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The Characteristics of Wirewound Resistors and Film Resistors

Writer:Microhm Page View:Date:2019-11-29
Wirewound precision resistors have some important characteristics. First, they can have very tight resistance tolerances. 0.005% is commonly achieved, such as PWP/PWS series of Microhm Electronics. More important, they are stable maintaining their precision over time because they are made with stable materials. Their TCR (Temperature Coefficient of Resistance) is low (<10ppm/°C) and can be controlled by selecting special wire alloys. 
In the past, the bulk of the resistor market went to carbon composition products. They are not accurate; you can sort them to tolerances of about 1% at best. They are unstable, have poor TCR (1000 ppm/C) and generate electrical noise. Low cost is their advantage. When customers still demand carbon resistor, NKT series is recommended. Now, film technology products are replacing carbon resistors. 

There are two common film types, thick film and metal or thin film. Thick film resistors are made by silk-screening conductive paint onto an insulating substrate. The paint is "fired" to make the assembly permanent. Thick film resistors have many carbon resistor characteristics. On the good side, they can be made small and are low cost in high quantities. However, they cannot have tight resistance tolerances (>0.1%), they have large TCR (50-100ppm/C) and they are unstable (500-1000ppm/yr.) Thick film technology is especially important in hybrid and integrated circuits because resistors can be "printed" onto the substrate, eliminating board loading and soldering steps.
Metal film resistors are a better technology. They are made by an evaporation/deposition process where a base metal is vaporized in a vacuum and deposited on an insulating substrate. Microhm Electronics' PMRY series, EE series and NLT series are typical metal film resistosrs.

Compared to precision wirewounds, they are not as accurate (>0.01%), have higher TCR (20-200ppm/C), and are not as stable (200-600ppm/yr.) They can be small for "chip" applications, are low cost in high quantities, and are excellent in high frequency and fast rise time applications.
Wirewound resistors have been around for a long time. They enjoy a long life because they have some very important features not duplicated by new technology products. If you must optimize circuit designs where precision is important, you should understand those characteristics.
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