The resistor which uses metal film to limit the flow of electric current to certain level is called metal film resistor.
Metal film resistor construction
The metal film resistor construction is similar to the carbon film resistor except that the material used for constructing the film is different.
In carbon film resistors, carbon is used to construct the film whereas in metal film resistors, tin and antimony or nickel chromium is used to construct the film.
The metal film resistor is made by placing the metal film on a ceramic substrate. The metal film resists the electric current flowing through it. Hence, it acts as resistive element to the electric current.
The ceramic substrate does not allow heat through it. Hence, it acts as insulating material to heat.
The metal film resistors are more preferred over carbon composition resistors because the metal film resistors generate less noise than the carbon composition resistors.
The metal film resistors have low tolerance value than the carbon composition resistors. The difference between our desired resistance value (300Ω) and the actual manufactured resistance value (302.4Ω) is called tolerance.
In metal film resistor, the desired resistance value can be achieved by either trimming the thickness of metal layer or by cutting the metal film in a helical manner along its length. This is generally done by using the lasers. Once the desired resistance value is obtained, the cutting of metal is stopped.
Metal film resistor has low temperature co-efficient of resistance
The rate at which the resistance of the material changes with change in temperature is called temperature co-efficient of resistance. Metal film resistors have low temperature co-efficient of resistance. Hence, the resistance of the metal film resistors does not change easily with change in temperature.
Resistance of metal film resistor depends on the thickness of metal film layer and width of helical metal film cut
The resistance of the metal film resistor is depends on the thickness of metal film layer and width of helical metal film cut
Metal film layer
The resistance of the metal film resistor is inversely proportional to the thickness of the metal film layer.
Metal film layer with more thickness provides large space for the free electrons to move freely from one place to another place. 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 and the remaining large number of free electrons moves freely by carrying the electric current. Therefore, the metal film resistors with very thick film layer allows large amount of electric current.
Metal film layer with small 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 and the remaining small number of free electrons moves freely from one place to another place by carrying the electric current. Therefore, the metal film resistors with very thin film layer allow only a small amount of electric current.
Width of helical metal film cut
The resistance of the metal film resistor is inversely proportional to the width of helical metal film cut.
The helical metal film cut with large width provide less resistance to the electric current because the free electrons 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 from one place to another place.
The large number of free electrons that moves freely from one place to another place will carry the electric current. Therefore, the metal film resistors with wide helical metal film cut allows large amount of electric current.
The helical metal film cut with less width provide more resistance to the electric current because the free electrons have to travel a large distance through the resistive path. Hence, the possibility of free electrons collision 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 metal film resistors with narrow helical metal film cut allows only a small amount of electric current.
Advantages of metal film resistor
Applications of metal film resistor
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