Rectifiers Theory with Circuit Diagrams

Rectifiers are defined as electronic or electrical devices which are used to convert alternating current of a.c. into direct current or d.c.

Rectifiers Theory

Current is defined as flow of electrons through conductor. An alternating current is one in which direction of flow of electrons or current flow reverses periodically. Whereas direct current is one in which direction of flow of electrons or current flow remains same i.e., current flows in one direction. A.C. has certain cycles per second which is called frequency and it is measured in Hertz (Hz). Each cycle has two half portions one of which is positive and another half is negative. In a.c. supply, as current flows in both directions periodically, therefore, polarity of terminals of a.c. supply also changes periodically i.e., a terminal which remains positive during first half cycle becomes negative during another half cycle, whereas in d.c. supply, polarity of output terminals does not change.

Rectifiers allow flow of electrons in one direction and block electrons flow in reverse direction. This is achieved by diodes. Diodes are electronic or electrical devices made from semiconductors and conduct only in one direction i.e., in forward biased condition. Diodes have  two terminal one is Anode and another is Cathode. Diode is said to be forward biased when its Anode is connected to positive terminal of battery and Cathode is connected to negative terminal of battery. A diode is said to be reverse biased when its Anode is connected to negative of battery and Cathode is connected to positive terminal of battery. Diodes conduct when they are forward biased and they do not conduct when they are reverse biased.


As we can see from above figure that a cycle of alternating current has both positive and negative portions and as we discussed above that diodes conduct when they are forward biased, so diodes conduct and allow a.c. to pass during half cycle when they are forward biased and block a.c. during next half cycle when they are reverse biased.


It may be noted that d.c. voltage shown in above figure is ideal d.c. voltage which has constant magnitude. Output of rectifiers is pulsating d.c. i.e., voltage with changing magnitude but fixed polarity. Capacitors may be introduced in the output circuits of rectifiers to convert pulsating output d.c. voltage into ripple d.c. voltage. A ripple d.c. voltage is one which has slight variations in magnitude and the variation is such that the voltage changes little above and down  periodically to a given straight line d.c. voltage.

CLASSIFICATION OF RESISTORS

     1.  Half Wave Rectifiers,
     2.  Full Wave Rectifiers,

     Full Wave rectifiers can be further classified as ;

          a)  Center Tapped Full Wave Rectifiers,
          b)  Bridge Rectifiers.

Have Wave Rectifier

A half wave rectifier blocks one half portion or cycle of alternating current and converts a.c. voltage into pulsating d.c. voltage using only another half portion or cycle of alternating current during each cycle of a.c. A simplest half wave rectifier circuit consists of a single P-N junction diode. A load is connected in series with diode. A simplest half wave rectifier circuit and wave forms of input a.c. and converted output d.c. is shown in figure below ;

WORKING OF HALF WAVE RECTIFIER

There are two portions of alternating current one is positive and another is negative portion and thus there are two switching states of diode, one is forward biased when diode conducts and another is reverse biased when it does not conduct and stops current to pass on due to which circuit remains open. During first half cycle, diode remains forward biased when it conducts and voltage is observed across the load as shown in figure above. During second half cycle, diode remains reverse biased when it does not conduct and no voltage is observed across the load.

CENTER TAPPED FULL WAVE RECTIFIER

A full wave rectifier converts a.c. voltage into pulsating d.c. voltage using both half portions of alternating current during each cycle of a.c. This is achieved using two diodes and two secondary coils of a transformer. A transformer known as center tapped transformer that has three secondary output leads may be used. One output lead  at center becomes common and two diodes are connected to two remaining leads. During first half cycle, one diode remains forward biased and during another half cycle another diode remains forward biased. In this way, we get pulsating output d.c. voltage during a complete cycle of a.c. A simplest full wave rectifier circuit consists of two P-N junction diodes. A simplest full wave rectifier circuit and wave forms of input a.c. voltage and converted output d.c. voltage is shown in figure below ;

WORKING OF FULL WAVE RECTIFIER

During first half cycle diode D1 becomes forward biased and diode D2 remains reverse biased. D1 then conducts, D2 does not conduct and central lead remains at zero, therefore, we observe voltage across load. During second half cycle diode D2 becomes forward  biased and diode D1 becomes reverse biased. D2 then conducts, D1 does not conduct, therefore, we observe voltage across load. In full wave rectifier voltage is observed during whole cycle of input a.c. unlike half wave rectifier in which voltage is observed across load during only positive half cycle of alternating current. As output of diodes D1 & D2 are common and both the diodes conduct one by one consecutively during positive half & negative half portions of each cycle of input alternating current, therefore, voltage is observed during whole cycle of input a.c.

FULL WAVE BRIDGE RECTIFIER

A full wave bridge rectifier also known as simply bridge rectifier converts a.c. voltage into pulsating d.c. voltage using both half negative & positive portions of alternating current. This is achieved using four diodes connected in bridge arrangement so that there becomes two inputs and two outputs. No center tapped transformer is required in bridge rectifier unlike simple two diodes full wave rectifier, however, the wave form of full wave bridge rectifier is same to that of simple two diodes full wave rectifier. During first half cycle, diodes D2 & D4 are forward biased and during another half cycle, diodes D1 & D3 are forward biased. In this way, we get pulsating output d.c. voltage during each complete cycle of input alternating current. A simplest full wave bridge rectifier circuit consists of four P-N junction diodes. A simplest full wave bridge rectifier circuit and wave forms of input a.c. voltage and converted output d.c. voltage is shown in figures below ;

WORKING OF FULL WAVE BRIDGE RECTIFIER

During first half cycle, diodes D2 & D4 becomes forward biased whereas diodes D1 & D3 becomes reverse biased. D2 & D4  then conduct and D1 & D3 do not conduct, therefore, voltage is observed across load. During second half cycle, diodes D1 & D3 becomes forward biased whereas diodes D2 & D4 becomes reverse biased. D1 &  D3 then conducts and D2 & D4 do not conduct, therefore, voltage is observed across load. In bridge rectifier, voltage is observed during whole cycle of input a.c. voltage unlike half wave rectifier in which voltage is only observed across load during only positive half cycle of a.c.  As output of diodes D2 & D3 are cathodes and they are common, so this output becomes positive terminal because they become forward biased when positive signal is applied to their anodes which are the inputs. Similarly as output of diodes D1 & D4 anodes and they are common, so this output becomes negative terminal because they become forward biased when negative signal is applied to their cathodes which are the inputs. Diodes D1 & D3 collectively conduct and diodes D2 & D4 collectively conduct one by one consecutively during negative half and positive half portions of each cycle of input alternating current, therefore, in this way voltage is observed during whole cycle.

Linear Variable Differential Transformer

Linear Variable Differential Transformer or LVDT is a displacement transducer, which is mainly used for measurement of linear displacements. LVDT works on the principle of transformer action i.e., mutual induction between primary and secondary coils. 

Construction of LVDT

LVDT essentially consists of a transformer which has one primary coil and two secondary coils with a movable core. Secondary coils are placed symmetrically relative to primary coil. Both secondary coils have equal number of turns and they are identical with each other. Secondary coils are connected in series opposition and when transformer is energized the two voltages induced in two coils have phase difference of 180⁰. Core is placed in between primary and secondary coils and allowed for free movement along its axis as shown in figure given. Core is made up of magnetic material and it facilitates magnetic flux linkage between primary coil and secondary coils. The objects or shafts whose displacements are to be measured are coupled with one of the ends of core. 

Working Principle of LVDT

Like a transformer, LVDT also works on the principle of mutual induction between primary and secondary coils as it's construction is just similar to a transformer but instead of two coils (one primary and one secondary) LVDT's transformer has three coils (one primary coil and two secondary coils). Further, two secondary coils are connected in series opposition in such a way that when transformer is energized the two voltages induced in two coils have phase difference of 180⁰ or simply we can say that two voltages induced in two secondary coils have reversed polarities so that addition of emfs induced in both coils because of linear displacements of core becomes net output of LVDT. When core moves up or down from central position about its axis, LVDT gives some output because magnetic flux linking each secondary coil changes and are not same which in turn produces voltages of different magnitudes and of reversed polarities in coils.

Working of LVDT

The object whose displacement need to be measured is attached to the core by suitable means. With the displacements of objects, core is moved either up or down. When core is moved towards down-side from central position, magnetic flux linking with secondary coil 2 (SC2) increases whereas magnetic flux linking with secondary coil 1 (SC1) decreases. Thus, the voltage induced in SC2 increases and in SC1 decreases. As discussed above, voltage induced in SC1 & SC2 are 180⁰ out of phase that is these voltages have reversed polarities than each other. Also these secondary coils are connected in series, therefore, addition of voltages in two coils is the net output voltage of LVDT. For example, say at any instance voltage induced in coils SC1 is -2 volts and in coil SC2 is 5 volts, so net output voltage will be 3 volts. This voltage will have same phase as SC2. Similarly, when core is moved up-side from central position, voltage induced in SC1 increases and in SC2 decreases. Thus net output will be the addition of two voltages. On the other hand, when core is at central position, magnetic flux linking with SC1 & SC2 are same and voltages induced in both coils are also same but with polarities reversed than each other. Thus, net output voltage of LVDT will be zero.


Because of residual voltages due to stay magnetic and capacitance effects, the net output voltage at central position may not zero at all the times. LVDTs are available in wide ranges. LVDTs have linear output over a wide range of input displacements.

Advantages of LVDT Instruments

• Minimum input force costs in sensing by LVDT because of negligible friction in movement of force.
• High sensitivity.
• Linear Output.

Resistors, Resistivity, Color Coding of Resistors

     Resistor is an electronic or electrical component that opposes the flow of current in a circuit. Such an oppose by resistors to flow of current is known as resistance. Practically all materials offer some resistance to flow of current.

     Resistors are most common components used in electronic circuits. Usually resistors have two leads which are connected in series with other components in the circuit to limit flow of current through components connected in circuit.

     Resistance of a Resistor is measured in "Ohm". The symbol used to represent Ohm is a Greek letter "Ω" (Omega). Symbol used to represent the Resistor is "R" and figure representation of Resistor is given below ;

Symbol of Resistor

Another way of representation symbol of Resistor

RESISTIVITY 

     Resistivity or Specific Resistance of a substance is defined as the resistance of a unit long wire having a unit cross-section area, which is kept at 20⁰ C. Symbol used to represent the resistivity is a Greek letter "ρ" (Rho). SI unit of measurement of Electrical Resistivity or Specific Resistance of a substance is "Ω-m".

IMPORTANT ; As the resistance of a substance is a function of size, shape and environmental conditions of the substance, thus Resistivity becomes a very important term because it gives a means of comparing resistance of various substances and help us determining best conductors among others.

RELATIONSHIP BETWEEN ELECTRICAL RESISTANCE AND ELECTRICAL RESISTIVITY OF A SUBSTANCE 

Let us assume a piece of substance having ;

1. Resistance = R  Ω
2. Resistivity = ρ  Ω-meters,
3. Length = L meters, &
4. Cross-sectional Area = A square meters

 {because resistance is proportional to length and

   inversely proportional to cross-sectional area}

FACTORS DETERMINING RESISTANCE 

     From the equation between electrical resistance & electrical resistivity of a substance explained above, now we can say that the factors determining resistance are resistivity (ρ), shape i.e. length & cross-sectional area. Also, environmental conditions i.e., temperature affects resistance. All these factors have been explained below ;

Resistivity ( ρ ) ;
     We know that resistivity or specific resistance of a substance is the resistance of a unit long wire having a unit cross-sectional area at 20⁰ C and resistivity is proportional to resistance. So higher the resistivity will be, the higher will be the resistance of that substance and vice versa.


Length ;
     Length is also proportional to the resistance. Therefore, a wire with a longer length say 15 meters will have higher resistance to electric current flow than a wire of not so long length say 10 meters, if other conditions like resistivity, cross-sectional area and temperature are kept unchanged. This is because the resistive path increases with the length.


Cross-sectional Area ;
     Cross-sectional Area is inversely proportional to the resistance. Therefore, a wire with a bigger cross-sectional area will have less resistance than a wire having comparatively smaller cross-sectional area, if other conditions like resistivity, length & temperature are kept unchanged. As we know that current is the flow of electrons through a conductor when we apply electro-motive force or voltage across a conductor. This means, number of loosely bounded electrons will be more if cross-sectional area is increased and conductor allows current flow with ease and thus resistance experienced by flow of current will also decrease.


Temperature ;
     Different materials have different properties and also change in temperature affects their resistivity differently. Resistance of some substances increase with increase in temperature and vice versa. Such substances are said to have positive temperature coefficient. Resistance of some other substances decrease with increase in temperature and vice versa. Such substances are said to have negative temperature coefficient.

CLASSIFICATION OF RESISTORS   

Classification based on applications ;

• Fixed Value Resistors 
• Variable Resistors

Classification based on constructional features ;

• Carbon composite filled resistors
• Wire-wound resistors
• Deposited film resistors

Classification based on input signal sensing principle

• Light Dependent Resistors (LDRs)
• Thermistors

All these resistors classified above have been explained below ;

CARBON COMPOSITE FILLED RESISTORS 

     These are fixed value resistors and most commonly used resistors. In these resistors, resistive path or resistance to current flow is obtained with the help of mixture of composite materials including fine carbon particles which are conductive in nature and fine particles of other suitable non-conductive materials which are used to bind mixture and hold them tightly together. This tightly bounded carbon composite looks like a cylindrical in shape which is protected inside ceramic coating and two connecting metallic leads, one at each end are joined to make connections in circuits.

     Desired resistance value is obtained by increasing or decreasing carbon contents in the mixture.

WIRE-WOUND RESISTORS 

     These resistors essentially consists of a length of wire of specific resistance (alloy of various suitable metals), which is wrapped around a core of non-conductive materials usually ceramic core. Wire is wrapped around in such a way that it runs from one end to another so that two connecting leads are obtained from core ends i.e., one connecting lead is available at each end. Also resistive wire is wrapped spirally around core in such a way that it does not make contact radially at any point throughout its length. Wire wound resistors are especially used where high wattage resistors are required. These resistors are available in various shapes and sizes. Wire-wound resistors are also made available as fixed value resistors as well as variable resistors. Examples of variable wire-wound resistors are Rheostats & Wire-wound potentiometers.

     Desired resistance value is obtained by selecting alloy metal for resistive wire to make resistive path and also by increasing or decreasing length of wire. Alloy metals used for wire and its cross-sectional area also determine wattage rating of resistors.

DEPOSITED FILM RESISTORS  

     These resistors, as name suggests, are made by depositing layer of resistive material, which may contain carbon film of metallic film, onto a core of some non-conductive material. Rest of its construction is similar to that carbon composite filled resistors.

RHEOSTATS RESISTORS 

     These are wire-wound type of resistors. These are variable resistors. Rheostats are analog devices. Various resistance values are obtained by a slider. A slider is a simple sliding mechanism which slides along the length of the core on which wire is wrapped with the help of shafts. Shafts are fixed parallel to core to facilitate movement of slider. Tip of the slider consists of a conductive material or a piece of metal which remains in contact with the wire wrapped around core. One output lead is connected to slider and another lead is taken from one of the extreme ends of resistive wire. As slider moves towards the end from where out put lead is taken out, length of resistive path is decreased and hence the resistance also decreases. Similarly, when slider moves away, length of resistive path increases which increases the resistance. In this way, resistance value is increased or decreased by moving slider along the core.

POTENTIOMETERS 

     These are variable resistors. Potentiometers may be made to take analog readings as well as readings in discrete steps. Various resistance values are obtained by a slider arm. One end of the slider arm is fixed for any eccentric or linear movements and only allowed to rotate at its axis. Another end remains in contact with the circular resistive path made on flat surface. Resistive path of potentiometers can be made by depositing layers of resistive materials onto a surface of non-conductive material or it can be made using wire by wrapping around a core like Rheostats. The only different between Rheostat and wire wound potentiometer is that core is not straight like a bar. In wire-wound potentiometers core is bend in such a way that it makes a circular path for slider. Slider can be made to move in discrete steps or it can be made to move continuously in a circular path.

LIGHT DEPENDENT RESISTORS (LDRs) 

     These are variable resistors. Resistance of LDRs varies with variation in light striking it. When light intensity striking LDR falls, its resistance increase and vice versa. LDRs are used in camera to switch ON flash in automatic mode while capturing pictures in low lights and also they are used to control street lights for automatic switching ON and OFF to save power consumption. LDRs also called Photo-resistors.

THERMISTORS  

     These are variable resistors. Resistance of Thermistors varies with variation in temperature of the atmosphere surrounding it. Both positive temperature coefficient (PTC) and negative temperature coefficient (NTC) substances are used in construction of Thermistors. Resistance of PTC Thermistors increases with increase in temperature and vice versa. On the other hand, resistance of NTC Thermistors decreases with increase in temperature and vice versa. Thermistors can be used to protect circuits from over current and they can be used in appliances like electrical geysers, electrical hotplates, etc., to protect overheating and damage to man and machine.

COLOR CODING OF RESISTORS

Following table is used to know the value of color bands of resistors ;

Following method is used to calculate resistance value from the colour bands of resistors using colour coding table given above ;

For example, we take resistor given below and calculate its value using colour bands ;

1. First band of resistor is Yellow ; Value of Yellow is 4,
2. Second band of resistor is Violet ; Value of Violet is 7,
3. Third band of resistor is Black ; Value of Black is 1,
4. Fourth band of resistor is Gold ; Value of Gold is 5%,
5. Write down value of first band i.e., 4,
6. Write down value of second band i.e., 7,
7. From first and second bands, we obtained digit 47,
8. Third band is multiplier band and value of third band is 1,
9. After multiplication, we obtained figure 47,
10. Calculate 5% of figure obtained from first, second & third bands i.e., 2.35,
11. We obtained value of above resistor as 47 Ω  +/- 2.35 Ω.

Let's take one more example of resistor given below and calculate its value using colour bands ;

1. First band of resistor is Red ; Value of Red is 2,
2. Second band of resistor is Black ; Value of Black is 0,
3. Third band of resistor is Brown ; Value of Brown is 10,
4. Fourth band of resistor is Gold ; Value of Gold is 5%,
5. Write down value of first band i.e., 2,
6. Write down value of second band i.e., 0,
7. From first and second bands, we obtained digit 20,
8. Third band is multiplier band and value of third band is 10,
9. After multiplication, we obtained figure 200,
10. Calculate 5% of figure obtained from first, second & third bands i.e., 10,
11. We obtained value of above resistor as  200 Ω  +/- 10 Ω.

SELECTION OF RESISTORS

Selection criterion of Resistors consists of three most important factors given below ;

1. Resistance value,
2. Wattage Rating, &
3. Tolerance / Precision

RESISTORS IN SERIES

     In circuits of resistors connected in series, voltage drops across each resistor and this voltage drop depends upon value of resistors. For example ;

Vtotal = V1 + V2                                                   .... (i)

As per Ohm's law ;

I = V/R

Or, V = I . R                                                 .... (ii)

Therefore, putting values of equation (ii) & in equation (i), we  get ;

Itotal . Rtotal = I1 . R1  + I2 . R2                .... (iii)

Or, Rtotal = R1 + R2                                       .... (iv)

                                                                                        (because Itotal = I1 = I2, as value of current remains

                                                                         same because there is only one path for flow of current)

R_total = R₁ + R₂ + R₃ + ........... + R_n          (universal form of equation (iv) for 'n' number of 

                                                                         resistors in series)

RESISTORS IN PARALLEL

     When resistors are connected in parallel with each other, current then have more than one path to flow through circuit as we can see in figure below ;

Therefore, 

Itotal = I1 + I2                                               .... (i)

As per Ohm's law ;

I = V/R                                                         .... (ii)

Therefore, putting value of equation (ii) in equation (i), we get equation (iii),

Vtotal /Rtotal = V1/R1 + V2/R2                   .... (iii)

Or, 1/Rtotal = 1/R1 + 1/R2                               .... (iv) 

                                                                                        (because Vtotal = V1 = V2, as value of voltage remains same)

Or Rtotal = (R1 . R2) / (R1 + R2)

When all the resistors connected in parallel are of same value, then total resistance will be ;

Rtotal = Resistor Value of One Resistor / Number of Resistors

RESISTORS CONNECTED IN SERIES-PARALLEL COMBINATIONS 

     Complex circuits have resistors connected in series and as well as parallel. Those circuits are reduced to simplify by first calculating resistance of two or more resistors either connected in series or parallel. For example, we take below circuit and calculate resistance ;

     In above problem, it can be observed from figure that resistors R2 & R3 are connected in parallel. First we will reduce circuit by calculating resistance value of these two resistors as given below ;

     We know that formula for calculating resistance of resistors connected in parallel is ;

R4 = (R2 . R3) / (R2 + R3)                        

We get R4 and circuit can now be drawn to simplest form as given below ;

    Now, it can be observed from above figure that resistors R1 & R4 are connected in series and there resistance can be calculated as given below ;

Therefore, Rtotal = R1 + R4

Logic gates & their Truth Table

     Logic gates are digital circuits that do processing of digital circuits. Gates have one or more inputs but only one output. There are three basic logic gates and they are named as OR gate, AND gate & NOT gate. These three gates can be combined in various ways to perform more complex arithmetic functions based on processed input digital signals.

     There are two types of gates named as sequential gates and combinational gates. One type of logic gates called sequential gates, which have memory function, can process sequence of input digital values and outputs are based on sequences of applied inputs. Example of sequential gates are Flip-flops, Counters & Registers. Another types of logic gates called combinational gates, which do not have memory function, can process only instantaneous digital inputs and outputs are based on inputs applied at the moment of time. Examples of combinational gates are OR gate, AND gate, NOT gate, XOR gate, NAND gate, etc.

     Different types of logic gates which will be discussed here are ;

1. OR gate,
2. AND gate,
3. NOT gate,
4. NOR gate,
5. NAND gate,
6. EXCLUSIVE OR gate, &
7. EXCLUSIVE NOR gate.

OR gate

     This gate can have two or more than two inputs but only one output. OR gate is named so because output signal will be high if any of the input signals are high. Truth table and symbol of OR gate with two inputs are given below ;

OR gate & Its Truth Table

Output of OR gate is "X = A + B".

AND gate

     This gate can have two or more than two inputs but only one output. AND gate shows output only when all the inputs are applied at same time. Truth table and symbol of AND gate with two inputs are given below ;

AND gate & its Truth Table

Output of AND gate is "X = A . B".

NOT gate

     This gate has only one input and one output. NOT gate always gives output opposite to that of input signal i.e., if input signal is 0 then output will be one and vice versa. Truth table and symbol of NOT gate are given below ;

NOT gate & Its Truth Table

Output of NOT gate is "X = Ā".

NOR gate

     This gate can have two or more than two inputs but only one output. NOR gate is a combination of OR gate and NOT gate. NOT gate is connected to the output of OR gate. So the output of NOR gate is always opposite to that of OR gate. Truth table and symbol of NOR gate with two inputs are given below ;

NOR gate & Its Truth Table

NAND gate

      This gate can have two or more than two inputs but only one output. NAND gate is a combination of AND gate and NOT gate. NOT gate is connected to the output of AND gate like NOR gate. So the output of NAND gate is always opposite to that of AND gate. Truth table and symbol of NAND gate with two inputs are given below ;

NAND gate & Its Truth Table

EXCLUSIVE OR (XOR) gate ;

Ex-OR gate & Its Truth Table

EXCLUSIVE NOR gate

Exclusive NOR gate & Its Truth Table

Few other posts seeking your attention are ;

1. Rectifiers Theory @ Industrial Items

2. Hooke's Law @ All Laws of Science & Imporant Formulas

CURRENT TRANSFORMER, POTENTIAL TRANSFORMER OR CT, PT

Current Transformer

    Current transformers and Potential transformers are widely used in industries to measure alternating currents and voltages, respectively, of high magnitudes and to operate and control protection devices. Current transformers and Potential transformers, like other transformers, have two windings i.e., Primary and Secondary windings. The output of these transformers, i.e., secondary winding remain connected to other instruments like energy meters and other protection devices. Current transformers and Potential transformers reduce high value alternating currents and voltages, respectively, flowing in main transmission line or supply system to very low values in proportions specified. Thus, these transformers provide economic, accurate, easy, simple and safe way of handling supply currents and voltages to get monitored and controlled. The magnitude of outputs of these transformers depends upon the ratio of the transformer. These transformers are also known as Instrument Transformers. Current transformer and potential transformer are also commonly called CT and PT respectively.

CONSTRUCTION OF CURRENT TRANSFORMERS ;

     Current transformers have primary windings, secondary windings and a core of magnetic material like normal transformers. Function of current transformers has to reduce transmission or supply current to low values in specific proportions to be sensed by other display, metering and protective instruments. For this reason, primary windings of current transformers have very few turns and some CTs have only one turn whereas, secondary windings of current transformers have large number of turns. Conductors of primary windings of CTs are of thicker gauges or heavy wires of higher diameters to handle high transmission currents. Primary windings are connected in Series of transmission lines. Some current transformers do not have primary windings instead a transmission line or supply line is passed through a hole of magnetic core wrapped over with conductors of secondary windings. These transformers are available in many shapes and sizes.

CONSTRUCTION OF POTENTIAL TRANSFORMERS ;

        Construction of Potential transformers is similar to Current Transformers, as discussed above, except the number of turns in secondary winding are much less than primary winding just like any step down transformer.

Types of Instrument Transformers ;

                

     Some of the common types of Instrument Transformers are Wire-wound and Ring or Toroidal type transformer. Rectangle CTs & Split-core CTs are also available, so that CTs can be fixed without removing or opening connections. Standard output of Instrument CTs is 1 Amp or 5 Amp and standard output of Instrument PTs are 110 V or 220 V. CTs can be of Oil-immersed as well.

                

      Important characteristics of Instrument transformers, which should be taken into consideration while selecting Instrument transformers are ; i) Input & Output ratio or Transformer Ratio, ii) Standard Load, iii) Accuracy, iv) Rated Voltage, etc.

    Some of the Applications of Instrument transformers are ; i) Metering Inputs, ii) Monitoring Loading and Un-loading of power transformers, heavy motors, etc., iii) Protection against over-loading of various equipment like power transformers, to operate protective relays, circuit breakers and switch gears, iv) Earth fault or leakage protection, etc.

     Current Transformer and Potential Transformer together can be used to measure Power with the help of Watt Meter.

      

SECONDARY OF CURRENT TRANSFORMERS CANNOT BE LEFT OPEN OR KEPT SHORT CIRCUITED ;

     Secondary terminals of Current transformers should not be left open because of high induced e.m.f. in secondary windings. Current transformers work as Step-up transformers which increase input e.m.f. as primary windings have only one or two turns of conductors whereas secondary windings have many hundreds of turns of conductor wrapped around magnetic core thereby magnify e.m.f. or voltage to many times which can be dangerous and source of accident. So secondary terminals of these transformers are kept short circuited when not connected with helping instruments.

     In the end, instrument transformers prove to be very useful in industries in sensing current flow and e.m.f. between two or more terminals of high magnitudes very efficiently, economically, safely and easily. 

     

Few other posts seeking your attention are ;

Coulomb's Law

Industrial Linear Motion Guides

INTRODUCTION TO LINEAR MOTION GUIDES ;

        Linear Motion Guides are those precision machine elements which are designed with friction less, noise less, vibration free and other working characteristics to permit accurate linear relative motion between two machine members. LM Guides are available in two parts, one is LM bearing or carriage and another is LM shaft or rail over which carriage moves. These guides are made in various shapes, sizes and materials depending upon load, length of travel, speed of travel, space availability and working conditions. Linear Motion Guides find applications in almost every industry from General Purpose Machine Tools to Special Purpose Machine, from Medical Equipment to Aerospace and from Transport to Construction. All the LM guides can be classified broadly into three categories depending upon the type of contact in the guide elements during motion i.e., (i) Sliding Contact, (ii) Rolling Contact and (iii) Magnetic field. 

Classification of Linear Motion Guides ;

            Classification based on technology incorporated to achieve motion or the type of contact in LM Guide elements ;

1.  Sliding Contact Linear Motion Guides ;

        In these guides, motion takes place because of sliding action between guide elements i.e., LM block and LM guide rails or rods. These are also known as Plain LM Guides. Sliding Contact LM Guides can be further classified in to two sub-categories, depending upon thickness of lubrication film in between sliding surfaces, as given below ;

a)       Thin Lubrication Film Sliding Contact LM Guides ; A partial contact remains in between guide elements as a thin layer of lubricant sweeps in between them during motion. Low friction coefficient materials are used in construction of this category of Guides. Despite of some advantages like simple in design, low cost, etc., these guides, however, least preferred because of certain disadvantages like  high friction, high heat generation, less efficiency, low life, low load carrying capacity, high maintenance.

b)        Hydrostatic LM Guides ; In these types of bearings a thick layer of pressurized fluid holds the carriage and there is no direct contact between the moving elements. To overcome disadvantages of high friction, high heat, etc. pressurized fluid is supplied in between moving elements forcibly from external source, which makes these guides costly because it requires additional unit to supply fluid, not suitable for applications in clean-rooms as there are always chances of leakage of pressurized fluid, etc.

Examples of Sliding Contact Linear Bearings are i) Spline Shaft & Spline Nut, ii) Slide Rails, iii) Lead Screws, etc.

2.    Rolling Contact Linear Motion Guides ; 

       These LM guides consists of balls or cylindrical rollers or wheels of various shapes or profiles which enable smooth and very low frictional motion at fairly high speeds in between carriages and their guides or rails. Despite of being comparatively costly these LM Guides are most preferred because of certain advantages like trouble free operations even at high speeds, compact design, low maintenance, high load carrying capacity, high efficiency, etc.

Examples of Rolling Contact Linear Motion Guides are ;

i)               LM Guide Blocks and Rails, and ii) Ball Screws, in which hardened spherical steel balls keep on circulating in a continuous end-less grooves of carriages while motion. These spherical balls support the carriage over rail or shaft which is firmly fixed to one of the machine members as well as facilitate smooth movement of loads,

iii)       LM Roller Guide Blocks have cylindrical rollers, which facilitate relative movement in between carriage with greater load and rail, and

iv)         Guide Wheels and Rails, in which wheels with a profile usually Circular groove (semi-circle) or V groove in its outer face along circumference is provided. Also a protrusion of similar profile like Guide Wheel’s groove profile in rail is provided which fits into Wheels and guides them. These are preferred where movements of comparatively greater loads at higher velocities are required.

LM Block

Advantages of Rolling Contact LM Guides ;

a)    High Positional Accuracy,

b)   High Efficiency,

c)    High Speed Motion of loads is possible,

d)   High Loads movement is possible, 

e)   Low Noise,

f)     Less Maintenance,

g)    Fairly good Life.

3.  Magnetic Field Linear Motion Guides ; 

       These guides preferably use electromagnets to provide relative linear movements in between two machine members. Their application is limited to only clean rooms in medical and other such similar clean and tidy places as no contact between parts having relative motion eliminates use of any kind of lubrication. Disadvantages like high cost, space constraints, etc. makes them less popular.

Types of Linear Motion Guides

Different types of LM Guides, which falls in any of the above mentioned categories, available in markets have been explained below ;

1.     Linear Motion Block & Rail ; 

          These blocks are highly precise and compact in design, in which 4 rows of steel balls keep on running in continuous end-less paths. The rolling balls remain in firm touch with 4 corresponding raceways on rail and supports block on rail and enable linear movement along the rail. The re-circulation of balls also permits the motion of load along desired length by introducing additional lengths of rails. These guides come in category of rolling contact LM guides and are available with either spherical hardened steel balls or hardened cylindrical rollers, which also re-circulate or guide wheels to achieve rolling action. These are most commonly used guides because of certain advantages like high accuracy, high efficiency, availability, high speed motion, etc.,

2.     Linear Motion Ball Bearings & Shafts ; 

         These are also a type of LM guides which permits linear relative motion. LM Ball Bearings are made in ring shape which carries continuous end-less raceways along its length to guide hardened spherical steel balls during motion. And as the bearings are in ring shape, they deliver motion through cylindrical shafts.  These bearings are available in various shaft sizes and types like closed-type and open-type, with or without flanges, Pillow blocks, etc. Open-type bearings are made cut open along its length to guide shafts mounting on fulcrums. These guides also come in Rolling Contact LM Guides.

3.    Linear Motion Stroke ; 

         These are also made in ring shape like LM bearings, except it has a drilled ball-cage in which holes are arranged spirally along the surface of cage and so the balls are and steel balls do not re-circulate unlike LM bearings. After inserting balls in holes of cage, holes are mechanically deformed through suitable process usually caulking, which narrows holes and cage retains balls and they do not fall. LM Strokes are also rolling contact type and they are suitable for rotational and reciprocating motions. These are used for small distance and high speed movements. 

4.  Linear Motion Spline Nut & Shaft ; 

         A Spline shaft is a round shaft in which a series of continuous equally spaced grooves are cut along the length of the shaft. Spline shafts, therefore, has teeth along its periphery and gives gear like structure when looking from cross-section area.  Thus, spline nut has similar teeth at its internal surfaces, which fits into corresponding grooves on shaft and hence provide stable sliding motion along shaft length. These are Sliding Contact type LM Guides. These are less efficient, they have high friction, low positioning accuracy. Advantages are ; Low Cost, Good Life, Less Maintenance, etc.

5.   Lead Screw ; 

      These screws normally have square threads instead of V threads as they are more efficient. Lead Screws are used to convert rotational motion of screw into linear motion of Nut. Backlash, low efficiency, high wear are the disadvantages of these screws.

6.   Ball Screw ; 

       These guides like Lead Screws convert rotational motion of screw into linear motion of Nut. Unlike lead screws, these are highly efficient because rolling motion of steel balls in between grooves cut in the nut and screw shaft. Steel balls re-circulate in the groove. Wear rate in these guides are much less.

Other LM Guides available in market are ; Ball Spline, Slide Rail, Guide Wheels and some LM Tables with or without drive.

Applications of Linear Motion Guides ;

Some common applications are ;

LM Block

• Machine Tools like Horizontal Machining Centres (HMC), Vertical Machining Centres (VMC), CNC machine tools, etc.,
• Industrial Robots,
• Medical Equipments,
• Electronic Equipments,
• Special Purpose Machineries.

Selection of Linear Motion Guides ;

 LM Guides are selected on the basis of following points ;

• Load to be moved,
• Moving Speed of Load,
• Space Availability,
• Accuracy,
• Length of Movement,
• Efficiency,
• Working Conditions, &
• Inter-changeability & Easy Availability

A thought from personal experience ; An LM Block was broken during my service to a Diaper Manufacturing Company. The constraint was that the machine manufacturer was Chinese and spare was not available with us or anywhere in Indian market. It was also not possible to get it delivered from China as it would have taken long time. So, I checked model number of LM Block engraved on it and searched it over internet. I found company's catalog and after comparing all the specifications of LM block with Indian manufacturer, I got part number of local brand Block. I ordered block and in that case rail also had to purchase. Just because of that I had put a point of Easy Availability in Points of Selection.

Guides also need to be protected from dust for which manufacturers provide some accessories like bellows to cover rail and open parts of carriage.

Few other posts seeking your attention are ;

1. Hooke's Law