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As the name indicates, metal film resistors axial leaded resistors are made from a thin film of metal that has been deposited into a ceramic rod.
The metal film resistor is widely used and gives superior performance in many respects to the metal oxide film resistors.
The term metal film resistor normally applies to the axial leaded components, although thin film surface mount chip resistors use the same technology to create the resistor.
The metal film resistor is widely used because its performance in terms of stability, accuracy, reliability and noise is better than other types like the carbon composition, carbon film and even the metal oxide film resistors that are also available.
What is a metal film resistor?
As the name indicates, the metal film resistor is made by depositing a thin layer of metal onto a ceramic former. The metal film acts in the same way as resistance wire, and as the thickness, width and length can be accurately controlled, the metal film resistor can be produced to a high tolerance.Also as the metal film resistor can be pre-aged and the metal protected to ensure it does not degrade over time, the long term stability is good.
Metal film resistor construction & manufacture
The metal film resistor is manufactured by vacuum depositing a metal layer onto a high purity ceramic cylindrical rod. Often, the thicker the metal film deposited, the more stable the resistor value. Typically film thicknesses between 50 and 250nm are used.The metal that is deposited is normally nickel Chromium, NiCr, but other metals including gold with platinum, or tantalum nitride may be used for specialised applications.
Once the film has been deposited, a metal end cap is pressed over the deposited metal. This makes contact with the resistive film and has the leads incorporated.
The next stage in the manufacture is trim the value of the resistance to the required figure. This is normally achieved by using lasers to cut a helix into the metal film. This extends the length of the metal element and also reduces its thickness. The accuracy of the lasers means that the metal film resistors can be manufactured to a very close tolerance.
The final stage in the manufacture of the metal film resistor is to add the protective coating and the markings. Typically the protective coating consists of a resin that is added in several layers which are individually baked on. Finally the marking rings are added to indicate the value of metal film resistor and the other relevant characteristics.
Typical metal film resistor specifications
Typical performance figures for metal film resistors are given below as a guide to the performance
|Metal Film Resistor Performance Guide|
|Metal Film Resistor Parameter||Metal Film Resistor Performance|
|Typical tolerance availability||±0.1%, ±0.25%, ±0.5%, ±1%, ±2%,|
|Value range||<1Ω - ~10MΩ|
|Load life (% change over 1000h)||1|
|Max noise (µV/V)||0.2|
|Temperature coefficient (ppm/°C)||±50 - >±100|
|Voltage coefficient (%/V)||0.0|
|Max resistor temperature (°C)||175|
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