This article is continuation of part 1 of this series. An important material parameter is the resistivity. The best-known resistor design is the cylindrical ceramic carrier with axial connecting wires. These connecting wires are routed through holes in circuit boards, for example, and soldered to the conductor track connections arranged there (through-hole mounting). A ceramic backing is coated with the resistance material, which gets its desired resistance value either through its composition, layer thickness or through indentations. The maximum power dissipation is between 0.1 W and 5 W.
The axial design with a square cross-section contains wire resistors and is filled with quartz sand. These resistors are suitable for higher power dissipation. A special design of the resistance layer is the meander shape, for example, such sheets are glued to an insulated carrier for low resistance values for high load-bearing capacity or the shape is used in the case of planar thick-film resistance layers to accommodate the required length on a compact surface or to increase the dielectric strength. Production on milling machines is also possible.
SMD resistors are applied on one side as a resistive layer on ceramic substrates and then separated. The resulting chip designs are small cuboids with an edge length of 1 mm × 2 mm × 0.5 mm, for example, which have metallizations at both ends or narrow sides as solderable contacts.
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The different materials of the resistive layers are selected according to the desired accuracy (tolerance) and temperature stability. Carbon layers have a negative temperature coefficient and are very inaccurate. Metal film resistors can be manufactured with the highest accuracies and, depending on the alloy, with very low temperature coefficients. Metals generally have a positive temperature coefficient. Metal film resistors are also manufactured as fuse resistors – these cause a safe interruption of the current flow in the event of overload.
For very high resistance values and high voltages, metal oxide film resistors are manufactured. These are particularly stable against the migration processes that occur at high voltages. Very small, heavy-duty resistors are made of metal foil. If these resistors are used for current measurement (shunts), they often have so-called Kelvin connections, i.e. two additional connections to avoid the measurement error due to the voltage drop at the contact.
Electrical resistors are available as electronic components in different designs, which differ, for example, in the type and shape of the resistor material:
Coating resistors:
- Carbon film resistor
- Metal Film Resistor
- Metal oxide (also MOX) film resistance
- Cermet (also thick-film) resistance
- Photoresistor (LDR for short, light-dependent resistor, e.g. made of a cadmium sulfide layer)
Foil resistors (planar) and solid metal resistors:
- Braking resistance (can be highly loaded for a short time)
- Shunt resistor (sheet metal or foil, also in the form of several parallel rods, often with Kelvin contacting)
Ground Resistors:
- Carbon Mass Resistance
- Ceramic Ground Resistor
Hot-Conductive (NTC) and Thermistor (PTC) Resistors
Varistor (VDR for short, voltage-dependent resistance)
- Axial
- Potentiometer (Changeable Resistance)
Classification of Resistors
Electrical resistors as a component can be grouped according to various criteria, for example:
- Achievement
- Resistance material
Another classification is according to use (decreasing requirements for accuracy and long-term stability from above):
- Precision resistance
- Measuring resistance
- Voltage divider, actuating resistor (fixed or variable in the form of a potentiometer or trim resistor)
- Working resistor, series resistor, general applications in electronic circuits
- Pull-up/pull-down resistor, digital circuitry (> 10%, often as resistor arrays)
