What is Pointer?

What is Pointer?

In computer science, a pointer is a programming language object, whose value refers to (or “points to”) another value stored elsewhere in the computer memory using its address. A pointer references a location in memory, and obtaining the value stored at that location is known as dereferencing the pointer.

As an analogy, a page number in a book’s index could be considered a pointer to the corresponding page; dereferencing such a pointer would be done by flipping to the page with the given page number.

The term “Pointer” can also be defined as

  1. A variable does not store a value but store the address of the memory space which contains the value.
  2. A variable that contains the address of a location in memory. The location is the starting point of an allocated object, such as an object or value type, or the element of an array.
  3. A value that designates the address (i.e., the location in memory), of some value.
  4. Variables that hold a memory location.
  5. A memory address.

In general, Pointer is a long thin piece of metal on a scale or dial that moves to indicate a figure or position.

What is Logical Block Addressing (LBA)?

Logical Block Address (LBA)

Logical block addressing is a technique that allows a computer to address a hard disk larger than 528 megabytes. A Logical Block Address (LBA) is a 28-bit value that maps to a specific cylinder-head-sector address on the disk. 28 bits allows sufficient variation to specify addresses on a hard disk up to 8.4 gigabytes in data storage capacity.

The term “Logical block addressing” can also be defined as

  1. An address that defines where data is stored on the hard drive.
  2. A common scheme used for specifying the location of blocks of data stored on computer storage devices.
  3. A run-time function of the system BIOS. The BIOS uses LBA for the following commands: read (with and without retries), read verify, read long, write (with and without retries), write verify, write long, read multiple, write multiple, read DMA, write DMA, seek, and format track.

What is Storage?

storage-device-computer

 

In a computer, storage is the place where data is held in an electromagnetic or optical form for access by a computer processor. There are two general usages.

  1. Storage is frequently used to mean the devices and data connected to the computer through input/output operations – that is, hard disk and tape systems and other forms of storage that don’t include computer memory and other in-computer storage. For the enterprise, the options for this kind of storage are of much greater variety and expense than that related to memory. This meaning is probably more common in the IT industry than meaning 2 (the following).
  2. In a more formal usage, storage has been divided into:
    1. Primary storage, which holds data in memory (sometimes called random access memory or RAM) and other “built-in” devices such as the processor’s L1 cache, and
    2. Ssecondary storage, which holds data on hard disks, tapes, and other devices requiring input/output operations.

Primary storage is much faster to access than secondary storage because of the proximity of the storage to the processor or because of the nature of the storage devices. On the other hand, secondary storage can hold much more data than primary storage.

In addition to RAM, primary storage includes read-only memory (ROM) and L1 and L2 cache memory. In addition to hard disks, secondary storage includes a range of device types and technologies, including diskettes, Zip drives, redundant array of independent disks (RAID) systems, and holographic storage. Devices that hold storage are collectively known as storage media.

A somewhat antiquated term for primary storage is main storage and a somewhat antiquated term for secondary storage is auxiliary storage. Note that, to add to the confusion, there is an additional meaning for primary storage that distinguishes actively used storage from backup storage.

Interview Question : What is Hard Disk?

what-is-hard-disk-drive-hdd

A hard disk is part of a unit, often called a “disk drive,” “hard drive,” or “hard disk drive (HDD),” that stores and provides relatively quick access to large amounts of data on an electromagnetically charged surface or set of surfaces. Today’s computers typically come with a hard disk that contains several billion bytes (gigabytes) of storage.

A Hard disk can also be defined as:

  1. a rigid (“hard”) non-removable magnetic disk with a large data storage capacity.
  2. a data storage device used for storing and retrieving digital information using one or more rigid (“hard”) rapidly rotating disks (platters) coated with magnetic material.
  3. A magnetic disk on which you can store computer data. The term hard is used to distinguish it from a soft, or floppy disk. Hard disks hold more data and are faster than floppy disks.

Extra Information

A hard disk is really a set of stacked “disks,” each of which, like phonograph records, has data recorded electromagnetically in concentric circles or “tracks” on the disk. A “head” (something like a phonograph arm but in a relatively fixed position) records (writes) or reads the information on the tracks. Two heads, one on each side of a disk, read or write the data as the disk spins. Each read or write operation requires that data be located, which is an operation called a “seek.” (Data already in a disk cache, however, will be located more quickly.)

A hard disk/drive unit comes with a set rotation speed varying from 4500 to 7200 rpm. Disk access time is measured in milliseconds. Although the physical location can be identified with cylinder, track, and sector locations, these are actually mapped to a logical block address (LBA) that works with the larger address range on today’s hard disks.

To know more regarding the terms follow the post about Difference between Disc and Disk click here.

 

Porsche’s New Electric Concept Car Could Give Tesla Trouble

 

[dropcap]Porsche[/dropcap] might not be saying it directly, but anyone taking even a quick glance can see the Germans are taking aim at Tesla with the freshly unveiled Mission E concept at the Frankfurt Motor Show. With over 590 horsepower, this electric sedan doesn’t have quite as much electric grunt as a Model S, but the claimed 310.7 miles of range would outdo even the latest take on Elon Musk’s four-door with a 90 kWh battery. For those keeping track, a Ludicrous Speed-equipped Tesla might still be the ultimate victor in a drag race because the E’s sprint to 62 takes “under 3.5 seconds.”

To power the Mission E, Porsche’s engineers use two permanent magnet synchronous motors, and they are similar to what’s found on the company’s 919 Hybrid LMP1 racecar. In a single unit, the devices can both accelerate and recover braking energy. All-wheel drive with torque vectoring and four-wheel steering help the electric sedan lap the Nordschleife in less than eight minutes, the company claims.

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Porsche-Missiohn-E-front-view

Drive System:

The drive system of the Mission E is entirely new, yet it is typical Porsche, i.e. proven in motor racing. Two permanent magnet synchronous motors (PMSM) – similar to those used in this year’s Le Mans victor, the 919 hybrid – accelerate the sports car and recover braking energy.

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Porsche-Missiohn-E-rear

Driving Range:

It is not just passionate sportiness that makes up a Porsche but also a high level of everyday practicality. Accordingly, the Mission E can travel over 500 km on one battery charge, and it can be charged with enough energy for around 400 km more driving range in about fifteen minutes.

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Porsche-Missiohn-E-at-frankfurt-2015

A moveable body segment on the front left wing in front of the driver’s door gives access to the charging port for the innovative “Porsche Turbo Charging” system. Via the 800-volt port, the battery can be charged to approximately 80 per cent of its capacity in around 15 minutes – a record time for electric vehicles

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Porsche-Missiohn-E-concept-frankfurt-1

Design:

A new type of matrix LED headlights in the brand’s typical four-point light design captures the viewer’s gaze. Integrated as an element hovering in the airflow of the air inlet, they lend a futuristic character to the front end.
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Porsche-Missiohn-E-dashboard

Display and Control Concept:

A new world based on an innovative display and control concept opens up before the driver. It is intuitive, fast and free of distractions – created for the sports car of tomorrow. The filigree driver’s display is curved, low-profile and free-standing. The instrument cluster shows five round instruments – they can be recognized as Porsche, but they are displayed virtually in OLED technology

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Porsche-Missiohn-E-foot-rest

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Porsche-Missiohn-E-interior

Holographic display:

The entire dashboard is chock full of new ideas. Its division into two three-dimensionally structuring layers reinforces the impression of lightness and clarity. The upper layer integrates the driver’s display, and between the levels there is a holographic display that extends far into the passenger’s side. It shows individually selectable apps, which are stacked in virtual space and arranged by priority with a three-dimensional effect.

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Porsche-Missiohn-E-interior-1

The driver – or passenger – can use these apps to touch-free control primary functions such as media, navigation, climate control, contacts and vehicle. The desired symbol is activated by gestures that are detected by sensors. A grasping gesture means select, while pulling means control. Moreover, driver or passenger can use a touch display on the centre console to control secondary functions such as detailed information menus.

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Porsche-Missiohn-E-side

Driving dynamics:

The battery mounted in the car’s underbody, which is based on the latest lithium-ion technology, runs the whole length between the front and rear axles. This distributes its weight to the two drive axles uniformly, resulting in exceptionally good balance. 

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Porsche-Missiohn-E-tail-light

The body as a whole is made up of a functional mix of aluminium, steel and carbon fibre reinforced polymer. The wheels are made of carbon: the Mission E has wide tyres mounted on 21-inch wheels in front and 22-inch wheels at the rear.

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Capable of hitting 124 mph in less than 12 seconds, the Mission E is undoubtedly quick, but its speed in charging is especially impressive too. The concept utilizes an 800-volt system that Porsche claims can charge the sedan to 80 percent in just 15 minutes, assuming you can find a source for that much energy. That would be enough to offer about 249 miles of range in less time than getting a cup of coffee at a busy Starbucks. The E could also use a traditional 400-volt charger or power up inductively.

The Mission E’s design looks like a futuristic Panamera as filtered through the style of the 918 and a next-gen Star Wars Stormtrooper helmet. In an especially cool touch, the charging port is hidden in a movable panel ahead of the driver’s door on the front fender. Also, like like lots of four-door concepts in recent memory, the design sports suicide doors with no B-pillars. To keep weight to a minimum, the body mixes aluminum, steel, and carbon-fiber reinforced polymer. Also present are carbon-fiber wheels. Inside, drivers get instruments that track their eyes so that the gauges always remain visible. There’s a video below, but stay tuned for a closer look at the Mission E from the show floor in Frankfurt.

Easy Lesson On : Active FTP vs Passive FTP

Contents:

 

Introduction

One of the most commonly seen questions when dealing with firewalls and other Internet connectivity issues is the difference between active and passive FTP and how best to support either or both of them. Hopefully the following text will help to clear up some of the confusion over how to support FTP in a firewalled environment.

 

The Basics

FTP is a TCP based service exclusively. There is no UDP component to FTP. FTP is an unusual service in that it utilizes two ports, a ‘data’ port and a ‘command’ port (also known as the control port). Traditionally these are port 21 for the command port and port 20 for the data port. The confusion begins however, when we find that depending on the mode, the data port is not always on port 20.

 

Active FTP

In active mode FTP the client connects from a random unprivileged port (N > 1023) to the FTP server’s command port, port 21. Then, the client starts listening to port N+1 and sends the FTP command PORT N+1 to the FTP server. The server will then connect back to the client’s specified data port from its local data port, which is port 20.

From the server-side firewall’s standpoint, to support active mode FTP the following communication channels need to be opened:

  • FTP server’s port 21 from anywhere (Client initiates connection)
  • FTP server’s port 21 to ports > 1023 (Server responds to client’s control port)
  • FTP server’s port 20 to ports > 1023 (Server initiates data connection to client’s data port)
  • FTP server’s port 20 from ports > 1023 (Client sends ACKs to server’s data port)

When drawn out, the connection appears as follows:active-ftp-winged-post

In step 1, the client’s command port contacts the server’s command port and sends the command PORT 1027. The server then sends an ACK back to the client’s command port in step 2. In step 3 the server initiates a connection on its local data port to the data port the client specified earlier. Finally, the client sends an ACK back as shown in step 4.

The main problem with active mode FTP actually falls on the client side. The FTP client doesn’t make the actual connection to the data port of the server–it simply tells the server what port it is listening on and the server connects back to the specified port on the client. From the client side firewall this appears to be an outside system initiating a connection to an internal client–something that is usually blocked.

 

Active FTP Example

Below is an actual example of an active FTP session. The only things that have been changed are the server names, IP addresses, and user names. In this example an FTP session is initiated from area1.wingedpost.com (192.168.150.80), a linux box running the standard FTP command line client, to area2.wingedpost.com (192.168.150.90), a linux box running ProFTPd 1.2.2RC2. The debugging (-d) flag is used with the FTP client to show what is going on behind the scenes. Everything in red is the debugging output which shows the actual FTP commands being sent to the server and the responses generated from those commands. Normal server output is shown in black, and user input is in bold.

There are a few interesting things to consider about this dialog. Notice that when the PORT command is issued, it specifies a port on the client (192.168.150.80) system, rather than the server. We will see the opposite behavior when we use passive FTP. While we are on the subject, a quick note about the format of the PORT command. As you can see in the example below it is formatted as a series of six numbers separated by commas. The first four octets are the IP address while the last two octets comprise the port that will be used for the data connection. To find the actual port multiply the fifth octet by 256 and then add the sixth octet to the total. Thus in the example below the port number is ( (14*256) + 178), or 3762. A quick check with netstat should confirm this information.

area1: {/home/p-t/wingedpost/public_html} % ftp -d area2
Connected to area2.wingedpost.com.
220 area2.wingedpost.com FTP server ready.
Name (area2:wingedpost): wingedpostuser
---> USER wingedpostuser
331 Password required for wingedpostuser.
Password: TmpPass
---> PASS XXXX
230 User wingedpostuser logged in.
---> SYST
215 UNIX Type: L8
Remote system type is UNIX.
Using binary mode to transfer files.
ftp> ls
ftp: setsockopt (ignored): Permission denied
---> PORT 192,168,150,80,14,178
200 PORT command successful.
---> LIST
150 Opening ASCII mode data connection for file list.
drwx------   3 wingedpostuser    users         104 Jul 27 01:45 public_html
226 Transfer complete.
ftp> quit
---> QUIT
221 Goodbye.

 

Passive FTP

In order to resolve the issue of the server initiating the connection to the client a different method for FTP connections was developed. This was known as passive mode, or PASV, after the command used by the client to tell the server it is in passive mode.

In passive mode FTP the client initiates both connections to the server, solving the problem of firewalls filtering the incoming data port connection to the client from the server. When opening an FTP connection, the client opens two random unprivileged ports locally (N > 1023 and N+1). The first port contacts the server on port 21, but instead of then issuing a PORT command and allowing the server to connect back to its data port, the client will issue the PASV command. The result of this is that the server then opens a random unprivileged port (P > 1023) and sends P back to the client in response to the PASV command. The client then initiates the connection from port N+1 to port P on the server to transfer data.

From the server-side firewall’s standpoint, to support passive mode FTP the following communication channels need to be opened:

  • FTP server’s port 21 from anywhere (Client initiates connection)
  • FTP server’s port 21 to ports > 1023 (Server responds to client’s control port)
  • FTP server’s ports > 1023 from anywhere (Client initiates data connection to random port specified by server)
  • FTP server’s ports > 1023 to remote ports > 1023 (Server sends ACKs (and data) to client’s data port)

When drawn, a passive mode FTP connection looks like this:passive-ftp-wingedpost

In step 1, the client contacts the server on the command port and issues the PASV command. The server then replies in step 2 with PORT 2024, telling the client which port it is listening to for the data connection. In step 3 the client then initiates the data connection from its data port to the specified server data port. Finally, the server sends back an ACK in step 4 to the client’s data port.

While passive mode FTP solves many of the problems from the client side, it opens up a whole range of problems on the server side. The biggest issue is the need to allow any remote connection to high numbered ports on the server. Fortunately, many FTP daemons, including the popular WU-FTPD allow the administrator to specify a range of ports which the FTP server will use.

The second issue involves supporting and troubleshooting clients which do (or do not) support passive mode. As an example, the command line FTP utility provided with Solaris does not support passive mode, necessitating a third-party FTP client, such as ncftp. 
NOTE: This is no longer the case–use the -p option with the Solaris FTP client to enable passive mode!

With the massive popularity of the World Wide Web, many people prefer to use their web browser as an FTP client. Most browsers only support passive mode when accessing ftp:// URLs. This can either be good or bad depending on what the servers and firewalls are configured to support.

 

Passive FTP Example

Below is an actual example of a passive FTP session. The only things that have been changed are the server names, IP addresses, and user names. In this example an FTP session is initiated from area1.wingedpost.com (192.168.150.80), a linux box running the standard FTP command line client, to area2.wingedpost.com (192.168.150.90), a linux box running ProFTPd 1.2.2RC2. The debugging (-d)  flag is used with the FTP client to show what is going on behind the scenes. Everything in red is the debugging output which shows the actual FTP commands being sent to the server and the responses generated from those commands. Normal server output is shown in black, and user input is in bold.

Notice the difference in the PORT command in this example as opposed to the active FTP example. Here, we see a port being opened on the server (192.168.150.90) system, rather than the client.

area1: {/home/p-t/wingedpost/public_html} % ftp -d area2
Connected to area2.wingedpost.com.
220 area2.wingedpost.com FTP server ready.
Name (area2:wingedpost): wingedpostuser
---> USER wingedpostuser
331 Password required for wingedpostuser.
Password: TmpPass
---> PASS XXXX
230 User wingedpostuser logged in.
---> SYST
215 UNIX Type: L8
Remote system type is UNIX.
Using binary mode to transfer files.
ftp> passive
Passive mode on.
ftp> ls
ftp: setsockopt (ignored): Permission denied
---> PASV
227 Entering Passive Mode (192,168,150,90,195,149).
---> LIST
150 Opening ASCII mode data connection for file list
drwx------   3 wingedpostuser    users         104 Jul 27 01:45 public_html
226 Transfer complete.
ftp> quit
---> QUIT
221 Goodbye.

 

Summary

The following chart should help admins remember how each FTP mode works:

 Active FTP :
     command : client >1023 -> server 21
     data    : client >1023 <- server 20

 Passive FTP :
     command : client >1023 -> server 21
     data    : client >1024 -> server >1023

A quick summary of the pros and cons of active vs. passive FTP is also in order:

Active FTP is beneficial to the FTP server admin, but detrimental to the client side admin. The FTP server attempts to make connections to random high ports on the client, which would almost certainly be blocked by a firewall on the client side. Passive FTP is beneficial to the client, but detrimental to the FTP server admin. The client will make both connections to the server, but one of them will be to a random high port, which would almost certainly be blocked by a firewall on the server side.

Luckily, there is somewhat of a compromise. Since admins running FTP servers will need to make their servers accessible to the greatest number of clients, they will almost certainly need to support passive FTP. The exposure of high level ports on the server can be minimized by specifying a limited port range for the FTP server to use. Thus, everything except for this range of ports can be firewalled on the server side. While this doesn’t eliminate all risk to the server, it decreases it tremendously.

References

An excellent reference on how various internet protocols work and the issues involved in firewalling them can be found in the O’Reilly and Associates book, Building Internet Firewalls, 2nd Ed, by Brent Chapman and Elizabeth Zwicky. 
Note : This book is VERY old and the information contained therein may be outdated!

Finally, the definitive reference on FTP would be RFC 959, which sets forth the official specifications of the FTP protocol. RFCs can be downloaded from numerous locations, including http://www.faqs.org/rfcs/rfc959.html.

What Is Electrical Conductivity?

Power lines are made of conducting materials.
Power lines are made of conducting materials.

Electrical conductivity (EC) is a property that is used to describe how well materials allow electrons to flow. It is determined using experiments and math equations. Conductivity is the reciprocal of resistivity, meaning the higher the conductivity, the lower the resistivity. A conductor is a material that has a high electrical conductivity, and an insulator is a material that has a high electrical resistivity. Both properties depend on the temperature and purity of materials.

Temperature dependence of electrical conductivity follows a general pattern. Metal is a conductor, and it has lower conductivity at higher temperatures. Glass is an insulator and shows higher conductivity at higher temperatures.

At very high temperatures, conductors behave like insulators, and insulators behave like conductors. This behavior of insulators and conductors is explained by the free electron model. In this model, conductors clearly show the ability to free electrons, and when a current, or electric force, is applied, the force can easily push around the extra electrons.

Soil is a mixture of minerals, salts and organic materials. It has a special electrical conductivity called soil electrical conductivity, which measures the amount of salt that is in a soil sample, which is called its salinity. The process can also measure other soil properties where salinity is low enough. These properties are related to the influence that purity has on EC data.

EC data of a soil sample can determine how much impurity is in the soil. Soil impurities are water, air and minerals. Each impurity influences the data differently, but a practiced soil scientist can determine this information from the gathered data. In general, more impurities lower the EC, with the exception of minerals that increase EC. Impurities also can explain the use of pure copper in electrical wiring.

Metals are often made of alloys, a mixture of two or more elements. This is not useful for conducting electricity. The metals in alloys are not the same elements and electrons cannot flow easily between different elements. Pure metals, such as copper wire, have high electricalconductivity. This applies only to solid metals because air pockets can lower the electricalconductivity of materials.

Materials that are not metals usually make good insulators. The best insulators are materials that naturally have air pockets in them, such as rubber. The air pockets are like impurities and disrupt the flow of electrons. Gases, such as air, are the best natural insulators. Modern chemistry has mastered insulators, creating materials that have thousands of times more resistivity than air.

 

Source / Courtesy : WiseGeek

What Is Electrical Resistivity?

Ohm's Law can be used to find the electrical resistance applied to a circuit by resistors.
Ohm’s Law can be used to find the electrical resistance applied to a circuit by resistors.

Electrical resistivity is the characteristic of a conductor, a semiconductor, or an insulator that limits the amount of current flow. It is determined by the atomic or molecular properties that may either allow or impede the flow of free electrons through the material. Electrical resistivity is almost the same as electrical resistance with the slight difference in the way electrical resistivity may refer to resistance of a specific length of a material. For instance, a basic unit of resistivity could refer to the amount of resistance per unit length of a copper cable.

Ohm’s law provides the relationship between the electrical resistance (R), the voltage (V), and the current flow in amperes (A). Resistance is the ratio of the voltage to the current. For the same voltage, a higher current is a result of a lower resistance. An electrical fuse is meant to have a very low voltage drop when placed in series with an electrical load. If the load is 9.999 ohms and the fuse has a resistance of 0.001 ohms, a 10-volt (V) supply voltage will produce a current of 1 A and the voltage across the fuse is negligible at 0.001 V.

Electrical resistivity tomography is an imaging tool that is able to present a three-dimensional profile of embedded materials. This is accomplished by using embedded electrodes and direct current (DC) to create a two-dimensional image. By using perpendicular image planes, it is possible to have an idea of the three-dimensional layout.

Various elements with notable electric resistivity have different uses in electrical applications. Silver and gold are very low-electrical resistivity elements that are used for special applications such as microbonding used in the semiconductor industry. Copper is the chosen commercial conductor sure to its acceptable electrical resistivity and relatively low price. Carbon is a low-cost material of choice for medium to high resistance resulting in huge varieties of carbon resistance in the market. The high stability of tungsten in relatively high temperatures makes it a common choice for incandescent and filament applications such as light bulbs, wire-wound variable resistors, and electric heaters.

Contact electrical resistance is usually very low when the conductive surfaces are not contaminated. In the case of relay contacts, the pressure that temporarily joins them determines how low the resistance will drop when the contact is closed. If the pressure is not enough and the current is high, it is possible for the contact to form plasma that can melt the contact. The spark generated due to repeated closures shortens the relay lifespan. In most cases, it is a good idea to use electronic DC switches such as the silicon-controlled rectifier (SCR) or use electronic alternating current (AC) switches like the three-terminal AC (TRIAC) switch.

 

Source / Courtesy : WiseGeek

What Is a Power Supply?

Solar panels are a type of power supply.
Solar panels are a type of power supply.

A power supply is a device that takes an incoming electrical current and amplifies it to levels required by various devices. In many instances, this type of device is also implemented to take the incoming electricity and deliver it across many other electronic devices, often at different preset levels. This device allows manufacturers to create electronics and machinery that can handle many different tasks from a single source of power, without the need for various adapters and additional hardware. Within other devices, a power supply is used to transform various types of power into a compatible format to be stored, like solar energy to electrical energy.

Perhaps the most common use of this type of device is within computer systems. As electricity enters the power supply, it is momentarily stored and then distributed to numerous functions throughout the system, allowing the motherboard, hard drive, and other various devices to receive electricity in order to function. Each one of these items requires a separate voltage, and it is delivered through specialized connectors that attach in a certain manner. For example, motherboards require either a 20-pin or a 24-pin power supply, and they are not interchangeable without the purchase of an additional adapter.

Modern vehicles also require a type of power supply in order to function, and it is referred to as an alternator. Although the wiring and design may be different, it essentially works in the exact same manner by taking incoming power and delivering it throughout the vehicle at the necessary levels. Alternators can be found on everything from lawn mowers to sea craft and industrial equipment, and without them, the devices would be rendered useless.

automobile alternator - Winged Post 0419515
An automobile engine’s alternator essentially serves as an electric generator.

Another common type of power supply can be found on windmills and solar panels, and its primary function is to convert various types of energy into electricity so that it can be stored and distributed across a grid. This is referred to as a generator, and it is often a free-standing object that is installed between the power source and the storage unit. Home and commercial generators, used during power outages, also work off of this same premise by transforming petroleum products into electrical energy by means of an engine. Many types of industrial tools also implement a type of generator. Other common types of power supplies are used within circuit breakers, battery-powered items and transformers.

 

Source / Courtesy : WiseGeek

What Is Diode Voltage Drop?

Diodes

 

A diode is a common semiconductor device used in many different types of electronic circuits. When an electrical signal passes through a diode, the diode consumes a small amount of the signal’s voltage in its operation. The difference between the voltage of the signal entering the diode and the voltage of the signal exiting the diode is the diode voltage drop. Although a diode voltage drop can refer to either the diode’s forward or reverse voltage drop, it typically describes the forward voltage drop.

The construction of a diode involves joining an anode and a cathode, two pieces of material with different electrical charges. The anode is positively charged and the cathode is negatively charged. At the point where these two different materials meet, called the junction, the two different opposing charges effectively cancel each other out. This area without a charge is the diode’s depletion layer, which forms an insulating layer within the diode between the anode and cathode.

When an electrical signal enters a diode’s cathode, the additional negative force increases the width of the depletion layer as it reacts with the positively charged anode. The wider depletion layer will block the signal from passing through the diode and consume all of the voltage in the process. For example, if 5 volts enter the diode, the diode voltage drop will also be 5 volts. A diode in this state is reverse biased, and the voltage drop is the diode’s reverse voltage drop.

An electrical signal entering a diode’s anode creates a different set of conditions within the diode. The negatively charged signal will bridge across the anode, meet the cathode, and pass through the diode, continuing on to the rest of the circuit. In the process, a relatively small amount of thevoltage is lost overcoming the anode’s positive charge. For a typical silicon diode, the voltage lost is approximately 0.7 volts. A diode in this state is forward biased, and the voltage drop is the diode’s forward voltage drop.

The difference between the forward and reverse states in a diode permits them to block a signal in one direction by dropping 100% of the voltage but allowing it to pass in the other by only dropping a small amount. As most diodes have a reverse voltage drop of 100%, the assumption is that the term “diode voltage drop” refers to the forward voltage drop; however, this is not always the case.

Specialty diodes exist that do not drop 100% of the reverse voltage, such as varicap or varactor diodes. In these diodes, the charges of the cathodes and anodes are not even across their widths. As a result, these diodes can allow part of the signal entering the cathode to pass through the diodes even though they are in a reverse biased state. When describing the voltage drop in these types of diodes, it is important to differentiate between the forward and reverse voltage drops.

 

Source / Courtesy : WiseGeek