Carbon
film resistor definition
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The carbon film resistor is a type of fixed
resistor that uses carbon film to restrict the electric current
to certain level. These types of resistors are widely used in
the electronic circuits.
Carbon
film resistor construction
The carbon film resistor is made by placing
the carbon film or carbon layer on a ceramic substrate. The
carbon film acts as the resistive material to the electric
current. Hence, the carbon film blocks some amount of electric
current. The ceramic substrate acts as the insulating material
to the heat or electricity. Hence, the ceramic substrate does
not allow heat through them. Therefore, these resistors can
withstand at higher temperatures without any damage.
Metallic end caps are fitted at both ends of
the resistive material. The leads made of copper are joined at
two ends of these metallic end caps. The resistive element of
the carbon film resistor is coated with epoxy for protection.
The carbon film resistors are more preferred
over carbon
composition resistors because the carbon film resistors
produce less noise than the carbon composition resistors.
The carbon film resistors have low tolerance
value than the carbon composition resistors. The tolerance of a
resistor is the difference between our desired resistance value
(200Ω)
and the actual manufactured resistance value (202.3Ω). The
carbon film resistors are available in the range from 1Ω (one
ohm) to 10MΩ (ten mega ohm).
In
carbon film resistor, the desired resistance value can be
achieved by either trimming the thickness of a carbon layer or
by cutting the carbon metal in a helical manner along its
length. This
is generally done by using the lasers. The cutting of carbon
metal is stopped once the desired resistance value is
obtained.
Carbon
film resistor has high negative temperature co-efficient
of resistance
The
rate at which the resistance of the material decreases with
increase in temperature is called negative temperature
co-efficient of resistance. Carbon film resistors have high
negative temperature co-efficient of resistance. Hence, the
resistance of the carbon film resistors decreases easily with
increase in temperature.
Resistance of
carbon film resistor depends on the thickness of carbon film
layer and width of helical carbon film cut
The resistance of the carbon film resistor is
depends on the thickness of carbon film layer and the width of
helical carbon film cut
Carbon
film layer
The resistance of the carbon film resistor is
inversely proportional to the thickness of the carbon film
layer.
Carbon film layer with large thickness
provides more space for the free
electrons to move freely. Hence, the possibility of free
electrons collision with the
atoms is low. Therefore, only a small number of free
electrons collide with the atoms. The small number of free
electrons, which collides with the atoms, loses their energy in
the form of heat and the remaining large number of free
electrons moves freely by carrying the electric current.
Therefore, the carbon film resistors with very thick film layer
allows large amount of electric current.
Carbon film layer with less thickness
provides less space for the free electrons. Hence, the
possibility of free electrons collision with the atoms is high.
Therefore, a large number of free electrons collide with the
atoms. The large number of free electrons, which collides with
the atoms, loses their energy in the form of heat and the
remaining small number of free electrons moves freely by
carrying the electric current. Therefore, the carbon film
resistors with very thin film layer allow only a small amount of
electric current.
Width
of helical carbon film cut
The resistance of the carbon film resistor is
inversely proportional to the width of helical carbon film cut.
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The helical carbon film cut with large width
provide less resistance to the electric current because the
electric current have to travel only a small distance through
the resistive path. Hence, the possibility of collision of free
electrons with the atoms is less. Therefore, only a small number
of free electrons collide with the atoms and the remaining large
number of free electrons moves freely.
The large number of free electrons that moves
freely from one place to another place will carry the electric
current. Therefore, the carbon film resistors with wide helical
carbon film cut allows large amount of electric current.
The helical carbon film cut with less width
provide more resistance to the electric current because the
electric current have to travel a large distance through the
resistive path. Hence, the possibility of collision of free
electrons with the atoms is high. Therefore, a large number of
free electrons collide with the atoms and the remaining small
number of free electrons moves freely.
The small number of free electrons that moves
freely from one place to another place will carry the electric
current. Therefore, the carbon film resistors with narrow
helical carbon film cut allow only a small amount of electric
current.
Advantages
and disadvantages of carbon film resistor
Advantages
of carbon film resistor
- Generates less noise than carbon composition resistor
- Low tolerance
- Low cost
- Wide operating range
Disadvantages
of carbon film resistor
- High negative temperature co-efficient of resistance
Applications
of carbon film resistor
- RADAR
- X-rays
- Power supplies
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