Important characteristics of resistor types include resistance ranges, usable temperature range, stability, noise level, and temperature coefficient.
Wire wound resistors are available in values that range from fractions of an ohm (usually 0R22) up to about 10 kW (though higher values up to 100 Kw are available). Carbon composition resistors can be obtained in ranges of around 2R2 to 1M0 and film resistors normally range from 1R0 to 1M0.
Table Letter and number codes for SM components
Typical usable temperature ranges are –40◦C to +105◦C for composition and –55◦C to +150◦C for metal oxide. Wire-wound resistors can be obtained that will operate at higher temperatures (up to 300◦C) depending on construction and resistance value.
The stability of value means the maximum change of value that can occur during shelf-life, on soldering, and in use in adverse conditions such as operating in high temperatures in damp conditions. These changes are in addition to normal tolerances. Composition resistors have the poorest figures for stability of value, with typical shelf-life change of 5%, soldering change of 2% and ‘damp-heat’ change of 15%. Metal-oxide resistors can, typically, have shelf-life changes of 0.1%, soldering changes of 0.15% and damp-heat changes of 1%. The noise level of a resistor is specified in terms of microvolts (mV) of noise signal generated per volt of DC across the resistor. Such noise levels range from 0.1 mV/V for metal oxide to a minimum of 2.0 mV/V for composition, and for composition resistors the value increases for higher values of resistance. The formula that is used to find the noise level of composition resistors is:
so, for example, a 680 kW resistor would have a predicted noise level of
Temperature Coefficient of Resistance
The temperature coefficient of resistance measures the change of resistance value as the surrounding temperature changes. The basic formula is:
The value of temperature coefficient is usually quoted in parts per million per ◦C (abbreviated to ppm/◦C) and this has to be converted to a fraction, by dividing by one million, to be used in the formula above.