TRANSISTORS

   

    Diodes are made up from two pieces of semiconductor material, either silicon or germanium to form a simple PN-junction and we also learnt about their properties and characteristics. If we now join together two individual signal diodes back-to-back, this will give us two PN-junctions connected together in series that share a common P or N terminal. The fusion of these two diodes produces a three layer, two junctions, and three terminal devices forming the basis of a Bipolar Junction
Transistor, or BJT for short.

Transistors are three terminal active devices made from different semiconductor materials
that can act as either an insulator or a conductor by the application of a small signal voltage.
The transistor's ability to change between these two states enables it to have two basic
functions: "switching" (digital electronics) or "amplification" (analogue electronics). Then
bipolar transistors have the ability to operate within three different regions:

1.Active Region - the transistor operates as an amplifier and Ic = β.Ib
2.Saturation - the transistor is "fully-ON" operating as a switch and Ic = I(saturation)
3.Cut-off - the transistor is "fully-OFF" operating as a switch and Ic = 0

Transistors (MOSFET , BJT or JFET) has two major applications

1. use it as a switch
2. use it as an amplifier (to amplify current or voltage)

we will now talk about using it as a switch.

why do we use transistor as a switch ? why not any other device ?

So here’s the answer

Its because of the property of base terminal to control the current from collector to emitter. (in case of BJT)

The input voltage is always applied to “base” of the transistor and the output voltage is taken out from the “collector” terminal to the ground (in case of a common emitter BJT)

Now,

There are two conditions,

• If I don’t apply any voltage to the base terminal, the base current (Ib) will be zero and the transistor will remain in cutoff region since collector current (Ic) will be zero and the output voltage (Vout) will be equal to Vcc(+5 volts) and transistor will act as open switch. i.e (logic 1)

• If I apply input voltage (Vin) to the base terminal, the base current (Ib) will flow. But if I apply “specifically very high input voltage” (very high Vin) , to the base terminal, the base current will increase and thus the collector current current will increase. This increase in collector current will be very high (Ic will be very high) thus the transistor will go in saturation region.

since collector current (Ic) is very high and output voltage (Vout) will be zero

transistor now will act as closed switch.

So basically, the conceptual meaning is that the base acts as a controller and it allows the transistor to be open or close.

Base controls the flow of current from collector to emitter terminal.

That is a property of a “switch” hence transistor proves that it can be used as a switch.

what I wrote here is for your conceptual understanding of why transistor is used as a switch. I didn’t explained it in detail as you can find many websites explaining the working of transistor.

Analog vs Digital انالوګ او ډيجيټل

#Analogue Versus #Digital

        There are two basic ways of representing the numerical values of the various physical quantities withwhich we constantly deal in our day-to-day lives. One of the ways, referred to as analogue, is to
express the numerical value of the quantity as a continuous range of values between the two expected
extreme values. For example, the temperature of an oven settable anywhere from 0 to 100 C may be
measured to be 65 C or 64.96 C or 64.958 C or even 64.9579 C and so on, depending upon the
accuracy of the measuring instrument. 

Similarly, voltage across a certain component in an electronic circuit may be measured as 6.5 V or 6.49 V or 6.487 V or 6.4869 V. The underlying concept in this mode of representation is that variation in the numerical value of the quantity is continuous and could have any of the infinite theoretically possible values between the two extremes.

The other possible way, referred to as digital, represents the numerical value of the quantity in steps
of discrete values. The numerical values are mostly represented using binary numbers. For example,
the temperature of the oven may be represented in steps of 1C as 64 C, 65 C, 66 C and so on.
To summarize, while an analogue representation gives a continuous output, a digital representation
produces a discrete output. Analogue systems contain devices that process or work on various physical quantities represented in analogue form. 

Digital systems contain devices that process the physical quantities represented in digital form.

Short Circuit شارټي

"Make sure you don't short the battery, otherwise it will burn" Language is important. If you know what "short" means, you can easily make sure the battery don't burn. If you don't know... ..well, you will have no idea if what you are doing will cause the battery to burn or not. To "short" something means to create a short circuit. A short circuit is a connection that was not meant to be there. For example, if you accidentally connect the plus to the minus of a battery, you'll have a short circuit between plus and minus of the battery. Which is not good because some batteries can actually explode if you short them. What Causes A Short Circuit? If you built the circuit, the most probable is that YOU caused the short circuit. I know that's hard to accept. I never accept it. ...until I find the error and I realize there's no way I can blame this on someone else. It's usually because I made one of these errors: * Connected something the wrong way * Added too much solder to a pad so that it flowed over to a neighbouring pad and created a solder bridge * Clipped of a component leg that landed on my circuit * Placed my circuit on top of something made of metal Also, if a component in your circuit is damaged, it can create a short circuit. What do you need to do to create less short circuits? And to find the short circuits you do create? Practice. Build many circuits.

د بریښنا انتقال او ویش Transmission and Distribution Lines

Transmission and Distribution Lines

       The power plants typically produce 50 cycle/second (Hertz), alternating-current (AC) electricity with voltages between 11kV and 33kV. At the power plant site, the 3-phase voltage is stepped up to a higher voltage for transmission on cables strung on cross-country towers. High voltage (HV) and extra high voltage (EHV) transmission is the next stage from power plant to transport A.C. power over long distances at voltages like; 220 kV & 400 kV. Where transmission is over 1000 kM, high voltage direct current transmission is also favoured to minimize the losses.

Sub-transmission network at 132 kV, 110 kV, 66 kV or 33 kV constitutes the next link towards the end user. Distribution at 11 kV / 6.6 kV / 3.3 kV constitutes the last link to the consumer, who is connected directly or through transformers depending upon the drawl level of service. The transmission and distribution network include sub-stations, lines and distribution transformers. High voltage transmission is used so that smaller, more economical wire sizes can be employed to carry the lower current and to reduce losses. Sub-stations, containing step-down transformers, reduce the voltage for distribution to industrial users. The voltage is further reduced for commercial facilities. Electricity must be generated, as and when it is needed since electricity cannot be stored virtually in the system.

There is no difference between a transmission line and a distribution line except for the voltage

level and power handling capability. Transmission lines are usually capable of transmitting large quantities of electric energy over great distances. They operate at high voltages.

Distribution lines carry limited quantities of power over shorter distances.

Voltage drops in line are in relation to the resistance and reactance of line, length and the current drawn. For the same quantity of power handled, lower the voltage, higher the current drawn and higher the voltage drop. The current drawn is inversely proportional to the voltage level for the same quantity of power handled. The power loss in line is proportional to resistance and square of current. (i.e. PLOSS=I2R). Higher voltage transmission and distribution thus would help to minimize line voltage drop in the ratio of voltages, and the line power loss in the ratio of square of voltages. For instance, if distribution of power is raised from 11 kV to 33 kV, the voltage drop would be lower by a factor 1/3 and the line loss would be lower by a factor (1/3)2 i.e., 1/9. Lower voltage transmission and distribution also calls for bigger size conductor on account of current handling capacity needed.

FERRANTI EFFECT

A long transmission line can be considered to compose a considerably high amount of capacitance and inductance distributed across the entire length of the line. Ferranti Effect occurs when current drawn by the distributed capacitance of the line itself is greater than the current associated with the load at the receiving end of the line(during light or no load). This capacitor charging current leads to voltage drop across the line inductor of the transmission system which is in phase with the sending end voltages. This voltage drop keeps on increasing additively as we move towards the load end of the line and subsequently the receiving end voltage tends to get larger than applied voltage leading to the phenomena called Ferranti effect in power system. It is illustrated with the help of a phasor diagram below.
Thus both the capacitance and inductor effect of transmission line are equally responsible for this particular phenomena to occur, and hence Ferranti effect is negligible in case of a short transmission line as the inductor of such a line is practically considered to be nearing zero. In general for a 300 Km line operating at a frequency of 50 Hz, the no load receiving end voltage has been found to be 5% higher than the sending end voltage.
Now for analysis of Ferranti effect let us consider the phasor diagrams shown above.
Here, Vr is considered to be the reference phasor, represented by OA.

This is represented by the phasor OC.

Now in case of a long transmission line, it has been practically observed that the line electrical resistance is negligibly small compared to the line reactance, hence we can assume the length of the phasor Ic R = 0, we can consider the rise in the voltage is only due to OA - OC = reactive drop in the line.

Now if we consider c0 and L0 are the values of capacitance and inductor per km of the transmission line, where l is the length of the line.

Since, in case of a long transmission line, the capacitance is distributed throughout its length, the average current flowing is,

Thus the rise in voltage due to line inductor is given by,

From the above equation it is absolutely evident, that the rise in voltage at the receiving end is directly proportional to the square of the line length, and hence in case of a long transmission line it keeps increasing with length and even goes beyond the applied sending end voltage at times, leading to the phenomena called Ferranti effect in power system.

TYPES OF DAMS

BASED ON FUNCTIONS

1.Storage Dam: 
They are constructed to store water during the rainy season when there is a large flow in the river.
2.Diversion Dam: 
A diversion dam is constructed for the purpose of diverting water of the river into an off-taking canal (or a conduit).
3.Detention Dam:
 Detention dams are constructed for flood control. A detention dam retards the flow in the river on its downstream during floods by storing some flood water.
4.Debris Dam: 
A debris dam is constructed to retain debris such as sand, gravel, and drift wood flowing in the river with water.
5.Coffer Dam: 
It is an enclosure constructed around the construction site to exclude water so that the construction can be done in dry.

BASED ON STRUCTURES

1.Gravity Dam:
 These are the dams which resist the horizontal thrust of the water entirely by their own weight.
2.Arch Dam:
These are designed so that the force of the water against it, known as hydrostatic pressure, presses against the arch, compressing and strengthening the structure as it pushes into its foundation or abutments.
3.Buttress Dam:
 A buttress dam or hollow dam is a dam with a solid, water-tight upstream side that is supported at intervals on the downstream side by a series of buttresses or supports.
4.Embankment Dam: 
These are typically created by the placement and compaction of a complex semi-plastic mound of various compositions of soil, sand, clay and/or rock.

Transformer

Transformers Help To Move Electricity Efficiently Over Long
Distances

To solve the problem of sending electricity over long distances, William Stanley developed a
device called a transformer. The transformer allowed electricity to be efficiently transmitted over
long distances. This increased delivery range made it possible to supply electricity to homes and
businesses located far from the electric generating plant.
The electricity produced by a generator travels along cables to a transformer, which changes
electricity from low voltage to high voltage. Electricity can be moved long distances more
efficiently using high voltage. Transmission lines are used to carry the electricity to a substation.
Substations have transformers that change the high voltage electricity into lower voltage
electricity. From the substation, distribution lines carry the electricity to homes, offices, and
factories, which require low voltage electricity.