Resistors

Guys its too hrd to type in sinhala..please try to understand,so now on i,m writing my bloog in english,please feel free to ask me any thing in sinhal ,if you dont understand anything.

So as you can see on the topic,its about resistors,but its not that much simple to study resistors in engineering turms,..so take this seriously.

Now simply, resistence is the resistence to current flow..(Much like friction force to moving object.)
   That resistence is call the electrical resistence.
                      We can calculate the voltage drop across a constant resistor by using OHMS LAW ,if we know the current flowing through it.
    OHMS LAW   V=I x R      
   V  is in volts,
    I in amps
   R in ohms

Also in the realworld,nothing is perfect so  resistens is no constant always,when a circuit is powered up(WORKING) it desipate power as heat,which means the resistor  temperature incresses with the time,so wenn it happens actual resistence of that perticulr resistor will also increase and reduce the current flowing through it..
In physical turms      resistence is inversly perposional to temperature
                                               R=k1/tempetature.
  Also we can calculate the power dissipation of a resistor by using  P=I^2 x R

These are the basic turms that we used in  A/L  and university first years.
But when we  considder practical  design aspects.there are various kinds of ressistors to  select from.depending uppon the appliction that we are working on.

There are several basic types of resistors.which is listed bellow.

                                                            (01)Carbon film resistors.
   These are the cheapest ones that we can buy  from anywhere in the world.those resistors basically have 4 colour bands.which represents the resistence value and the tolerence of that resistor.( basically how much the actual resistence is changed from the marked values  )5% resistors are the most available ones.
Also there are no marks to identyfy the temperature coefficients.(temperature coefficient is the turm which indicate ,resistence  change with temperature.
If this themperature coefficent modulus is very high ,they are call thermistors.

Threare 2 types of  resistors deppending uppon their temperature coefficent.
    PTC- positive temperature coefficient
                   Resistors which their resistence increases with the  increasing temperature.
     NTC- negetive temperature coefficient
                  Resistors which their resistence decrease with the increasing temperature.

If need to have very pression resistors in a design  ,then we have to take care of this PTC,NTC resistors

                                                               ( 02 )Metal film resistors
Those resistors are  most probably have alt least 6 colour  bands,where 3rd band represent the temperature coefficeient of that resistor..last band is for resistor tolerence,if it is brown colour,they are 1% resistors.altho those resistors are marked as 1% they are not actually 1%off from the marked value.it might chnge depending on the manufacturere and the batch of resistors.

                                                                ( 03) wire wound resistors.
Those are the ones with very high power dissipation capabilities.verry good for power applications.also thos resistors are very stable due to their high thermal mass.these  resistors can wound by our selves if we wanted presise value resistor...also there are wire wound potentiometers..those are also very good in turms of  stability.

                                                                    (04)Carbon composite resistors
Those resistors can be manufactured directly to the PCB ,which reducess assembling cost of the project...but those resistors are mostly 5% tollerence,
Also they are available only in 0.25W power ratings.so noot good for high power applications.

Abouve mentioned resistors are the most used industrial standard ones, of course there are othere ones as well ,but unless a without a spacific resason there is no need for it.PArty comes in when we need to purchasing resistors to a project..because there are no exact  place to that,most of the resistors are manufactured in chaina,and asian countries.but as you might expact those resistors might very from manufacturere to manufacturer.and date to date,but its not a must to not to purchas from those chaina companies,its your choice if you want to keep your project cost down.but i recomend to buy from a well known manufacturer like philips,panasonic,sony,samsung...etc...
The bast plece where you can buy without worry is from DIGIKEY.COM or MOUSER.COM.  those websites offer parametric search for components,which lets the designer to select any components to an application depending on the required parameters like,voltage,current,operation frequency..etc.
I recomend not to buy from ebay,because those sellers are mostly from china for electronics.

Ok guys im stopping now,because its its useless if i just keep saying about resistors...lets design a simple project and lets see how to choose resistors and other components to that aplication..hopefully complete the project☺.