Precision and power wirewound resistors

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					                    Precision and power wirewound resistors
Wire wound resistors all are designed with the same principle: a very long wire which has a high resistivity that is
winded around an insulating body. However, the way they are made has got a significant influence on the area of
use. Wire wound resistors can roughly be categorized in 2 sorts: power and precision they may be changed to be
used in temperature and current sensors and potentiometers. These types of multi-purpose resistors can be
utilized in a wide range of circuits.

Precision resistors
Some popular uses for high accuracy wire wound resistors are measuring bridges
and attenuators. It's standard for such resistors to have a production tolerance of
0.1 pct. They've got a rather low temperature coefficient of about five ppm/K
compared to the metal film resistors (around 25 ppm/K). Generally it is combined
with a good stability. For twelve months of operation at maximum power rating,
the shift is around 30 ppm. They are produced for increases in temperature of
commonly less than 30°C. For that reason they can be protected by epoxy

Power resistors
A method to classify these power resistors in different varieties is by
breaking them down according to the coating type. Silicone resins can be
used for the smallest dissipation ranges. They're compact resistors that
are able to endure temperature increases to a maximum of 300 degrees
above the normal conditions. Another type of coating is vitreous enamel.
This traditional coating has reasonable insulation qualities at lower
temps, but at higher temperatures the insulation is considerably less. This
property makes it less and less common. Four hundred degrees is
approximately the highest working temperature for this coating style. TCR goes from 80 till 200 ppm/K. The range
of the resistance value is usually from 1 Ohm until ten kilo ohm. The third type of coating is the ceramic coating.
This is basically the most widespread coating type and they feature a ceramic core and coating to cover the wire.
The coating provides effective heat dissipation, yet has got still good insulation properties. It offers a good
protection against damage to the winding. Regular power ratings are somewhere between 5 and 17 watts. These
resistors can warm up to somewhere around 300 degrees without any damage. The temperature coefficient differs
from more or less 200 until ppm/°C. Between ten and twenty kilo ohm lay the resistance values. Generally they are
manufactured with leads that allow for vertical or horizontal installation. The resistors with the largest power
ratings are encased in a aluminum case with fins. Because of the bigger surface, the energy can better be
transferred. The surface is anodized to keep a good insulation. These resistors feature a typical power rating of 20 -
50 watts. In order to achieve this power rating, normally the resistor should be attached to a metallic surface to
achieve an ideal transfer of heat. The highest surface temperatures are around 300°C and the temperature
coefficient is quite low with around 30 ppm/°C for ohmic values higher than 50 ohms. The TCR is usually low for
small resistance values.

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