I am trying to understand why there is variations in resistance values, in other words, why there is difference in the value of the resistance between color code and the measured resistance. Also, how to manufacture more precise resistors?
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4\$\begingroup\$ If you buy a hundred tins of tomatoes would you expect every tin to weigh EXACTLY the same weight? \$\endgroup\$– Andy akaCommented Mar 8, 2017 at 8:58
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3\$\begingroup\$ As with most things in life accuracy is a function of price. You can control the manufacturing processes and materials very well and get good tolerances, you can then test and reject or trim them to ensure an even tighter tolerance. But all of that costs more. Most of the time you don't need the accuracy so why pay for it? \$\endgroup\$– AndrewCommented Mar 8, 2017 at 9:03
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\$\begingroup\$ See electronics.stackexchange.com/questions/39344/… \$\endgroup\$– pjc50Commented Mar 8, 2017 at 9:29
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1\$\begingroup\$ how to manufacture more precise resistors? - basically produce something that is near enough and then trim the value using lasers to obtain the accuracy required. \$\endgroup\$– JIm DeardenCommented Mar 8, 2017 at 10:21
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1\$\begingroup\$ first of all note that the tolerance is indicated as a part of the color code \$\endgroup\$– dlatikayCommented Mar 8, 2017 at 10:53
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2 Answers
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Modern carbon-film resistors are actually manufactured to a nominal 0Ω, then laser-trimmed to the desired value. A laser burns off bits of carbon to increase the resistance, then the resistor is encapsulated, printed, and packaged on tape.
However, the laser doesn't burn off a consistent amount of carbon. The manufacturer increases precision by testing the resistor and burning off more carbon to approach the target value. The more tests and adjustments are made, the higher the precision.
Of course, repeatedly adjusting the resistor consumes time, which makes precision resistors more expensive to produce.
To maintain precision, the resistor is also tested after being encapsulated and fitted with pads, in case a manufacturing defect pushed the value out of spec.
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Andy aka is quite right above, statistically if you manufacture hundreds of resistors there will be some statitical variation in the output. Thats said, resistor precision depends on several such as resistor techonology (wire-wound, chip, etc), temperature of operation (the resistance of a straight piece of wire will vary with temperature) and frequency of operation.
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\$\begingroup\$ Some technology also has a really measurable voltage coefficient, that you seldom see in the short form datasheets, but the better manufacturers will fess up to in the full data. \$\endgroup\$ Commented Mar 8, 2017 at 22:52