Power Resistors Features and Classification

2018-07-20 18:47:21 post by microhm

A power resistor is a resistor designed and manufactured to dissipate large amounts of power in a compact physical package. Generally speaking, they have a power rating of at least 5 Watt.

Materials of power resistors, allowing efficient cooling, are with a high thermal conductivity. Heat sink are often coupled with power resistor to dissipate high amount of power. Some might even need liquid cooling or forced air while working under max. load. Power resistors including wire wound type, winding type, grid resistor, Chip/SMD resistors and more. They are designed to dissipate the most power while keeping sizes as small as possible. An example use for power resistors are load banks used to dissipate power generated during engine braking in vehicles using electrical motors, such as locomotives or trams.

 Wire wound resistors are made by winding a metal wire, Nichrome ususally, around a ceramic, fiberglass or plastic form. Metal caps are attached to the end of the winding and metallic leads are attached to the ends. The body of the device is then coated with a non-conductive paint, enamel or plastic. The end product is often coated with a non-conductive paint or enamel offering some protection from the environment. Wire wound resistors can be built to withstand high temperatures up to 450 °C sometimes.

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There are several winding methods for Winding type resistor  including helical winding, edge-winding and bifilar winding. The helical type is the ordinary winding in which a wire is wound in a helix around a cylindrical core. Since the wire is coil-shaped, this type of resistor also has a certain inductance, then a bifilar winding resistor wounding in two directions are made to reduce the electromagnetic fields. Edge-wound resistors are made by winding a strip of metal by its wider edge. These are usually coreless, air-cooled and can dissipate more power than the helical type.

Grid resistors are large matrices of metal strips connected between two electrodes. They vary in size, but can be as large as a refrigerator. It is not uncommon to see a grid resistors value at under 0.04Ω and can withstand currents of over 500 amperes are also produced. They are applicable to brake resistors and load banks for railroad vehicles, neutral grounding resistors, load testing of generators and harmonic filtering for electric substations.

SMD resistors are actually smaller form, surface mounted chip resistors. The resistor itself consists of a metal oxide film deposited onto a ceramic substrate. SMD resistors are resistors which look like integrated circuit chips. Surface mount power resistors are made from many different materials, such as pressed carbon, ceramics and metal (cermet resistors) or metal foil. The thickness and length of film determines the resistance. Their power dissipation ratings are much lower than those of grid resistors and can usually dissipate a few watts if they have appropriate cooling.

 


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TAG:   wire resistor power resistor SMD resistor