CARBON FILM RESISTORS

Carbon Film Resistors have many of the same characteristics as carbon composition resistors. The material is similar therefore they have noise, a voltage coefficient, the TCR can be much lower because the formula can be varied to achieve this, the tolerance is much tighter due to the difference in manufacturing processes. The Carbon Film Resistor is made by coating ceramic rods with a mixture of carbon materials. This material is applied to these rods in a variety of means, the one most familiar to me are dipping, rolling, printing , or spraying the rods in the appropriate solution. The thickness of the coating can be determined by the viscosity of the solution. This as well as the material composition will determine the ohms / square. Some of you may not be familiar with this term. It simply means that if a material has a resistivity of 100 ohms / square, one square inch with have the same resistance as 1 square mm, or 1 square foot or 1 square yard or 1 square mile all equalling 100 ohms but the power handling capability is proportional to the size. One batch of material can produce resistors in a wide range of values. These rods are cut to the length required for a specific size of resistor. These rods can then be spiral cut to a wide range of values. The original method of spiralling these was done with grinding wheels on a machine similar to a lathe. I am sure that later processes use lasers that are programmed to cut to specific values. The maximum ohmic value of this group is the highest in the discrete resistor group. Tolerance of 1% can be achieved with out measuring and selecting. Tolerance of less than 1% can be achieved by measuring and selecting. You should use caution in getting tight tolerances in this type because the temperature coefficient, voltage coefficient and stability may mean that it is only good for that tolerance at the time it was installed. The TCR of carbon film resistors is in the neighbourhood of 100 to 200 ppm and is generally negative. Measuring and selecting can yield even tighter TCRs. The frequency response of this type of resistor is among the best, far better than Wirewounds, and much better that carbon composition. The wirewound resistors are inductive at lower frequencies and values and somewhat capacitive at higher frequencies regardless of value. Also wirewound resistors will have a resonant frequency. Carbon Composition Resistors will be predominately capacitive .