In my previous post, I mentioned
about active and passive devices which are used to play with electrons for
creating a world with electronics.
Now we first discuss about
passive devices.
We discussed earlier that passive
devices are devices which doesn’t have any source of their own and they consume
the supplied power.
In this post we will discuss
about some of the passive devices.
1. Resistors:
·
All materials in this earth are resistors in one
form or other
·
Resistors have 3 forms
1. Conductor
2. Semiconductor
3.
Insulator
· Conductor: They have more number of free electrons and
as a result very low electric field can make electrons to move in the direction
of electric field. Here electrons are in conduction band and very less
attracted to nucleus of atom. Conduction band is nothing but representation of
what state an electron is in an atom. In the fig below we can see that
electrons are not attracted and its movement is random over the surface of the
conductor based on the imbalance of the atoms in the conductor.
·
Semiconductor: This is the place where electronics gained
its name and fame. It is a state which conducts as well as insulates but both
are in a particular ration to make it a semiconductor. Normally, at low
electric field the electrons in this material (semiconductor) are not in
conduction band to orient electrons in the direction of electric field. But
after a threshold voltage is achieved this material acts as a conductor.
·
Insulator:
This type of material does not allow electrons to flow through it. In ideal
case, an insulator material will not allow flow of electrons even though
applied electric field is infinity. But practically there is no 100% insulator
in this world. Every material in this earth will conduct after a particular
threshold voltage.
·
Now, let us see the types of resistors
1. Fixed
2. Variable
·
Fixed:
These type of resistors have a fixed value of resistance and they are made up
of specific material and their resistance value is designed during
manufacturing and their value is specified in certain format (like colour
coding, coded values etc.)
·
Variable:
This type of resistor has a range of resistance. The values specified in these
resistors correspond to the maximum range and the minimum is obviously zero
ohms. They usually have a knob to adjust the resistance manually (in steps or
continuous).
·
Types of
fixed resistors:
1. Carbon
composition: Made by mixing carbon with resin or china clay.
2. Carbon film:
Made by depositing carbon film over resin or china clay. The films have certain amount of resistance and based on deposition of film the resistance value is designated.
3. Metal film:
Made by depositing metal film (alloy of tin and antimony) over resin or china
clay.
4. Wire wound:
Made by winding a long wire of a particular material over a ceramic or epoxy
substrate (mould). The wire will offer a particular resistance per unit length
with known wire diameter.
·
Types of
variable resistors (potentiometer):
1. Wire wound
potentiometers: It is made in the same way as fixed wire wound resistors,
but with the difference that there will be a contact which slides over the wire
wound on a substrate (mould). This varies the length of the wire when
resistance is measured between one end of wire and the slide which is also
known as rheostat. Copper alloy wire used for low resistance and nickel
chromium for high resistance.
2. Trimming
potentiometer: These are resistors which are used for calibration purposes.
Usually screw drivers are used to vary the resistance. Carbon coating over
which a slide contact moves and any one end of carbon coating can be used as
another contact.
3. Carbon
potentiometer: These are resistors which are manufactured either in film or
moulded track. An annular ring of carbon resistance coating is done over the
film. A slip ring contact over the carbon track will slide which will vary the
resistance when measured from contact and one end of resistive coating.
Some useful symbols for resistors
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