Resistors can be divided in construction type as well as resistance material. The following breakdown for the type can be made:
· Fixed resistors
· Variable resistors, such as the:
o Potentiometer
o Rheostat
o Trimpot
· Resistance dependent on a physical quantity:
o Photo resistor (LDR) as a result of a changing light level
o Thermistors (NTC and PTC) as a result of temperature change
o Magneto resistor (MDR) as a result of a changing magnetic field
o Varistor (VDR) as a result of a changing voltage
o Strain Gauges as a result of mechanical load
For each of these types a standard symbol exists. Another breakdown based on the material and manufacturing process can be made:
· Carbon film
· Carbon composition
· Metal oxide film
· Foil
The choice of material technology is a specific to the purpose. Often it is a trade-off between costs, precision and other requirements. Carbon composition resistors have a body of a mixture of fine carbon particles and a non-conductive ceramic. The carbon film technique has a better tolerance. These are made of a non-conductive rod with a thin carbon film layer around it. This layer is treated with a spiral cut to increase and control the resistance value. Metal and metal oxide film are widely used nowadays, and have better properties for stability and tolerance. Metal oxide film is generally more durable. Wirewound resistors are probably the oldest type and can be used for both high precision as well as high power applications. They are constructed by winding a special metal alloy wire, such as nickel chrome, around a non-conductive core. They are durable, accurate and can have very low resistance value. For the highest requirements on precision and stability, metal foil resistors are used. They are constructed by cementing a special alloy cold rolled film onto a ceramic substrate.
