Resistors Fixed Composition
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On: 04 Jul, 2017
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Resistors Fixed Composition including:
- Color Code
Standard resistors are furnished in percent tole...
Resistors Fixed Composition including:
- Color Code
Standard resistors are furnished in percent tolerances, and in the preferred-value series of Table 2. “Even” values, such as 50 000 ohms, may be found in old equipment, but they are seldom used in new designs.
- Packaging Styles
In addition to the familiar axial-lead configuration, resistors are also available in SIP (single in-line package), DIP (dual in-line package), flat-pack, and leadless chip configurations.
The SIP and DIP types are designed for thru-hole or socket mounting on printed circuit boards. Flat-pack and chip configurations are surface mounted.
Flat-packs, SIPs, and DIPs contain multiple resistors, which may be internally connected to form networks or to have a common terminal pin. The chip configuration allows increased component density and is auto-insert able. Since there are no leads, basic circuit reliability and high-frequency performance are improved.
- Temperature and voltage Coefficients
Resistors are rated for maximum wattage at an ambient temperature of 70 above these temperatures up to maximum allowable hot-spot temperature of 130 or 150 , it is necessary to operate at reduced wattage ratings. Resistance values are a function of voltage as well as temperature; present specifications allow a maximum voltage coefficient of resistance as given in Table 9 and permit a resistance-temperature characteristic as in Table 10.
A 1000-hour rated-load life test should not cause a change in resistance greater than 12% for watt resistors and 10% for all other ratings. A severe cycling humidity test may cause resistance changes of 10% average and 15% maximum; 250 hours at 40 and 95% relative humidity may cause up to 10% change. Five temperature-change cycles, -55 to +85 , should not change the resistance value by more than 4 % from the 25 value. Soldering the resistor in place may cause a resistance change of 3 %. Always allow -inch minimum lead length; use heat dissipating clamps summary indicates that close tolerances cannot be maintained over a wide range of load and ambient conditions.
Composition resistors above 1 megohm have high Johnson noise levels, precluding their use in critical applications.
The end-to-end shunted capacitance effect may be noticeable because of the short resistor bodies and small internal distance between the ends. Operation at VHF or higher frequencies reduces the effective resistance because of dielectric losses.
Good Design Practice
Operate at one-half the allowable wattage dissipation for the expected ambient temperature. Provide an adequate heat sink. Mount no other heat-dissipating parts within one diameter. Use only in applications where a 15% change from the installed value is permissible or where the environment is controlled to reduce the resistance-value change.
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