to interconnect large portions of national power grids. All operate with the same basic principles and with many similarities in their parts.

Single phase pole-mounted step-down transformer

Transformers alone cannot do the following:

Convert DC to AC or vice versa Change the voltage or current of DC Change the AC supply frequency.

However, transformers are components of the systems that perform all these functions.

An analogy

The transformer may be considered as a simple two-wheel 'gearbox' for electrical voltage and current. The primary winding is analogous to the input shaft and the secondary winding to the output shaft. In this analogy, current is equivalent to shaft speed, voltage to shaft torque. In a gearbox, mechanical power (torque multiplied by speed) is constant (neglecting losses) and is equivalent to electrical power (voltage multiplied by current) which is also constant.

The gear ratio is equivalent to the transformer step-up or step-down ratio. A step-up transformer acts analogously to a reduction gear (in which mechanical power is transferred from a small, rapidly rotating gear to a large, slowly rotating gear): it trades current (speed) for voltage (torque), by transferring power from a primary coil to a secondary coil having more turns. A step-down transformer acts analogously to a multiplier gear (in which mechanical power is transferred from a large gear to a small gear): it trades voltage (torque) for current (speed), by transferring power from a primary coil to a secondary coil having fewer turns.

Coupling by mutual induction

A simple transformer consists of two electrical conductors called the primary winding and the secondary winding. Energy is coupled between the windings by the time-varying magnetic flux that passes through (links) both primary and secondary windings. When the current in a coil is switched on or off or changed, a voltage is induced in a neighboring coil. The effect, called mutual inductance, is an example of electromagnetic induction.

Simplified analysis

A practical step-down transformer showing magnetising flux in the core

If a time-varying voltage is applied to the primary winding of turns, a current will flow in it producing a magnetomotive force (MMF). Just as an electromotive force (EMF) drives current around an electric circuit, so MMF tries to drive magnetic flux through a magnetic circuit. The primary MMF produces a varying magnetic flux in the core, and, with an open circuit secondary winding, induces a back electromotive force (EMF) in opposition

to . In accordance with Faraday's law of induction, the voltage induced across the primary winding is proportional to the rate of change of flux:

     and     

where

vP and vS are the voltages across the primary winding and secondary winding,

NP and NS are the numbers of turns in the primary winding and secondary winding,

dΦP / dt and dΦS / dt are the derivatives of the flux with respect to time of the primary and secondary windings.

Saying that the primary and secondary windings are perfectly coupled is equivalent to saying that . Substituting and solving for the voltages shows that:

    

where

vp and vs are voltages across primary and secondary, Np and Ns are the numbers of turns in the primary and secondary,

respectively.

Hence in an ideal transformer, the ratio of the primary and secondary voltages is equal to the ratio of the number of turns in their windings, or alternatively, the voltage per turn is the same for both windings. The ratio of the currents in the primary and secondary circuits is inversely proportional to the turns ratio. This leads to the most common use of the transformer: to convert electrical energy at one voltage to energy at a different voltage by means of windings with different numbers of turns. In a practical transformer, the higher-voltage winding will have more turns, of smaller conductor cross-section, than the lower-voltage windings.

The EMF in the secondary winding, if connected to an electrical circuit, will cause current to flow in the secondary circuit. The MMF produced by

current in the secondary opposes the MMF of the primary and so tends to cancel the flux in the core. Since the reduced flux reduces the EMF induced in the primary winding, increased current flows in the primary circuit. The resulting increase in MMF due to the primary current offsets the effect of the opposing secondary MMF. In this way, the electrical energy fed into the primary winding is delivered to the secondary winding. Also because of this, the flux density will always stay the same as long as the primary voltage is steady.

For example, suppose a power of 50 watts is supplied to a resistive load from a transformer with a turns ratio of 25:2.

P = EI (power = electromotive force × current)

50 W = 2 V × 25 A in the primary circuit if the load is a resistive load. (See note 1)

Now with transformer change:

50 W = 25 V × 2 A in the secondary circuit.

ConstructionCores

Steel cores

Laminated core transformer showing edge of laminations at top of unit.

Transformers for use at power or audio frequencies have cores made of many thin laminations of silicon steel. By concentrating the magnetic flux, more of it is usefully linked by both primary and secondary windings. Since the steel core is conductive, it, too, has currents induced in it by the changing magnetic flux. Each layer is insulated from the adjacent layer to reduce the energy lost to eddy current heating of the core. The thin laminations are used to reduce the eddy currents, and the insulation is used to keep the laminations from acting as a solid piece of steel. The thinner the laminations, the lower the eddy currents, and the lower the losses. Very thin laminations are generally used on high frequency transformers. The cost goes up when using thinner laminations mainly over the labor in stacking them.

A typical laminated core is made from E-shaped and I-shaped pieces, leading to the name "EI transformer". In the EI transformer, the laminations are stacked in what is known as an interleaved fashion. Due to this interleaving a second gap in parallel (in an analogy to electronic circuits) to the gap between E and I is formed between the E-pieces. The E-pieces are pressed together to reduce the gap width to that of the insulation. The gap area is very large, so that the effective gap width is very small (in analogy to a capacitor). For this to work the flux has to gradually flow from one E to the other. That means that on one end all flux is only on every second E. That means saturation occurs at half the flux density. Using a longer E and wedging it with two small Is will increase the overlap and additionally make the grains more parallel to the flux (think of a wooden frame for a window). If an air gap is needed (which is unlikely considering the low remanence available for steel), all the E's are stacked on one side, and all the I's on the other creating a gap.

The cut core or C-core is made by winding a silicon steel strip around a rectangular form. After the required thickness is achieved, it is removed from the form and the laminations are bonded together. It is then cut in two forming two C shapes. The faces of the cuts are then ground smooth so they fit very tight with a very small gap to reduce losses. The core is then assembled by placing the two C halves together, and holding them closed by a steel strap. Usually two C-cores are used to shorten the return path for the magnetic flux resulting in a form similar to the EI. More cores would necessitate a triangular cross-section. Like toroidal cores they have the advantage, that the flux is always in the oriented parallel the grains. Due to the bending of the core some area is lost for a rectangular winging.

A steel core's remanence means that it retains a static magnetic field when power is removed. When power is then reapplied, the residual field will cause a high inrush current until the effect of the remanent magnetism is reduced, usually after a few cycles of the applied alternating current. Overcurrent protection devices such as fuses must be selected to allow this harmless inrush to pass. On transformers connected to long overhead power transmission lines, induced currents due to geomagnetic disturbances during solar storms can cause saturation of the core, and false operation of transformer protection devices.

Distribution transformers can achieve low off-load losses by using cores made with low loss high permeability silicon steel and amorphous (non-crystalline) steel, so-called "metal glasses" — the high cost of the core material is offset by the lower losses incurred at light load, over the life of the transformer. In order to maintain good voltage regulation, distribution transformers are designed to have very low leakage inductance.

Certain special purpose transformers use long magnetic paths, insert air gaps, or add magnetic shunts (which bypass a portion of magnetic flux that would otherwise link the primary and secondary windings) in order to intentionally add leakage inductance. The additional leakage inductance limits the secondary winding's short circuit current to a safe, or a controlled, level. This technique is used to stabilize the output current for loads that exhibit negative resistance such as electric arcs, mercury vapor lamps, and neon signs, or safely handle loads that may become periodically short-circuited such as electric arc welders. Gaps are also used to keep a transformer from saturating, especially audio transformers which have a DC component added.

Solid cores

Powdered iron cores are used in circuits (such as switch-mode power supplies) that operate above mains frequencies and up to a few tens of kilohertz. These materials combine high magnetic permeability with high bulk electrical resistivity.

At even higher, radio-frequencies (RF), other types of cores made from non-conductive magnetic ceramic materials, called ferrites, are common. Some RF transformers also have moveable cores (sometimes called slugs) which

allow adjustment of the coupling coefficient (and bandwidth) of tuned radio-frequency circuits.

Air cores

High-frequency transformers may also use air cores. These eliminate the loss due to hysteresis in the core material. Such transformers maintain high coupling efficiency (low stray field loss) by overlapping the primary and secondary windings.

Toroidal cores

Various transformers. The top right is toroidal. The bottom right is from a 12 VAC wall wart supply.

Toroidal transformers are built around a ring-shaped core, which is made from a long strip of silicon steel or permalloy wound into a coil, from powdered iron, or ferrite, depending on operating frequency. The strip construction ensures that the grain boundaries are optimally aligned, improving the transformer's efficiency by reducing the core's reluctance. The closed ring shape eliminates air gaps inherent in the construction of an EI core. The cross-section of the ring is usually square or rectangular, but more expensive cores with circular cross-sections are also available. The primary and secondary coils are often wound concentrically to cover the entire surface of the core. This minimises the length of wire needed, and also provides screening to minimize the core's magnetic field from generating electromagnetic interference.

Ferrite toroid cores are used at higher frequencies, typically between a few tens of kilohertz to a megahertz, to reduce losses, physical size, and weight of switch-mode power supplies.

Toroidal transformers are more efficient than the cheaper laminated EI types of similar power level. Other advantages, compared to EI types, include smaller size (about half), lower weight (about half), less mechanical hum (making them superior in audio amplifiers), lower exterior magnetic field (about one tenth), low off-load losses (making them more efficient in standby circuits), single-bolt mounting, and more choice of shapes. This last point means that, for a given power output, either a wide, flat toroid or a tall, narrow one with the same electrical properties can be chosen, depending on the space available. The main disadvantages are higher cost and limited size.

A drawback of toroidal transformer construction is the higher cost of windings. As a consequence, toroidal transformers are uncommon above ratings of a few kVA. Small distribution transformers may achieve some of the benefits of a toroidal core by splitting it and forcing it open, then inserting a bobbin containing primary and secondary windings.

When fitting a toroidal transformer, it is important to avoid making an unintentional short-circuit through the core. This can happen if the steel mounting bolt in the middle of the core is allowed to touch metalwork at both ends, making a loop of conductive material which passes through the hole in the toroid. Such a loop could result in a dangerously large current flowing in the bolt.

Windings

The wire of the adjacent turns in a coil, and in the different windings, must be electrically insulated from each other. The wire used is generally magnet wire. Magnet wire is a copper wire with a coating of varnish or some other synthetic coating. Transformers for years have used Formvar wire which is a varnished type of magnet wire.

The conducting material used for the winding depends upon the application. Small power and signal transformers are wound with solid copper wire, insulated usually with enamel, and sometimes additional insulation. Larger power transformers may be wound with wire, copper, or aluminum rectangular conductors. Strip conductors are used for very heavy currents. High frequency transformers operating in the tens to hundreds of kilohertz will have windings made of Litz wire to minimize the skin effect losses in the conductors. Large power transformers use multiple-stranded conductors as well, since even at low power frequencies non-uniform distribution of

current would otherwise exist in high-current windings. Each strand is insulated from the other, and the strands are arranged so that at certain points in the winding, or throughout the whole winding, each portion occupies different relative positions in the complete conductor. This "transposition" equalizes the current flowing in each strand of the conductor, and reduces eddy current losses in the winding itself. The stranded conductor is also more flexible than a solid conductor of similar size. (see reference (1) below)

For signal transformers, the windings may be arranged in a way to minimise leakage inductance and stray capacitance to improve high-frequency response. This can be done by splitting up each coil into sections, and those sections placed in layers between the sections of the other winding. This is known as a stacked type or interleaved winding.

Windings on both the primary and secondary of power transformers may have external connections (called taps) to intermediate points on the winding to allow adjustment of the voltage ratio. Taps may be connected to an automatic, on-load tap changer type of switchgear for voltage regulation of distribution circuits. Audio-frequency transformers, used for the distribution of audio to public address loudspeakers, have taps to allow adjustment of impedance to each speaker. A center-tapped transformer is often used in the output stage of an audio power amplifier in a push-pull type circuit. Modulation transformers in AM transmitters are very similar. Tapped transformers are also used as components of amplifiers, oscillators, and for feedback linearization of amplifier circuits.

Insulation

The turns of the windings must be insulated from each other to ensure that the current travels through the entire winding. The potential difference between adjacent turns is usually small, so that enamel insulation is usually sufficient for small power transformers. Supplemental sheet or tape insulation is usually employed between winding layers in larger transformers.

The transformer may also be immersed in transformer oil that provides further insulation. Although the oil is primarily used to cool the transformer, it also helps to reduce the formation of corona discharge within high voltage transformers. By cooling the windings, the insulation will not break down as

easily due to heat. To ensure that the insulating capability of the transformer oil does not deteriorate, the transformer casing is completely sealed against moisture ingress. Thus the oil serves as both a cooling medium to remove heat from the core and coil, and as part of the insulation system.

Certain power transformers have the windings protected by epoxy resin. By impregnating the transformer with epoxy under a vacuum, air spaces within the windings are replaced with epoxy, thereby sealing the windings and helping to prevent the possible formation of corona and absorption of dirt or water. This produces transformers suitable for damp or dirty environments, but at increased manufacturing cost.

Shielding

Where transformers are intended for minimum electrostatic coupling between primary and secondary circuits, an electrostatic shield can be placed between windings to reduce the capacitance between primary and secondary windings. The shield may be a single layer of metal foil, insulated where it overlaps to prevent it acting as a shorted turn, or a single layer winding between primary and secondary. The shield is connected to earth ground.

Transformers may also be enclosed by magnetic shields, electrostatic shields, or both to prevent outside interference from affecting the operation of the transformer, or to prevent the transformer from affecting the operation of nearby devices that may be sensitive to stray fields such as CRTs.

Coolant

Three phase dry-type transformer with cover removed; rated about 200 KVA, 480 V.

Small signal transformers do not generate significant amounts of heat. Power transformers rated up to a few kilowatts rely on natural convective air cooling. Specific provision must be made for cooling of high-power transformers. Transformers handling higher power, or having a high duty cycle can be fan-cooled.

Some dry transformers are enclosed in pressurized tanks and are cooled by nitrogen or sulfur hexafluoride gas.

The windings of high-power or high-voltage transformers are immersed in transformer oil — a highly-refined mineral oil, that is stable at high temperatures. Large transformers to be used indoors must use a non-flammable liquid. Formerly, polychlorinated biphenyl (PCB) was used as it was not a fire hazard in indoor power transformers and it is highly stable. Due to the stability and toxic effects of PCB byproducts, and its accumulation in the environment, it is no longer permitted in new equipment. Old transformers which still contain PCB should be examined on a weekly basis for leakage. If found to be leaking, it should be changed out, and professionally decontaminated or scrapped in an environmentally safe manner. Today, nontoxic, stable silicone-based oils, or fluorinated hydrocarbons may be used where the expense of a fire-resistant liquid offsets additional building cost for a transformer vault. Other less-flammable fluids such as canola oil may be used but all fire resistant fluids have some drawbacks in performance, cost, or toxicity compared with mineral oil.

The oil cools the transformer, and provides part of the electrical insulation between internal live parts. It has to be stable at high temperatures so that a small short or arc will not cause a breakdown or fire. The oil-filled tank may have radiators through which the oil circulates by natural convection. Very large or high-power transformers (with capacities of millions of watts) may have cooling fans, oil pumps and even oil to water heat exchangers. Oil-filled transformers undergo prolonged drying processes, using vapor-phase heat transfer, electrical self-heating, the application of a vacuum, or combinations of these, to ensure that the transformer is completely free of water vapor before the cooling oil is introduced. This helps prevent electrical breakdown under load.

Oil-filled power transformers may be equipped with Buchholz relays which are safety devices that sense gas build-up inside the transformer (a side

effect of an electric arc inside the windings), and thus switches off the transformer.

Experimental power transformers in the 2 MVA range have been built with superconducting windings which eliminates the copper losses, but not the core steel loss. These are cooled by liquid nitrogen or helium.

Terminals

Very small transformers will have wire leads connected directly to the ends of the coils, and brought out to the base of the unit for circuit connections. Larger transformers may have heavy bolted terminals, bus bars or high-voltage insulated bushings made of polymers or porcelain. A large bushing can be a complex structure since it must provide electrical insulation without letting the transformer leak oil.

EnclosureSmall transformers often have no enclosure. Transformers may have a shield enclosure, as described above. Larger units may be enclosed to prevent contact with live parts, and to contain the cooling medium (oil or pressurized gas).

REFERENCES:

1. WWW.GOOGLE.COM2. WWW. EN.WIKIPEDIA.ORG3. WWW. SCIENCE.HOWSTUFFWORKS.COM4. WWW. WWW.ROBOTS.COM