Engineering Dictionary
"A" "B" "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M",
"N", "O", "P", "Q", "R", "S", "T", "U", "V", "W", "X", "Y", "Z"

Dictionary of Resistor Terms
'A', 'B', 'C', 'D/E', 'F-L', 'M', 'N', 'O', 'P', 'R', 'S', 'T', 'V-Z'

Resistor Tolerance

Tolerance: A permissible deviation from a specified value. A maximum electrical or mechanical variation of specifications that can be tolerated without impairing the operation of a device.

Resistor Tolerance: is expressed as the deviation from nominal value in percent and is measured at 25C with no appreciable load applied. A resistors value will also change with temperature [TCR].
For networks, absolute resistor tolerance refers to the overall tolerance of the network. The most common resistor tolerance values are 1% and 5%, but other possible tolerance values include 2%, 3%, 4%, 10% and 20%. The resistors tolerance is indicated by the fourth color band for through-hole and large power resistors. Smaller Surface Mount Devices [SMD] do not use a printed indication of the tolerance.

Resistor Ratio Tolerance: For a resistor divider or network, this is the tolerance of the ratio of components within the network. It is the relationship of each resistor to the others. It is often practical to specify tight ratio tolerances and loose absolute tolerances.
Also see Resistor Networks Derating Curves.

Temperature Coefficient of Resistance [TCR]: TCR causes the value of the resistance to vary with change in temperature. The change in temperature of the resistor can occur from two causes:
1) the change in ambient temperature and
2) self-heating due to power dissipation of the resistor.
The ambient temperature can vary due to causes such as the heating of a PC board by components near the resistor or the increase or decrease of the temperature of the air around the resistor.
The following formula expresses the rate of change in resistance value per 1 degree C in a prescribed temperature range:

TCR (ppm/C) = (R-Ro)/Ro X 1/(T-To) X 106

R: Measured resistance (ohms) at T in Centigrade
Ro: Measured resistance (ohms) at temperature To
T: Measured test temperature, Reference Temperature (C)
To: Measured test temperature, Test Temperature (C)

Self-heating occurs as power is dissipated by the resistor in response to current flowing through the resistor. Resistors have Temperature Derating Curves from which their self-heating may be determined.
Rate of self-heating =
(125C - 70C) / (Power rating at +70C) - (Power rating at +125C)

MIL-STD-202 Method 304 is often referenced as the standard for measuring TCR. Resistor TCR values may be zero [at 25C], negative [below 25C], or positive [above 25C].

Total Deviation of Resistance from its nominal value; Percent change from normal resistor tolerance + percent change from TCR + percent change from self-heating.

MIL-STD-202 Method 304 [Resistance-Temperature Characteristic]; The reference temperature shall be 25C or as specified. There shall be two standard series of test temperatures. The first series shall be 25C, 0C, -15C, and -55C; the second series shall be 25C, 50C, 75C, 100C, 125C, 200C, 275C, and 350C. The tolerance on each temperature in both series shall be +/-3C. The lowest test temperature in the first series, and the highest test temperature in the second series, shall be as specified. Measurements for each series of temperatures shall be performed in the order shown without interruption. However, a lapse of time not to exceed 24 hours is permitted between the end of the first series and the start of the second series.

Dictionary of Resistor Terms, Main Resistor Applications page.
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