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Precision and Power Wire Wound Resistors

Writer:Microhm Page View:Date:2019-07-12
Wire wound resistors can roughly be classified in two types: power and precision. They can be modified for use in current and temperature sensors and potentiometers. These versatile resistors can be used in a wide range of applications.
Wire wound resistors exist for very high power applications. The range varies from 0.5 watts till more than 1000 watts. Power wirewound resistors can be divided in types according to the coating type. Silicone resins are used for the lowest dissipation levels. They are compact resistors that can withstand temperature rises up to 300°C above the ambient temperature.
Another type of coating is vitreous enamel. This traditional coating has good insulating properties at low temperatures, but at full rated temperature the insulation is considerably less. This property makes it less and less common. The maximum working surface temperature is up to 400°C. TCR varies from 75 till 200 ppm/°C. Typical resistance values are in the range from 1Ω til 10 kΩ. Microhm Nual series is a kind of typical power  metal-clad wirewound resistor.

For the highest dissipation values the resistor has a aluminum case with fins. These fins give a larger surface area from which to dissipate heat, letting the resistor handle more power without being damaged. These resistors have a ceramic core and a silicone resin coating, encased in an aluminum extrusion. The surface is anodized to maintain a good insulation resistance. These resistors have a typical power rating of 25 to 50 watts. This assumes that the resistor will be mounted on a metal surface, so that the heat can dissipate better. The maximum surface temperature is around 300°C and the TCR is low with around 25 ppm/°C for the ohmic values above 50Ω. Usually the TCR is higher for lower resistance values.
 High precision wire wound resistors typically used in precision AF attenuators, measuring bridges and calibration equipment. Typical values for the tolerance of the resistance value is 0.1% or better. The temperature coefficient of resistance lies around 5 ppm/°C, which is considerably better than most metal film resistors (around 25 ppm/°C). The stability is fairly good, with values like 35 ppm change for a year of operation at full power rating. The temperature rise of these resistors is usually below 30°C. Therefore they can be coated by epoxy resin materials.In practice, a designer might decide that a resistor needs to be within ±0.05% of the design value for a particular circuit application. To account for aging, TCR and other parameters, the designer might then specify a tolerance of ±0.01%. This ensures that the resistor stays within the required resistance range over time and varying circuit conditions. These kind of resistors extremely fix the application of precision instrument, such as Microhm's wirewound precision resistors MPF seriese and EE series.

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