The different purposes of wire wound (WW) resistors

The different purposes of wire wound (WW) resistors The wire wound resistor is one of the oldest resistor types that is still in use today. All wirewound resistors are made with the same principle of a metal wire that has been winded around an insulating core. Never the less, the applications differ widely. Wirewound resistors can roughly be categorized in two sorts. They are the power and the precision wirewound resistor. These kinds of multi-purpose resistors can be used in many electrical circuits. They sometimes are designed to be used in current and temperature sensors.

The range goes from under one watt till above a kilowatt. Power wirewound resistors are usually split in accordance to the coating type.

The smallest power resistors are generally layered with silicon resins.

They're compact resistors that can undergo temperature rises up to 300 degrees higher than the normal level. Another kind of covering is porcelain enamel. This conventional coating provides good insulating qualities at lower temperatures, but at high temperature the insulation is much less. This makes it less and less used. The maximum operating surface temperature is up to four hundred degrees Celsius.

Common resistance values are approximately between 0.1-10 kOhm. The third coating kind is the ceramic coating. This is basically the most common coating type and these resistors have a ceramic body and coating to secure the wire. The coating creates effective heat dissipation, and yet has good insulation properties furthermore it offers a good physical protection. Standard power ratings are between 1 and 20 watts. These types of resistors possibly can warm up to roughly 300 degrees with no problems. The TCR differs from approximately 200 till 400 ppm/K and the resistance values lie between 10 and twenty kilo ohm. They could be placed vertically or horizontally, depending on the direction of the leads. The resistors having the highest power feature an aluminum enclosure with fins, for the maximum dissipation values. The fins provide a bigger surface than can dissipate heat, permitting the resistor handle more power without getting weakened. A lot of these resistors have a body of a ceramic material and coating of silicone resin, encapsulated in an aluminum extruded part. The aluminium is anodized in order to keep the best insulation resistance. Generally, these resistors have a power rating of 20 to 50W. In order to achieve this high power dissipation, the resistor will have to be placed on a metal surface to reach an ideal heat transfer. The highest surface temperature is around 300째C. The TCR is quite small with around 20 ppm/K for values above fifty Ohm. Precision wire wound resistors are typically used in accurate measuring bridges, calibration equipment and AF attenuators. Such precise resistors can have a tolerance of less than 0.1 %. The stability is pretty good, with figures like 40 ppm change for a year of functioning at maximum power rating. The temperature coefficient is around 5 ppm/째C, and that is considerably better than most metal film resistors (approximately 25 ppm/째C). These resistors are designed for temperature raises of usually under thirthy degrees. Because of this they might be coated by epoxy resin materials.