Carbon film resistor

 
Carbon film resistor

What is a carbon film resistor? Carbon film resistors are a fixed form type resistor. They are constructed out of a ceramic carrier with a thin pure carbon film around it, that functions as resistive material. Advantages and drawbacks Carbon film resistors are a significant improvement on carbon composition. However, in comparison to metal film and metal oxide film, the commercially available range steadily decreases. Metal and oxide film are not more expensive to produce, and have overall better properties. Carbon film resistor applications Typical use for carbon film resistors is in high voltage and temperature applications. Operating temperatures are up to 15kV with a nominal temperature of 350°C. Examples are high voltage power supplies, radar, x-rays and laser. Manufacturing Carbon film resistors are made with a deposition process. At high temperature and under a high pressure, a ceramic carrier is held in hydrocarbon gas. The gas (methane or benzene) is cracked at a temperature of 1000°C. The crystalline carbon is pyrolytically deposited on the ceramic substrate. Because of the precise distribution of the pure graphite without binding, these carbon resistors have a low noise. The desired resistance value can be obtained by choosing the right layer thickness, and by cutting a spiral shape in the carbon layer. The helical cut in the film increases the length of the current path. By decreasing the pitch of the helix, the length of the resistive path increases, and therewith the resistance value increases. Furthermore, by fine tuning the cutting of the spiral the resistor can have a higher accuracy of resistance value. Typical tolerance values for carbon film resistors are 2, 5, 10 and 20%. Because of the use of pure carbon, the carbon film resistor has a higher negative temperature coefficient than carbon composition. The resistive temperature coefficient lies between 2.5×10^-4 [... read more]

Resistor types

 
Resistor types

There are a lot of different resistor types, all with their own applications, characteristics and construction. This page lists the different resistor types which are described on the resistor guide. Fixed resistors The fixed resistor type is the most common resistor, when people talk about a resistor they most probably mean a fixed resistor. The picture to the right shows an axial carbon film resistor, the most common type. Fixed resistors have a constant resistance value. Different resistor materials are used for fixed resistors. For all resistor types the used materials has influence on the resistor properties like the tolerance, cost and noise. Fixed resistors are available in axial and SMD packages. Variable resistors There are several variable resistor types, their general property is that the resistance value is adjustable. Most variable resistors are adjusted by mechanical movement (linear or rotary). When they are used as a variable voltage divider, they are called potentiometers. When they are used as a variable resistance to control the current in a circuit, they are called rheostats. Digital potentiometers are controlled electronically instead of by mechanical action. In general variable resistors have a higher tolerance of 20% compared to fixed resistors where 5% is the most common. Thermistors Thermistors are resistors of which the resistance changes significantly when temperature changes. Different thermistor types exist, the two most common types are the NTC and PTC thermistor. NTC thermistors decrease in resistance when the temperature rises, while PTC thermistors increase in resistance when the temperature rises. Thermistors are often used as temperature sensors or thermal protection devices. Varistors Varistors have a non-linear resistance which is dependent on the voltage over the varistor. Above a certain threshold voltage the resistance of a varistor drops very fast. Different varistor types exist, but nowadays the MOV varistor is the [... read more]

Resistor materials

 
Resistor materials

Resistors are produced with a wide variety of materials and manufacturing processes. Each resistor material has its typical properties and specific areas of use. The main types that are used in electrical engineering are summed below. Wirewound (WW) These types are made by winding resistance wire in a spiral around a non-conductive core. The resistance wire is usually a nickel-chromium alloy and the core is often ceramic or fiberglass. A coating such as vitreous enamel is used for protection. The spiral winding has capacitive and inductive effects that makes it not suitable for applications higher than 50 kHz. Often other winding techniques are used to reduce the undesired high frequency effects. Wirewound resistors are essentially produced for high precision or for high power applications. They have low noise, are robust, and are temperature stable. Resistance values are available from 0.1 up to 100 kW, with accuracies between 0.001 and 20%. Carbon Composition (CCR) The resistive element is made from a mixture of fine carbon particles and a non-conductive ceramic material. The substance is pressed in a cylindrical shape and baked. The resistance value depends on the dimensions of the body and the ratio between carbon and ceramic material. More carbon means a lower resistance. Carbon composition resistors are remarkably reliable, but have a poor accuracy with a maximum tolerance around 5%. Until the 1960s they were the standard for general applications. They quickly lost market share as other resistor types came on the market with better properties for tolerance, voltage coefficient, temperature coefficient, stability and finally cost. However, their ability to withstand high energy pulses and their high reliability makes them still useful for certain applications. Examples are power supplies and welding controls. Carbon Film A thin, pure carbon film is deposited on an insulating cylindrical core.  A spiral cut [... read more]