What is a Fixed Film Resistors Used for?
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On: 05 Jul, 2017
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Film-type resistors use a thin layer of resistive material deposited on an insulting core. The low-power types are more stable than the usual composition resistors. Except for very high-precious requirements, film type resistor are a good alternat...
Film-type resistors use a thin layer of resistive material deposited on an insulting core. The low-power types are more stable than the usual composition resistors. Except for very high-precious requirements, film type resistor are a good alternative to accurate wire wound resistors, being both smaller and less expensive and having excellent noise characteristics.
The power types are similar in size and performance to conventional wire wound power resistors. While their 200 maximum operating temperature limits power rating, the maximum resistance value available for a given physical size is much higher than that of the corresponding wire wound resistor.
For low resistance values, a continuous film is applied to the core, a range of values being obtained by varying the film thickness. Higher resistances are achieved by the use of a spiral pattern, a coarse spiral for intermediate values and a fine spiral for high resistance. Thus, the inductance is greater in high values, but it is likely to be far less than in wirewound resistors. Special high-frequency units having greatly reduced inductance are available.
Resistive-material films presently used are microcrystalline carbon, boron-carbon, and various metallic oxides or precious metals.
Deposited-carbon resistors have a negative temperature coefficient of 0.01 to 0.05 percent/°C for low resistance values and somewhat larger for higher values. Cumulative permanent resistance changes of 1 to 5 percent may result from soldering, overload, low- temperature exposure, and aging. Additional changes up to 5 percent are possible from moisture penetration and temperature cycling.
The introduction of a small percentage of boron into the deposited-carbon film results in a more stable unit. A negative temperature coefficient of 0.005 to 0.02 percent/°C is typical. Similarly, a metallic dispersion in the carbon film provides a negative coefficient of 0.015 to 0.03 percent/°C. In other respects, these materials are similar to standard deposited carbon. Carbon and boron-carbon resistive elements have the highest random noise of the film-type resistors.
Metallic-oxide and precious-metal-alloy films permit higher operating temperatures. Their noise characteristics are excellent. Temperature coefficients are predominantly positive, varying from 0.03 to as little as 0.0025 percent/°C.
Power ratings of film resistors are based on continuo us direct-current operation or on root-mean-square operation. Power derating is necessary for operation at ambient temperatures above the rated temperature. In pulse applications, the power dissipated during each pulse and the pulse duration are more significant than average power conditions. Short high-power pulses may cause instantaneous local heating sufficient to alter or destroy the film. Excessive peak voltages may result in flashover between turns of the film element. Derating under these conditions must be determined experimentally.
Film resistors are fairly stable up to about 10 megahertz. Because of the extremely thin resistive film, skin effect is small. At frequencies above 10 megahertz, it is advisable to use only un-spiraled units if inductive effects are to be minimized (these are available in low resistance values only).
Under extreme exposure, deposited-carbon resistors deteriorate rapidly unless the element is protected. Encapsulated or hermetically sealed units are preferred for such applications. Open-circuiting in storage as the result of corrosion under the end caps has been reported in all types of film resistors. Silver-plated caps and core ends effectively overcome this problem.
Some technical characteristics of film resistors are given in Tables 11, 12, and 13.
The MIL “RN” series of film resistors is more stable than the “RL” series and is available in a wider range of ratings. Commercial equivalents are also offered. Where stability and reliability are desired, the “RN” series is economically very competitive with the “RC” or “RL” series.
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