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What Are Current, Resistance and Voltage?

Voltage, current, and resistance are three properties that are fundamental to almost everything you will do in electrical and electronics engineering. They are intimately related. 

In this article we used water in a river analogy to explain them. This method will help the beginners to imagine and understand these fundamentals much easily.


The problem with electrons is that you cannot see them, so you just have to imagine how they do things. I like to think of electrons as little balls flowing through pipes.

Any physicists reading this will probably be clutching their heads in disgust now. But it works for me.

Each electron has a charge and it’s always the same — lots of electrons, lots of charge, few electrons, and a little bit of charge.

Current, rather like the current in a river, is measured by counting how much charge passes you per second.


A resistor’s job is to provide resistance to the flow of current. So, if we keep thinking about our river, it is like a constriction in a river.

The resistor has reduced the amount of charge that can pass by a point. And it doesn’t matter which point you measure at (A, B, or C) because, if you look upstream of the resistor, the charge is hanging around waiting to move through the resistor. Therefore, less is moving past A per second. In the resistor (B), it’s restricted.

The “speed” analogy does not really hold true for electrons, but one important point is that the current will be the same wherever you measure it.

Imagine what happens when a resistor stops too much current from flowing through an LED.


Voltage is the final part of the equation (that we will come to in a minute). If we persist with the water-in-a-river analogy, then voltage is like the height that the river drops over a given distance.

As everyone knows, a river that loses height quickly flows fast and furious, whereas a relatively gently sloped river will have a correspondingly gentle current.

This analogy helps with the concept of voltage being relative. That is, it does not matter if the river is falling from 10,000 ft to 5,000 ft or from 5,000 ft to 0 ft. The drop is the same and so will be the rate of flow.

Introduction to Gas Insulated Substations (GIS)

Gas insulated substations (GIS) have been used in power systems over the last three decades because of their high reliability, easy maintenance, small ground space requirement etc.

Gas Insulated Substation (GIS) also called SF6 Gas Insulated Metalclad Switchgear is preferred for 12kV, 36kV, 72.5kV, 145 kV, 245 kV, 420 kV and above voltages.

In a GIS substation, the various equipment like Circuit Breakers, Bus bars, Isolators, Load break switches, Current transformers, Voltage transformers, Earthing switches etc. are housed in separate metal enclosed modules filled with SF6 gas. The SF6 gas provides the phase to ground insulation.

As the dielectric strength of SF6 gas is higher than air, the clearances required are smaller. Hence the overall size of each equipment and the complete sub-station is reduced.

The various modules are factory assembled and are filled with SF6 gas. Thereafter, they are taken to site for final assembly.

SF6 Gas Insulated Substations are compact and can be installed conveniently on any floor of a multi storeyed building or in an underground sub-station.  As the units are factory assembled, the installation time is substantially reduced.

Such installations are preferred in composition cities, industrial townships, hydro stations where land is very costly.

Higher cost of SF6 insulated switchgear is justified by saving to reduction in floor-area requirement.

SF6 insulated switchgear is also preferred in heavily polluted areas where dust, chemical fumes and salt layers can cause frequent flashovers in conventional outdoor sub-stations.

The GIS require less number of lightning arresters than a conventional one. This is mainly because of its compactness.

Why SF6 is Used?

SF6 is used in GIS at pressures from 400 to 600 kPa absolute. The pressure is chosen so that the SF6 will not condense into a liquid at the lowest temperatures the equipment experiences. 

SF6 has two to three times the insulating ability of air at the same pressure. SF6 is about 100 times better than air for interrupting arcs.

Inside a Gas Insulated Substation (Video)

Watch the video below to understand how different components are arranged in a GIS substation.

Advantages of Gas Insulated Substations

The following are the main advantages of Gas Insulated Substations over Air Insulated Substations and Hybrid Substations.

Compactness of GIS

The space occupied by SF6 installation is only about 10% of that of a conventional outdoor substation. High cost is partly compensated by saving in cost of space. 

Protection from pollution

The moisture, pollution, dust etc., have little influence on SF6 insulated sub-stations. However, to facilitate installation and maintenance, such substations are generally housed inside a small building. 

The construction of the building need not be very strong like conventional power houses. 

Reduced Switching over voltages

The over voltages while closing and opening line, cables motors capacitors etc. are low. 

Reduced Installation Time

The principle of building-block construction (modular construction) reduces the installation time to a few weeks. Conventional sub-stations require a few months for installation. 

Superior Arc Interruption

SF6 gas is used in the circuit-breaker unit for arc quenching. This type of breaker can interrupt current without overvoltages and with minimum acing time. Contacts have long life and the breaker is maintenance free. 

Gas Pressure

The gas pressure (4 kgf/cm2) is relatively low and does not pose serious leakage problems. 

Increased Safety

As the enclosures are at earth potential, there is no possibility of accidental contact by service personnel to live parts. 

Demerits of GIS

The following are the main disadvantages of Gas Insulated Substations over Air Insulated Substations and Hybrid Substations.

(a) High cost compared to conventional outdoor sub-station. 

(b) Excessive damage in case of internal fault. Long outage periods as repair of damaged part at site may be difficult. 

(c) Requirements of cleanliness are very stringent. Dust or moisture can cause internal flashovers. 

(d) Such sub-stations generally indoor. They need a separate building. This is generally not required for conventional outdoor sub-stations. 

(e) Procurement of gas and supply of gas to site is problematic. Adequate stock of gas must be maintained. 

MCQs on Alternator

1. In an alternator, voltage drops occurs in

2. The magnitude of various voltage drops that occur in an alternator, depends on

3. In an alternator, at lagging power factor, the generated voltage per phase, as compared to that at unity power factor

4. The power factor of an alternator depends on

5. Which kind of rotor is most suitable for turbo alternators which arc designed to run at high speed ?

6. Salient poles are generally used on

7. The frequency of voltage generated in an alternator depends on

8. The frequency of voltage generated by an alternator having 8 poles and rotating at 250 rpm is

9. An alternator is generating power at 210 V per phase while running at 1500 rpm. If the need of the alternator drops to 1000 rpm, the generated voltage per phase will be

10. A 10 pole AC generator rotates at 1200 rpm. The frequency of AC voltage in cycles per second will be

11. The number of electrical degrees passed through in one revolution of a six pole synchronous alternator is

12. Fleming's left hand rule may be applied to an electric generator to find out

13. If the input to the prime mover of an alternator is kept constant but the excitation is changed, then the

14. An alternator is said to be over excited when it is operating at

15. When an alternator is running on no load the power supplied by the prime mover is mainly consumed

MCQs on Synchronous Motors

1. Synchronous motor can operate at

2. An unexcited single phase synchronous motor is

3. The maximum power developed in the synchronous motor will depend on

4. In case the field of a synchronous motor is under excited, the power factor will be

5. A synchronous motor is switched on to supply with its field windings shorted on themselves. It will

6. When the excitation of an unloaded salient pole synchronous motor gets disconnected

7. The damping winding in a synchronous motor is generally used

8. The back emf set up in the stator of a synchronous motor will depend on

9. A synchronous motor is a useful industrial machine on account of which of the following reasons ?
I. It improves the power factor of the complete installation
II. Its speed is constant at all loads, provided mains frequency remains constant
III. It can always be adjusted to operate at unity power factor for optimum efficiency and economy.

10. Which of the following is an unexcited single phase synchronous motor ?

How Multicolor (Red Green Yellow) LEDs Work?

A LED that emits one color when forward biased and another color when reverse biased is called a multicolor LED.

Related Article: Working of Light Emitting Diode (LED)

One commonly used schematic symbol for these LEDs is shown below.

Working of Multicolor LEDs

Multicolor LEDs actually contain two pn junctions that are connected in reverse-parallel i.e. they are in parallel with anode of one being connected to the cathode of the other.

If positive potential is applied to the to terminal as shown below, the pn junction on the left will light.
Note that the device current  passes through the left pn junction.

If the polarity of the voltage source is reversed as shown in figure below, the pn junction on the right will light.

Note that the direction of device current has  reversed and is now passing through the right pn junction.

How Colors are Formed?

Multicolour LEDs are typically red when biased in one direction and green when biased in the other. 

If a multicolour LED is switched fast enough between two polarities, the LED will produce a third colour. 

A red/green LED will produce a yellow light when rapidly switched back and forth between biasing polarities.

Working of Light Emitting Diode (LED)

A light-emitting diode (LED) is a diode that gives off visible light when forward biased.

Light-emitting diodes are not made from silicon or germanium but are made by using elements like gallium, phosphorus and arsenic. By varying the quantities of these elements, it is possible to produce light of different wavelengths with colours that include red, green, yellow and blue.

For example, when a LED is manufactured using gallium arsenide, it will produce a red light. If the LED is made with gallium phosphide, it will produce a green light.

Working Theory of LED

When light-emitting diode (LED) is forward biased as shown in figure below, the electrons
from the n-type material cross the pn junction and recombine with holes in the p-type material. 

We know that these free electrons are in the conduction band and at a higher energy level than the holes in the valence band. 

When recombination takes place, the recombining electrons release energy in the form of heat and light. In germanium and silicon diodes, almost the entire energy is given up in the form of heat and emitted light is insignificant. 

However, in materials like gallium arsenide, the number of photons of light energy is sufficient to produce quite intense visible light.

The schematic symbol for a LED is shown in the above figure. The arrows are shown as pointing away from the diode, indicating that light is being emitted by the device when forward biased. 

Although LEDs are available in several colours (red, green, yellow and orange are the most common), the schematic symbol is the same for all LEDs. There is nothing in the symbol to indicate the colour of a particular LED. 

This is a graph between radiated light and the forward current of the LED. It is clear from the graph that the intensity of radiated light is directly proportional to the forward current of LED.

LED Voltage and Current

The forward voltage ratings of most LEDs is from 1V to 3V and forward current ratings range from 20 mA to 100 mA. 

In order that current  through the LED does not exceed the safe value, a resistor Rs is connected in series with it. The input voltage is Vs and the voltage across LED is Vⅆ.

Core Type and Shell Type Transformers

Depending upon the type of construction used, the transformers are classified into two categories.
  1. Core type Transformer
  2. Shell type Transformer

Core Type Transformer

In core type construction, as shown in figure, the coils are wound around the two limbs of a rectangular magnetic core.

Each limb carries one half of the primary winding and one half of the secondary winding so as to reduce the leakage reactance to the minimum possible. 

The LV (low voltage) winding is wound on the inside nearer to the core while the HV (high  voltage) winding is wound over the LV winding away from the core in order to reduce the amount  of insulation materials required.
Small transformers may have cores of rectangular or square cross section with rectangular or circular coils but it is wasteful in case of large capacity transformers.

In case of large sized transformers stepped cruciform core with circular cylindrical coils is employed, as illustrated in figure below

Stepped cruciform core employs laminations of different sizes.

Though the cost of manufacturing of such a cruciform core is much greater, but the circular coils that are used are easier to wind and provide more mechanical strength, especially when short-circuit occurs.

Other advantages of using cruciform core are,high space factor and reduced mean length of turns resulting in reduced copper loss. 

Shell type Transformer

In shell type construction, the coils are wound on the central limb of a three limb core. The entire flux passes through the central limb and divides into two parts going to side limbs, as shown in figure.

Consequently, the x-sectional area (and hence width) of the central limb is twice of that of each of the side limbs. 

Sandwich type winding is used in such a construction. 

Comparison of Core & Shell Type Transformers

Core Type Shell Type
Easy in design and construction. Comparatively complex
Has low mechanical strength due to non bracing of windings High mechanical strength.
Reduction of leakage reactance is not easily possible. Reduction of leakage reactance is highly possible
The assembly can be easily dismantled for repair work. It cannot be easily dismantled for repair work.
Better heat dissipation from windings. Heat is not easily dissipated from windings since it is surrounded by core.
Has longer mean length of core and shorter mean length of coil turn. Hence best suited for EHV (Extra High Voltage) requirements. It is not suitable for EHV (Extra High Voltage) requirements.

Application of Core & Shell Type Transformers

In shell type transformers advantage is gained through the core being used to protect the windings from mechanical damage. 

The shell type construction gives better support against electromagnetic forces between current carrying conductors. These forces are of considerable magnitude under short-circuit conditions. 

The shell type construction is commonly used for small transformers where a square or rectangular core cross-section is suitable for economic consideration. 

The shell type construction needs more specialized fabrication facilities than core type,while the latter offers an additional advantage of permitting visual inspection of coils in the-case of a fault and ease of repair it substation site. 

For these reasons, the present practice is to use core type transformers in large high voltage installations.

Why Core Type in HV and Shell Type in LV?

Core type transformers are popular in High voltage applications like Distribution transformers, Power transformers, and obviously auto transformers. 

Reasons are, High voltage corresponds to high flux. So, for keeping iron loss down we have to use thicker core. So core type is better choice. At high voltage we require heavy insulation. In core type winding, putting insulation is easier. In fact LV(low voltage) winding it self acts as an insulation between HV (high voltage) winding and core.

Where as, Shell type transformers are popular in Low voltage applications like transformers used in electronic circuits and power electronic converters etc. 

Reasons are, at low voltage, comparatively you require more volume for the copper wires than that of iron core. So the windows cut on the laminated sheets have to be of bigger proportion with respect to the whole size of the transformer. So, shell type is a better choice.

In shell type we don't care about the insulation much and insulation is thin and light. So we can put the winding anyway we want in the shell.

Low Voltage Switchgear

As per IEC 60947, switchgears with rated voltages up to 1000 V ac and 1500 V dc are termed as low voltage (LV) switchgear.

The term ‘switchgear’ is a generic term encompassing a wide range of products like circuit breakers, switches,  switch fuse units, off-load isolators, HRC fuses, contactors, earth leakage circuit breakers (ELCBS), miniature circuit breakers (MCBS), and moulded case circuit breakers (MCCBS), among others.

A commonly followed network combination in LV distribution boards in shown in the figure below.

It is a combination of power control centres and motor control centres or load distribution boards including lighting distribution boards.

The incomer/sub-incomer/ distribution network generally depends on the capacity of the source and the distribution of load centres.
Conventional Low Voltage Distribution Network
The characteristics and feature of load controlling and protecting devices vary on the basis of the locations. The system being most commonly followed currently is detailed below.

Conventional Incomer

The devices used in the incomer should be capable of:

1. Switching and carrying normal currents (generally above 1200 A);

2. Withstanding abnormal currents for a short duration in order to allow downstream devices  to operate;

3. Interrupting the maximum value of the fault current generated in the system;

4. Ensuring the safety to the operating personnel;

5. Inter-locking with downstream equipment; and

6. Facilitating easy maintenance.

ln the past, oil circuit breakers (OCBS), re-wireable fused isolators and air circuit breakers (ACBs) were the commonly used devices.

However, ACB has been acknowledged as an idea device for incomer in terms of the safety, reliability and maintenance needs of the system.

This is mainly due to its various characteristics like quick-make, quick-break stored energy type reliable mechanism, safety interlocks/ indications, ease of maintenance and its ability to withstand fault currents for a specified duration (1-3 secs) thereby allowing the feeder device to isolate the faulty branch of the network and ensuring reliable supply to healthy areas.

Conventional Sub-Incomer

The devices installed as parts of a sub-incomer should have the following characteristics:

  • Ability to achieve economy without sacrificing protection and safety
  • Capability to withstand abnormal currents; and
  • Need for relatively less number of inter-locking indicating accessories since it covers a limited area of network.

ACB’s and switch fuse units (SFUS) are, to a large extent, being used as sub-incomers along with modern devices like moulded case circuit breakers (MCCBs).

Conventional Feeder Protection

Feeder protection covers all load centres like motor control centres, lighting switchboards and industrial load centres. 

The choice of feeder protection device based on the different conventional feeder load centres are discussed below.

 1. Motor Control

The motor feeder needs to be protected against the following eventualities in addition to normal switching control:

  • Short-circuit;
  • Over-currents up to locked rotor condition; and
  • Single-phasing.

Requirements for Motor Control

Switching normal currentsContactors
Single-phasing sensingBi-metallic Relay
Over—current (up to locked rotor condition) sensingBi-metallic Relay
Switching over-currents including single-phasing current as aboveContactors
Short-circuit currentMCCB/SFU
LogicTimers/Auxiliary ; Contactors and other Accessories

2. Other Industrial Load Control

Loads like oven, pre-treatment and electroplating baths fall under this category and the feeders need to be protected against faulty over-currents. 

Presently MCCBS and SFUS are being commonly used for this purpose.

3. Lighting/Domestic Load Control

The requirements of domestic load control are similar to those listed in other industrial load control with the addition of earth leakage current protection in order to reduce any damages to life and property that could be caused by harmful leakages of electric current and fire.

In a low voltage power distribution system, electrical appliances are protected against damages from over-loads or short-circuits by fuses or circuit breakers. 

However, the human operator is not adequately protected when a fault occurs within the appliance itself. Hence the need for fast acting ELCBS operating on low leakage currents arises.

The device, which detects leakage current as low as 100 mA and is capable of disconnecting
equipment in less than 100 msec is called an earth leakage circuit breaker (ELCB). 

The following two types of ELCBS are used depending upon the parameters to be detected:

1. Voltage-operated ELCB; and
2. Current-operated ELCB.

Arduino DC Digital Voltmeter

A voltmeter is an instrument used for measuring electrical potential difference between two points in an electric circuit. Analog voltmeters move a pointer across a scale in proportion to the voltage of the circuit; digital voltmeters give a numerical display of voltage by use of an analog to digital converter. 

We are using internal ADC of Arduino to make Digital Voltmeter capable to display 0 to 5V. You can increase its input voltage capacity by using voltage divider circuit.

What you will learn?

1. How to connect 7-Segment Display to Arduino?
2. How to read analog input?
3. Use of shift register to reduce IOs.
4. Measurement of DC voltage using Arduino.

Components Required

1. Arduino Uno
2. 4 digit 7-segment Display Common Cathode.
3. Variable Resistor.
4. 1K Resistors
5. 75HC595

Digital Voltmeter Circuit

Main components of object counter circuit are 
  • 4 digit 7 segment display, 
  • Arduino Uno, 
  • 74HC595.
Digital Voltmeter Circuit Diagram
4 digit 7 segment display

Digital Voltmeter Arduino Code

Program is constructed using “TimerOne” library. Program have different modules, Setup, Loop. 
In setup we initialize all the IO connections and Timer, Display.
In main loop we are taking Analog input and constantly updating display to show voltage value.

#include <TimerOne.h>
//Define 74HC595 Connections with arduio
const int Data =7;
const int Clock=8;
const int Latch=6;
const int SEG0 =5;
const int SEG1 =4;
const int SEG2 =3;
const int SEG3 =2;
int cc =0;
char Value [4];
const char SegData []={0x3F,0x06,0x5B,0x4F,0x66,0x6D,0x7D,0x07,0x7F,0x6F};
// Setup
void setup () {
// initialize the digital pin as an output.
Serial . begin(9600);
pinMode ( Data , OUTPUT );
pinMode ( Clock, OUTPUT );
pinMode ( Latch, OUTPUT );
pinMode ( SEG0 , OUTPUT );
pinMode ( SEG1 , OUTPUT );
pinMode ( SEG2 , OUTPUT );
pinMode ( SEG3 , OUTPUT );
//Initialize Display Scanner
 cc =0;
Timer1 . initialize (10000); // set a timer of length 100000 microseconds (or 0.1 sec - or 10Hz
=> the led will blink 5 times, 5 cycles of on-and-off, per second)
Timer1 . attachInterrupt ( timerIsr ); // attach the service routine here
// Loop
void loop () {
char Volt [4];
int Voltage = analogRead ( A0 );
//To get fixed point decimal point we multiply it by 100
Voltage = (500/1024.0) * Voltage ; //Scaling of 0 to 5V i.e. 0 to 1023 to 0 to 10 (in 10 steps)
//Display Voltage on Segments
sprintf ( Volt ,"%04d", Voltage ); //We get ASCII array in Volt
Serial . println( Volt );
Value [0]= Volt [0] & 0x0F; //Anding with 0x0F to remove upper nibble
Value [1]= Volt [1] & 0x0F; //Ex. number 2 in ASCII is 0x32 we want only 2
Value [2]= Volt [2] & 0x0F;
Value [3]= Volt [3] & 0x0F;
// Generates Bargraph
void DisplayDigit (char d )
int i ;
for( i =0; i <8; i ++) //Shift bit by bit data in shift register
if(( d & 0x80)==0x80)
digitalWrite ( Data , HIGH );
digitalWrite ( Data , LOW );
d = d <<1;
 //Give Clock pulse
digitalWrite ( Clock, LOW );
digitalWrite ( Clock, HIGH );
//Latch the data
digitalWrite ( Latch, LOW );
digitalWrite ( Latch, HIGH );
void timerIsr ()
cc ++;
if( cc ==5) //We have only 4 digits
{ cc =1;}
Scanner ();
TCNT0 =0xCC;
void Scanner ()
switch ( cc ) //Depending on which digit is selcted give output
case 1:
digitalWrite ( SEG3 , HIGH );
DisplayDigit ( SegData [ Value [0]]);
digitalWrite ( SEG0 , LOW );
case 2:
digitalWrite ( SEG0 , HIGH );
DisplayDigit ( SegData [ Value [1]] | 0x80); //0x80 to turn on decimal point
digitalWrite ( SEG1 , LOW );
case 3:
digitalWrite ( SEG1 , HIGH );
DisplayDigit ( SegData [ Value [2]]);
digitalWrite ( SEG2 , LOW );
case 4:
 digitalWrite ( SEG2 , HIGH );
DisplayDigit ( SegData [ Value [3]]);
digitalWrite ( SEG3 , LOW );

What is a FUSE and How it Works?

A fuse protects an electrical circuit or device from excessive current when a metal element inside it melts to create an open circuit. With the excep­tion of resettable fuses, a fuse must be dis­carded and replaced after it has fulfilled its func­tion.

When high current melts a fuse, it is said to blow or trip the fuse. (In the case of a resettable fuse, only the word trip is used.) 

A fuse can work with either AC or DC voltage, and can be designed for almost any current. In residential and commercial buildings, circuit break­ers have become common, but a large cartridge fuse may still be used to protect the whole sys­tem from short-circuits or from overcurrent caused by lightning strikes on exposed power lines.

In electronic devices, the power supply is al­most always fused.

Schematic symbols for a fuse are shown in figure. Those at the right and second from right are most frequently used. The one in the center is approved by ANSI, IEC, and IEEE but is seldom seen. To the left of that is the fuse symbol understood by electrical contractors in architec­tural plans. The symbol at far left used to be com­mon but has fallen into disuse.

How a FUSE Works?

The element in a fuse is usually a wire or thin metal strip mounted between two terminals. In a cartridge fuse, it is enclosed in a glass or ceramic cylinder with a contact at each end, or in a small metallic can. (Old-style, large, high-amperage fuses may be packaged in a paper or cardboard tube.) The traditional glass cartridge allows vis­ual inspection to confirm that the fuse has blown.

A fuse responds only to current, not to voltage. When choosing a fuse that will be reliable in conditions of steady current consumption, a safe rule is to figure the maximum amperage when all components are functioning and add 50%. 

How­ever, if current surges or spikes are likely, their duration will be relevant. If I is the current surge in amps and t is its duration in seconds, the surge sensitivity of a fuse—which is often referred to verbally or in printed format as I2t—is given by the formula:

I2t = I² * t

Some semiconductors also have an I2t rating, and should be protected with a similarly rated fuse.

Any fuse will present some resistance to the cur­rent flowing through it. Otherwise, the current would not generate the heat that blows the fuse. Manufacturer datasheets list the voltage drop that the internal resistance of a fuse is likely to introduce into a circuit.

Types of Rechargeable Batteries

Rechargeable batteries are everywhere these days: cordless tools, laptop computers, cordless phones, and cell phones, just to name a few.

Rechargeable batteries for use with consumer electronic products are of four basic types: 

  1. Nickel cadmium (Ni-Cd), 
  2. Nickel metal hydride (Ni-MH) 
  3. Lithium ion (Li-Ion). 
  4. Lithium polymer (Li-Po)

Although these four types of batteries will not look much different from the outside, there are significant differences among them. We will explain a bit about each of them now.

Ni-Cd Batteries

Ni-Cd Batteries have been around the longest of these three types. There are many Ni-Cd batteries out there in use today. They have a good capacity and hold a very stable voltage between charges as they are being discharged.

The major Disadvantage of Ni-Cd batteries is the memory effect ( The decreasing useful time between charges for a rechargeable battery is due to what is called the memory effect )

Also, Ni-Cd batteries can only undergo a limited number of discharge-recharge cycles before they need to be replaced. They often last only one to two years.

Ni-MH (Nickel-metal hydride) Batteries

Ni-MH (Nickel-metal hydride) Batteries are a more recent development in rechargeable batteries.

They have many of the same advantages that the Ni-Cad batteries have. However they suffer much less from the memory effect than Ni-Cd batteries. There is some memory effect with Ni-MH batteries, but not nearly as much as with Ni-Cd. 

Also they can go through more discharge-recharge cycles than Ni-Cd batteries. Their typical useful life is more like 3 to 4 years. On the down side, Ni-MH batteries discharge more when not in use than Ni-Cds. 

After about a week of not being used, a fully charged Ni-MH battery will have lost about 20% of its charge. Also Ni-MH batteries cost more than Ni-Cd batteries, but their longer life tends to more than compensate for that.

Li-Ion (Lithium Ion) batteries

Li-Ion (Lithium Ion) batteries are the next newest type rechargeable battery to be commercially available.

They have all the advantages mentioned above for Ni-Cd and Ni-MH batteries and have a longer useful life than either of them. They do not suffer at all from the memory effect that is a problem for Ni-Cad and to a lesser extent for Ni-MH batteries. 

The main problem with Li-Ion batteries is that they lose about 10% of their useful capacity each year of use. This loss is due to chemical breakdown in the cells and currently there is no way to prevent or reverse this.

Li-Ion batteries typically last through about 300 to 500 discharge cycles, or about four to five years.

Li-Po (Lithium Polymer) Batteries

The next evolution of rechargeable battery technology is the Lithium Polymer Battery. 

Lithium Polymer or LiPo batteries are a great new way of storing energy for portable devices from cell phones, home electronics to RC hobby battery packs used in cars, boats and flight. 

They're great because they can store 350% (approximately) more energy than a typical NiCd/NiHm battery pack and weigh 10% - 20% less. They can also discharge much more current than a NiCd/NiHm battery and can be fully charged in about an hour. 

LiPo batteries also don't develop memory or voltage depression characteristics like NiCd/NiHm batteries, and do not need to be discharged before being charged.

But they're not without their downside. Mishandling of these batteries can lead to fire, explosions and toxic smoke inhalation.

Classification of Analog Instruments

An analog device is one in which the output or display is a continuous function of time and bears a constant relation to its input.

The analog instruments find extensive use in present day applications although digital instruments are increasing in number and applications. The areas of application which are common to both analog and digital instruments are fairly limited at present. 

Hence, it can safely be predicted that the analog instruments will remain in extensive use for a number of years and are not likely to be completely replaced by digital instruments for certain applications.

Classification of Analog Instruments

Broadly, the analog instruments (and for that matter digital instruments) may be classified according to the quantity they measure.  For example an instrument meant for measurement of current is classified as an Ammeter while an instrument that measures voltage is classified as a Voltmeter. Thus we have wattmeters, power factor meters, frequency meters etc...

Electrical instruments may also he classified according to the kind of current that can be measured by them. Electrical instruments may be classified as instruments for:

  1. Direct Current (DC)
  2. Alternating Current (AC)
  3. Direct and Alternating Curent (DC/AC)

Instruments depend for their operation on one of the many effects produced by current and voltage and thus can be classified according to which of the effects is used for their working.
The analog instruments are also classified as : 
  • (a) Indicating 
  • (b) Recording, 
  • (c) Integrating.

Indicating Instruments

Indicating Instruments are those instruments which indicate the magnitude of a quantity being measured. They generally make use of a dial and a pointer for this purpose. Ordinary voltmeters, ammeters and wattmeters belong to this category.

The analog indicating instruments may be divided into two groups :
  • (i) electromechanical instruments, 
  • (ii) electronic instruments. ‘
Electronic instruments are constructed by addition of electronic circuits to electromagnetic indicators in order to increase the sensitivity and input impedance.

Recording Instruments

Recording Instruments give a continuous record of the quantity being measured over a specified period. 

The variations of the quantity being measured are recorded by a pen (attached to the moving system of the instrument ; the moving system is operated by the quantity being measured) on a sheet of paper carried by it rotating drum. 

For example, we may have a recording voltmeter in a sub-station which keeps record of the variations of supply voltage during the day.

Integrating Instruments

Integrating Instruments totalize events over a specified period of time. The summation, which they give is the product of time and an electrical quantity. 

Ampere hour and watt hour (energy) meters are examples of this category. 

The integration (summation value) is generally given by a register consisting of a set of at pointers and dials.

The analog instruments may also be classified on the basis of method used for comparing the unknown quantity (measurand) with the unit of measurement. The two categories of instruments based upon this classification are:
  1. Direct Measuring Instruments
  2. Comparison Instruments

Direct Measuring Instruments

These instruments convert the energy of the measurand directly into energy that actuates the instrument and the value of the unknown quantity is measured or displayed or recorded directly. 

The examples of this class of instruments are ammeters, voltmeters, wattmeters and energy metres.

Comparison Instruments

These instruments measure the unknown quantity by comparison with a standard. 

Direct measuring instruments are the most commonly used in engineering practice because they are the most simple and inexpensive. Also their use makes the measurement possible in the shortest time

The examples of comparison type instruments are DC and AC bridges.
Comparison instruments are used in cases where a higher accuracy of measurement is required.

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