All About Resistors part-2

1.3 Metal Film resistor:  

These resistors are made from small rods of ceramic coated with metal (such as a nickel alloy) or metal oxide (such as tin oxide). The value of resistance is controlled mainly by the thickness of the coating layer (the thicker the layer, the lower is the value of resistance). A fine spiral groove can be cut along the rod using a laser to split the carbon or metal coating effectively into a long and spiral strip, which forms the resistor.

 

 

Metal film resistors can be obtained in a wide range of resistance values from a few Ohms to tens of millions of Ohms with a very small tolerance. For example, for a stated value of 100K Ohm, the actual value will be between 99K Ohm and 101K Ohm. Small carbon, metal and oxide resistors come in various colors such as dark red, brown, blue, green, grey or white.  

1.4 Wire wound resistor: 

Wire wound resistors vary in size and physical appearance. Their resistive elements are commonly lengths of wire, usually an alloy such as Nickel/Chromium or Manganin wrapped around a small ceramic or glass fiber rod and coated in an insulating flameproof cement film. They are normally available in low values of resistance but are capable of dissipating large amounts of power.

 

These resistors can get very hot during use. For this reason, these resistors are housed in a finned metal case that can be bolted to a metal chassis to dissipate the heat generated. Protection from fire is important and fireproof cases or coatings are vital. Lead-out wires are normally welded rather than soldered to the resistor. Enamel resistors are used in scenarios where high power is involved and are encapsulated in heat proof bases.

 

Since wire wound resistors are primarily coils, they have more undesirable inductance than other types of resistor, although winding the wire in sections with alternately reversed directions can minimize inductance. Other techniques employ bifilar winding to reduce cross-section area of the coil. For the most demanding circuits, resistors with Ayrton-Perry windings are used.

 

source: engineersgarage.com