PRECISION WIREWOUND

The Precision Wirewound is a highly accurate resistor with a very low TCR and can be accurate within .005%. A temperature coefficient of resistance (TCR) of as little a 3 part per million per degree Celsius (3ppm/oC) can be achieved. However these components are too expensive for general use and are normally used in highly accurate DC applications. The frequency response of this type is not good. When used in an rf application all Precision Wirewound Resistors will have a low Q resonant frequency. The power handling capability is very small. These are generally used in highly accurate DC measuring equipment, and reference resistors for voltage regulators and decoding networks. The accuracy is maintained at 25oC(degrees Celsius) and will change with temperature. The maximum value available is dependent upon physical size and is much lower than most other types of resistor. Their power rating is approximately 1/10 of a similar physical size in a carbon composition. They are rated for operation at +85oC or +125oC with maximum operating temperature not to exceed +145oC. This means that full rated power can be applied at +85 ( 125) oC with no degradation in performance. It may be operated above +125 (85) oC if the load is reduced. The derating is linear, rated load at +125(85) oC and no load at +145oC. Life is generally rated for 10,000 hours at rated temperature and rated load. The allowable change in resistance under these conditions is 0.10%. Extended life can be achieved if operated at lower temperatures and reduced power levels. End of life requirements are generally defined by the manufacturer or in some case by user specification. Some degradation in performance can be expected. In some cases, particularly if the tolerance is very low and the TC is low, the rated power is reduced to improve resistor stability through life. Precision Resistors regardless of type, are designed for maximum accuracy and not to carry power. The materials used in these resistors are highly stable heat treated materials that do change under extended heat and mechanical stress. The manufacturing processes are designed to remove any stresses induced during manufacture. There is little detectable noise in this type of resistor. The stability and reliability of these resistors is very good and their accuracy can be enhanced by matching the absolute value and the temperature coefficient over their operating range to achieve very accurate voltage division.