Short version: Some capacitors (and presumably some other components) are sold with imbalanced/asymmetrical tolerances. Why?
Explanation:
Many ceramic capacitors are marked with, for example, +80% -20% tolerance or something similarly imbalanced.
For example, let's say that I have a capacitor with the (admittedly contrived) value of 17pF and a tolerance of +80%, -20%.
(Please ignore abuse of significant figures.)
- Maximum value: 17pF * (1 + 80%) = 17pF * 1.8 ≈ 30.6pF
- Minimum value: 17pF * (1 – 20%) = 17pF * 0.8 ≈ 13.6pF
- Mean value: (30.6pF + 13.6pF) / 2 ≈ 22.1pF
- Tolerance above mean: (30.6pF – 22.1pF)/22.1pF ≈ +38.5%
- Tolerance below mean: (13.6pF – 22.1pF)/22.1pF ≈ -38.5%
It would be fair to say that this supposedly "17pF" capacitor is virtually identical to a 22pF capacitor with ±40% tolerance.
By a similar process, a 10000pF +80% -20% capacitor (a real value from a catalog, not contrived) should be the same as a 13000pF around ±40%.
So, if I say I want a component of a given value, why am I being sold something that's quite a bit more likely to overshoot than undershoot this value? Is this imbalance useful to anyone?
Best Answer
Unlike resistors, whose price is essentially independent of resistance except at extreme values which represent less than 0.01% of the market, most types of capacitors have a cost which is tied strongly to capacitance--it costs more to make a large cap than a small one. Further, capacitors are often used in circumstances were a cap which is larger than specified might work better than the specified one, up to a certain limit, but the bigger cap might not be worth a higher price.
Suppose a designer determines that a device needs a minimum of 8uF to work correctly in a particular situation, but anything up to 20uF would work just as well. Some manufacturers can produce devices within +/-20% of their target; other manufacturers are capable of +/-33% of their target. If published tolerances were symmetrical, one would have to specify that the part could be either a 10uF+/-20% or a 12uF+/-33%--a bit awkward. If, however, manufacturers by convention use -20% for the lower tolerance and adjust the upper tolerance as needed, then it's possible to directly compare and substitute parts with different tolerances without affecting circuit operation.