Overview of the Film Type Resistors

Film Resistor is a general term referring to different types such as Carbon Film, Metal Film, and Metal Oxide Film resistors. They are generally manufactured by depositing pure metals (e.g., nickel) or oxide film (e.g., tin-oxide) onto an insulating ceramic or substrate.

Carbon Film Resistor

This low wattage, fixed type of resistor is the most commonly used. It incorporates a solid ceramic core coated with carbon film that has electrical resistant properties. It has a helix cut in it to create a long, narrow resistive path. The resistive material is made of carbon clay composition while the leads are made of tinned copper. A thin film of carbon is deposited onto a small ceramic rod during the manufacture. The resistive coating is spiraled away in an automatic machine until the resistance between the two ends of the rod is as close as possible to the correct value.

Carbon film resistors have been most popular for low-power applications from a safety point of view, considering their heat radiation and fire retardation. They have a disadvantage though, of tending to be electrically noisy. They are easily available with values within +10% or +5% of their nominal value.

Metal Film Resistor

These resistors are made from small rods of ceramic coated with metal such as nickel alloy. They excel carbon film resistors in temperature characteristics, current-noise suppression level, linearity, and accuracy. However, the metal film resistors are more expensive than carbon film resistors. They are manufactured by depositing a homogeneous film of pure carbon over a glass, ceramic, or other insulating material core.

The value of resistance is controlled mainly by the thickness of the coating layer or by a fine spiral groove cut along the rod using a laser or diamond cutter to cut the carbon or metal coating effectively into a long spiral strip. The desired value can be obtained by either trimming the layer thickness or by cutting helical grooves of suitable pitch along its length. Metal film resistors are used when a high tolerance or more accurate value is needed. They are used for bridge circuits, filter circuits, and low-noise analog signal circuits.

Metal Oxide Film Resistor

The metal oxide film resistor is similar to the metal film resistor but incorporates a stable ceramic core coated with a metal oxide such as tin oxide. This results in a higher operating temperature and greater stability and reliability than metal film. It is comparatively small in size with low annual shift for the strengthened metal oxide film. It can withstand a few watts because the metal oxide does not burn. However, when mounting metal oxide film resistors, utmost attention is required because they radiate heat.

They are used in applications with high endurance demands. Some features include the availability of very low or very high ohmic value, excellent flame retardant coating, high purity ceramic core, stable performance in diverse environments, and high safety standard. Some companies manufacture these by demands.