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March 31, 2017

What is a Resistor? What are the Types of Resistors? What are the Applications of Resistor?

What is a Resistor?

A resistor is a passive two-terminal electrical or electronic component that resists an electric current by producing a voltage drop between its terminals in accordance with Ohm's law. The electrical resistance is equal to the voltage drop across the resistor divided by the current through the resistor. 

Figure 1: A typical axial-lead resistor.

Figure 2: Two common schematic symbols of resistor.
In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active elements, and terminate transmission lines, among other uses. High-power resistors that can dissipate many watts of electrical power as heat may be used as part of motor controls, in power distribution systems, or as test loads for generators. Fixed resistors have resistances that only change slightly with temperature, time or operating voltage. Variable resistors can be used to adjust circuit elements (such as a volume control or a lamp dimmer), or as sensing devices for heat, light, humidity, force, or chemical activity.

Theory of Operation:

Ohm's law: The behavior of an ideal resistor is dictated by the relationship specified by Ohm's law:
             V = I/R

Ohm's law states that the voltage (V) across a resistor is proportional to the current (I), where the constant of proportionality is the resistance (R). For example, if a 300 ohm resistor is attached across the terminals of a 12 volt battery, then a current of 12 / 300 = 0.04 amperes flows through that resistor.

Practical resistors also have some inductance and capacitance which affect the relation between voltage and current in alternating current circuits.

The ohm (symbol: Ω) is the SI unit of electrical resistance, named after Georg Simon Ohm. An ohm is equivalent to a volt per ampere. Since resistors are specified and manufactured over a very large range of values, the derived units of milliohm (1 mΩ = 10-3 Ω), kilohm (1 kΩ = 103 Ω), and megohm (1 MΩ = 106 Ω) are also in common usage. 

Figure 3: A few types of resistors.

Types of Resistors:

1.     Linear resistors.
                                 i.         Fixed resistors
a)    Led arrangement
b)    Carbon composition
c)     Carbon Pile
d)    Carbon film
e)    Printed carbon resistor
f)      Thick and thin film
g)    Metal film
h)    Metal oxide film
i)      Wire wound
j)      Foil resistor
k)    Ammeter shunt
l)      Grid resistor
m) Special verities
                                    ii.         Variable resistor
a)       Adjustable resistor
b)       Potentiometers
c)        Resistance and decade boxes
d)       Special devices.
2.     Non-linear resistors.

Applications of Resistors:

  • In general, a resistor is used to create a known voltage-to-current ratio in an electric circuit. If the current in a circuit is known, then a resistor can be used to create a known potential difference proportional to that current. Conversely, if the potential difference between two points in a circuit is known, a resistor can be used to create a known current proportional to that difference.  
  • Current-limiting. By placing a resistor in series with another component, such as a light-emitting diode, the current through that component is reduced to a known safe value.  
  • A series resistor can be used for speed regulation of DC motors, such as used on locomotives and train sets.  
  • An attenuator is a network of two or more resistors (a voltage divider) used to reduce the voltage of a signal.  
  • A line terminator is a resistor at the end of a transmission line or daisy chain bus (such as in SCSI), designed to match impedance and hence minimize reflections of the signal.  
  • All resistors dissipate heat. This is the principle behind electric heaters.  

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Describe a Starter-Motor Circuit.

Figure 1: Typical Starter-Motor Circuit.

A Starter-Motor Circuit: 

Figure 1 shows a very simple diagram of an engine starter-motor circuit. If one day you turn on the ignition switch, push the start button, and nothing happens, this is the circuit diagram you will need. A complete engine wiring diagram would probably contain the same information, but it would also contain all of the meters, idiot lights, and alternator wiring, adding considerable visual confusion.

For the starter-motor circuit all you need to see are the:

  • Battery switch
  • Fuse between battery switch common terminal and engine panel
  • On/Off switch
  • Momentary start switch
  • Wire from starting switch to solenoid
  • Heavy positive cable from battery-select common terminal to solenoid
  • Heavy negative cable from battery negative terminal to engine negative terminal.

If the engine won’t turn over, the problem probably lies somewhere in this diagram. 

Wiring Symbols

The Discovery of Electricity Timeline.

The Discovery of Electricity:

Schematic diagram of a copper–zinc voltaic pile.

Democritus (460?–370? BC) proposes an “atomic theory” wherein all matter is made up of indivisible particles, or atoms.
Charles de Coulomb (1736–1806) discovers that the force of attraction between electric charges is proportional to the product of the two charges and inversely proportional to the distance between them.
Luigi Galvani (1737–1798) discovers that two unlike metals immersed in blood cause the muscles of a frog’s legs to twitch.
Alessandro Volta (1745–1827) discovers that a current ows between two connected unlike metals in a salt solution and, thus, invents the battery.
John Dalton (1766–1844) proposes the rst table of atomic weights of elements.
André Ampere (1775–1836) develops the theory of magnetic lines of force and quanties electric current for the rst time.
Hans Christian Ørsted (1777–1851) discovers a connection between electric current and magnetism and a way to measure electric current by the deection of a magnet.
Georg Ohm (1787–1854) discovers the relationship (Ohm’s Law) between voltage, current, and resistance in a circuit.
Michael Faraday (1791–1867) analyzes the chemical reactions in batteries and denes the terms “electrode,” “anode,” “cathode,” and “electrolyte.”
James Clerk Maxwell (1831–1879) develops the mathematical equations relating electricity and magnetism.
Joseph Thomson (1856–1940) proves that electricity consists of electrons.

March 17, 2017

What is the difference between Electric Motor and Electric Generator?

Difference between Electric Motor and Electric Generator: 

  • A generator converts mechanical energy to electrical energy, while motor converts mechanical energy to electrical energy. 
  • In a generator, a shaft attached to the rotor is driven by a mechanical force and electric current is produced in the armature windings, while the shaft of a motor is driven by the magnetic forces developed between the armature and field; current has to be supplied to the armature winding. 
  • Motors (generally a moving charge in a magnetic field) obey Fleming`s left-hand rule, while the generator obeys Fleming’s right-hand rule. 

March 14, 2017

What are the Advantages of Integrated Circuits (ICs)?

Advantages of Integrated Circuits:

The major advantages of integrated circuits over those made by interconnecting discrete components are as follows:
  1. Extremely small size – Thousands times smaller than discrete circuits. It is because of fabrication of various circuit elements in a single chip of semiconductor material.
  2. Very small weight owing to miniaturised circuit.
  3. Very low cost because of simultaneous production of hundreds of similar circuits on a small semiconductor wafer. Owing to mass production of an IC costs as much as an individual transistor.
  4. More reliable because of elimination of soldered joints and need for fewer interconnections.
  5. Lower power consumption because of their smaller size.
  6. Easy replacement as it is more economical to replace them than to repair them.
  7. Increased operating speed because of absence of parasitic capacitance effect.
  8. Close matching of components and temperature coefficients because of bulk production in batches.
  9. Improved functional performance as more complex circuits can be fabricated for achieving better characteristics.
  10. Greater ability of operating at extreme temperatures.
  11. Suitable for small signal operation because of no chance of stray electrical pickup as various components of an INC are located very close to each other on a silicon wafer.
  12. No component project above the chip surface in an INC as all the components are formed within the chip.

What are the limitations of integrated circuits?

The integrated circuits have few limitations also, as listed below:

  1. In an IC the various components are part of a small semi-conductor and the individual component or components cannot be removed, replaced, therefore, if any component in an IC fails, the whole IC has replaced by the new one. 
  2. Limited power rating as it is not possible to manufacture high power greater than 10 Watt) ICs.
  3. Need of connecting inductors and transformers exterior to the conductor chip as it is not possible to fabricate inductors and transform on the semi-conductor chip surface. 
  4. Operations at low voltage as ICs function at fairly low voltage.
  5. Quite delicate in handling as these cannot withstand rough handling or excessive heat. 
  6. Need of connecting capacitor exterior to the semi-conductor chip as it is neither convenient nor economical to fabricate capacitances exceeding 30pF. Therefore, for higher values of capacitance, discrete components, exterior to IC chip are connected. 
  7. High grade P-N-P assembly is not possible.
  8. Low temperature coefficient is difficult to be achieved.
  9. Difficult to fabricate an IC with low noise.
  10. Large value of saturation resistance of transistors.
  11. Voltage dependence of resistors and capacitors.
  12. The diffusion processes and other related procedures used in the fabrication process are not good enough to permit a precise control of the parameter values for the circuit elements. However, control of the ratios is at a sufficiently acceptable level. 

What is Schmitt trigger? How it works? Where is it used?

Schmitt trigger:

Schmitt trigger is an electronic circuit with positive feedback which holds the output level till the input signal to the comparator is higher than the threshold. It converts a sinusoidal or any analog signal to digital signal. It exhibits hysteresis by which the output transition from high to low and low to high will occur at different thresholds.


The Schmitt trigger was invented by American scientist Otto H. Schmitt in 1934. By that time, Otto Schmitt was a student. In the year 1937, he published his invention in his doctoral. The name he gave was "thermionic trigger".

Schmitt Trigger Types:

The two different Schmitt trigger types are:
  1. Non-inverting type, in which the input and output are both high / both low at the same time (no phase shift).
  2. Inverting type, in which there is 180° phase shift between input and output.


There are basically two symbols for the Schmitt Trigger. The symbol is a triangle with an input and an output, just like the one used for the non-inverting buffers. Inside there is the hysteresis symbol. Depending on the type of Schmitt Trigger, inverting or non-inverting (standard), the hysteresis curve sign differs.

Figure 1: Logic Symbols of Schimitt Trigger.

Operations of Schimitt Trigger:

The Schmitt Trigger is a type of comparator with two different threshold voltage levels. Whenever the input voltage goes over the High Threshold Level, the output of the comparator is switched HIGH (if is a standard ST) or LOW (if is an inverting ST). The output will remain in this state, as long as the input voltage is above the second threshold level, the Low Threshold Level. When the input voltage goes below this level, the output of the Schmitt Trigger will switch.

The HIGH and LOW output voltages are actually the POSITIVE and NEGATIVE power supply voltages of the comparator. The comparator needs to have positive and negative power supply (like + and -) to operate as a Schmitt Trigger normally. The following drawing shows how a Schmitt Trigger would react to an AC voltage input:

Figure 2: Basic operation of a Schimitt Trigger.

The orange line is the AC input. The horizontal RED line indicates the High Threshold Level, while the BLUE horizontal line indicates the Low Threshold Level. The green line is the output of the Schmitt Trigger. When the input voltage level goes above the High Threshold Level, then the output of the ST goes High. When the input voltage level goes below the Low Threshold Level, then the output of the ST goes Low. This is the basic operation of a Schmitt Trigger.

Figure 3: The most simple Schmitt Trigger circuit is implemented with a comparator with a positive feedback.

Applications of Schimitt Trigger:  

March 11, 2017

Why should Dry-Type Transformers never be overloaded?

Overloading of a transformer results in excessive temperature. This excessive temperature causes overheating which will result in rapid deterioration of the insulation and cause complete failure of the transformer coils.

Can transformers be used in parallel?

Figure: Transformers in Parallel

Single phase transformers can be used in parallel only when their impedances and voltages are equal. If unequal voltages are used, a circulating current exists in the closed network between the two transformers, which will cause excess heating and result in a shorter life of the transformer. In addition, impedance values of each transformer must be within 7.5% of each other. For example: Transformer A has an impedance of 4%, transformer B which is to be parallel to A must have an impedance between the limits of 3.7% and 4.3%. When paralleling three phase transformers, the same precautions must be observed as listed above, plus the angular displacement and phasing between the two transformers must be identical.

Can Single Phase Transformers be used for Three Phase applications?

Yes. Three phase transformers are sometimes not readily available, whereas single phase transformers can generally be found in stock. Three single phase transformers can be used in delta connected primary and wye or delta connected secondary. They should never be connected wye primary to wye secondary, since this will result in unstable secondary voltage. The equivalent three phase capacity when properly connected of three single phase transformers is three times the nameplate rating of each single phase transformer. 

For example: Three 10 kVA  single phase transformers will accommodate a 30 kVA three phase load.

State Possible Troubles in a DC Motor with Reasons.

Several troubles may arise in a d.c. motor and a few of them are discussed below:

i) Failure to start: This may be due to-

  1. Ground fault                                                            
  2. Open or short-circuit fault 
  3. Wrong connections                                                 
  4. Too low supply voltage 
  5. Frozen bearing ore                                                    
  6. Excessive load.

ii) Sparking at brushes: This may be due to-

  1. Troubles in brushes 
  2. Troubles in commutator 
  3. Troubles in armature or 
  4. Excessive load.
    • Brush troubles may arise due to insufficient contact surface, too short abrush, too little spring tension or wrong brush setting.
    • Commutator troubles may be due to dirt on the commutator, high mica, rough surface or eccentricity.
    • Armature troubles may be due to an open armature coil. An open armature coil will cause sparking each time the open coil passes the brush. The location of this open coil is noticeable by a burnt line between segments connecting the coil.

iii). Vibrations and pounding noises: These may be due to-

  1. Worn bearings                                                              
  2. Loose parts 
  3. Rotating parts hitting stationary parts               
  4. Armature unbalanced 
  5. Misalignment of machine                                      
  6. Loose coupling, etc.

iv) Overheating: The overheating of the motor may be due to-

  1. Overloads                                                                  
  2. Sparking at the brushes 
  3. Short-circuited armature or field coils             
  4. Too frequent starts or reversals 
  5. Poor ventilation                                                     
  6. Incorrect voltage.

What is an Autotransformer? Explain Autotransformer Operation. What are the Limitations, Advantages, Disadvantages and Applications of Autotransformer?


An autotransformer (sometimes called autostep down transformer) is an electrical transformer with only one winding. In an autotransformer, portions of the same winding act as both the primary and secondary sides of the transformer. The winding has at least three taps where electrical connections are made. 

On load condition, a part of the load current is obtained directly from the supply and the remaining part is obtained by transformer action. An Auto transformer works as a voltage regulator.

Figure-1: Single-phase tapped autotransformer with an output voltage range of 40%–115% of input.


An autotransformer has a single winding with two end terminals, and one or more terminals at intermediate tap points, or it is a transformer in which the primary and secondary coils have part of, or all of their turns in common. The primary voltage is applied across two of the terminals, and the secondary voltage taken from two terminals, almost always having one terminal in common with the primary voltage. The primary and secondary circuits therefore have a number of windings turns in common. Since the volts-per-turn is the same in both windings, each develops a voltage in proportion to its number of turns. In an autotransformer part of the current flows directly from the input to the output, and only part is transferred inductively, allowing a smaller, lighter, cheaper core to be used as well as requiring only a single winding. However the voltage and current ratio of autotransformers can be formulated the same as other two-winding transformers:

One end of the winding is usually connected in common to both the voltage source and the electrical load. The other end of the source and load are connected to taps along the winding. Different taps on the winding correspond to different voltages, measured from the common end. In a step-down transformer the source is usually connected across the entire winding while the load is connected by a tap across only a portion of the winding. In a step-up transformer, conversely, the load is attached across the full winding while the source is connected to a tap across a portion of the winding.

As in a two-winding transformer, the ratio of secondary to primary voltages is equal to the ratio of the number of turns of the winding they connect to. For example, connecting the load between the middle and bottom of the autotransformer will reduce the voltage by 50%. Depending on the application, that portion of the winding used solely in the higher-voltage (lower current) portion may be wound with wire of a smaller gauge, though the entire winding is directly connected.

If one of the center-taps is used for the ground, then the autotransformer can be used as a balun to convert a balanced line (connected to the two end taps) to an unbalanced line (the side with the ground).


An autotransformer does not provide electrical isolation between its windings as an ordinary transformer does; if the neutral side of the input is not at ground voltage, the neutral side of the output will not be either. A failure of the isolation of the windings of an autotransformer can result in full input voltage applied to the output. Also, a break in the part of the winding that is used as both primary and secondary will result in the transformer acting as an inductor in series with the load (which under light load conditions may result in near full input voltage being applied to the output). These are important safety considerations when deciding to use an autotransformer in a given application.

Because it requires both fewer windings and a smaller core, an autotransformer for power applications is typically lighter and less costly than a two-winding transformer, up to a voltage ratio of about 3:1; beyond that range, a two-winding transformer is usually more economical.

In three phase power transmission applications, autotransformers have the limitations of not suppressing harmonic currents and as acting as another source of ground fault currents. A large three-phase autotransformer may have a "buried" delta winding, not connected to the outside of the tank, to absorb some harmonic currents.

In practice, losses mean that both standard transformers and autotransformers are not perfectly reversible; one designed for stepping down a voltage will deliver slightly less voltage than required if it is used to step up. The difference is usually slight enough to allow reversal where the actual voltage level is not critical.

Like multiple-winding transformers, autotransformers use time-varying magnetic fields to transfer power. They require alternating currents to operate properly and will not function on direct current.


  1. An autotransformer requires less Cu than a two-winding transformer of similar rating.
  2. An autotransformer operates at a higher efficiency than a two-winding transformer of similar rating.
  3. An autotransformer has better voltage regulation than a two-winding transformer of the same rating.
  4. An autotransformer has smaller size than a two-winding transformer of the same rating.
  5. An autotransformer requires smaller exciting current than a two-winding transformer of the same rating. 
It may be noted that these advantages of the autotransformer decrease as the ratio of transformation increases. Therefore, an autotransformer has marked advantages only for relatively low values of transformation ratio (i.e. values approaching 1).


  1. There is a direct connection between the primary and secondary. Therefore, the output is no longer d.c. isolated from the input.
  2. An autotransformer is not safe for stepping down a high voltage to a low voltage. As an illustration, Figure-2 shows 11000/230 V step-down autotransformer. If an open circuit develops in the common portion 2-3 of the winding, then full-primary voltage (i.e., 11000 V in this case) will appear across the load. In such a case, any one coming in contact with the secondary is subjected to high voltage. This could be dangerous to both the persons and equipment. For this reason, autotransformers are prohibited for general use.
  3. The short-circuit current is much larger than for the two-winding transformer of the same rating. It can be seen from Figure-2 that a short-circuited secondary causes part of the primary also to be short circuited. This reduces the effective resistance and reactance.
Figure-2: 11000/230 V step-down autotransformer.

Applications of Autotransformers:

(i) Power transmission and distribution

Autotransformers are frequently used in power applications to interconnect systems operating at different voltage classes, for example 132 kV to 66 kV for transmission. Another application in industry is to adapt machinery built (for example) for 480 V supplies to operate on a 600 V supply. They are also often used for providing conversions between the two common domestic mains voltage bands in the world (100 V—130 V and 200 V—250 V). The links between the UK 400 kV and 275 kV 'Super Grid' networks are normally three phase autotransformers with taps at the common neutral end.

On long rural power distribution lines, special autotransformers with automatic tap-changing equipment are inserted as voltage regulators, so that customers at the far end of the line receive the same average voltage as those closer to the source. The variable ratio of the autotransformer compensates for the voltage drop along the line.

A special form of autotransformer called a zig zag is used to provide grounding on three-phase systems that otherwise have no connection to ground. A zig-zag transformer provides a path for current that is common to all three phases (so-called zero sequence current).

(ii) Audio system

In audio applications, tapped autotransformers are used to adapt speakers to constant-voltage audio distribution systems, and for impedance matching such as between a low-impedance microphone and a high-impedance amplifier input.

(iii) Railways

In UK railway applications, it is common to power the trains at 25 kV AC. To increase the distance between electricity supply Grid feeder points they can be arranged to supply a 25-0-25 kV supply with the third wire (opposite phase) out of reach of the train's overhead collector pantograph. The 0 V point of the supply is connected to the rail while one 25 kV point is connected to the overhead contact wire. At frequent (about 10 km) intervals, an autotransformer links the contact wire to rail and to the second (antiphase) supply conductor. This system increases usable transmission distance, reduces induced interference into external equipment and reduces cost. A variant is occasionally seen where the supply conductor is at a different voltage to the contact wire with the autotransformer ratio modified to suit.

(iv) Autotransformers are used for reducing the voltage supplied to a.c. motors during the starting period.

(v) Autotransformers are used as a voltage regulator.

Download this Article as PDF:
Autotransformer – Operation, Limitations, Advantages, Disadvantages & Applications.

March 9, 2017

State the main effects of electric current? What are the applications of it?

The three main effects of an electric current are:

  1. Magnetic effect
  2. Chemical effect and
  3. Heating effect.

Some practical applications of the effects of an electric current include:

Magnetic effect: 

When electric current flows through a wire, it behaves like a magnet. This is called magnetic effect of electric current.

Figure 1: Electric bell - examples of magnetic effect.

Applications of magnetic effect:

  • Bells
  • Relays
  • Motors
  • Generators
  • Transformers
  • Telephones
  • Car-ignition and 
  • Lifting magnets.

Chemical effect: 

The passage of an electric current through a conducting liquid causes chemical reactions. The resulting effects are called chemical effects of electric current.
Figure 2: Battery - Example of chemecal effect.

Applications of chemical effect:

  • Primary and secondary cells and 
  • Electroplating.

Heating effect:  

When electric current flows through a wire, the wire gets heated. This is called the heating effect of electric current.

Figure 3: Irons - example of heating effects.

Applications of heating effect:

  • Cookers
  • Water heaters
  • Electric fires
  • Irons
  • Furnaces
  • Kettles and 
  • Soldering irons

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