What are Metal Film Resistors ?
Metal film resistors are one of the most common types of axial resistors. They use a thin metal layer as a resistive element on an otherwise non-conducting body. The first metal film resistor was manufactured in the 1930s by Dubilier.
At the base of the resistor, there is a non-conducting ceramic rod. This rod prevents contaminants from deteriorating the metal. A thin piece of metal is placed on the rod using vacuum deposition. Various materials such as nickel chromium, tin, and gold may also be used. The thickness of the material ranges from 50 to 250 nm. Resistance and stability are both based on the thickness of the metal. Thicker metal films result in lower resistance and better stability.
The desired resistance is achieved using lasers. These lasers cut a spiral-shaped slot in the metal. This lengthens the conductor and makes the conduction path narrower. Doing this increases the resistance.
Caps are placed on either end of the of the rod which allows wire connections to be made. The resistor is covered in a protective coating which prevents mechanical damage and any moisture or other contaminants from entering.
Resistors are available in different tolerance percentages of 0.1%, 0.25%, 0.5%, 1%, and 2%. Metal film resistors have very low noise and high linearity due to their low voltage coefficient.
In order to ensure reliability, these resistors are used at between 20% and 80% of their power rating. It is important to take caution as these resistors are easily damaged by voltage surges and power overloads.