Vamistor Carbon Film Resistors
Source: Tepro of Florida, Inc.
Carbon film resistors are designed for both high temperature and high voltage applications. They are actually made in a vacuum by breaking down hydrocarbon gases at high temperatures. This in turn forms a carbon deposit on the substrate. The process is accomplished by heating methane or propane gas in a process called high temperature pyrolysis. When the heat is applied, molecular condensation results and hydrogen is released. The end result is carbon.
The main advantage and reason for using the carbon film resistor is that these components are designed to withstand high voltages as well as high temperatures. The highest voltage the devices can operate up to is 15kV with a nominal temperature of 350°C. They are available with tolerances of 2 percent, 5 percent, 10 percent, and 20 percent. Cutting a helical grove in the carbon film creates the resistive tolerances. The desired resistance value is achieved by regulating the pitch of the helix. The thinner the carbon layer, the finer the pitch and the higher the resistance value. After the helixing or spiraling is completed special alloy contact caps and tinned electrolytic copper connecting wires are pressed onto the ends of the resistor body. The final step involves coating several layers of tan lacquer or using a glass film as a shield. This is done primarily for electrical protection, but also for shielding from the climate.
Carbon film resistors are a good choice because they have a small size for such high ratings and they have a wide resistance range as well. The resistance can be anywhere from 500 Ohms to 100M Ohms. Another plus is that they have no outgassing. This means they will not deposit any vapor into the environment during operation. Outgassing is a condition that causes high material vapor pressure in the device to emit contamination deposits into the air. It corresponds directly to the temperature.
These deposits can cause problems with the surrounding equipment. Some of the other advantages are that they have less stray capacitance and inductance, so they are better at high frequencies. Also, they have a high stability of performance when compared to carbon composition resistors and, of course, they are flame proof.
Electrical noise is another factor when choosing a resistor. There are two main types of electrical noise, thermal and shot. Thermal noise is the product of the Brownian motion of ionized molecules. This noise cannot be eliminated because it is fundamental to resistance.
Carbon film resistors have less of this thermal noise than the carbon composition resistor. The lower values tend to be noise free while increasing with the higher values. Shot noise usually results from the flow of electrons through a highly charged field. It is more prevalent in solid state devices. It is not significant in carbon film resistors and until recently was not even known to exist, with respect to resistors.
There are also some disadvantages when using carbon film resistors. In the first place, they are limited to about 1 percent accuracy. Secondly, they exhibit drift with temperature and vibration. The resistors can have a TCR (temperature coefficient of resistance) range of -250 to -1000 ppm/°C, depending on the resistance value. The advantages, however, seem to outweigh most of the disadvantages, especially in specific applications.