Why is ParSer needed anyway?

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Why is ParSer needed?


I've been asked a few times, why ParSer is needed at all? The nominal values of the components are not precise anyway and the IEC sets were created to account for these imprecisions. E12 was made for +/-10%, E24 for +/-5% and E96 for +/- 1% tolerance.


Well, that is all true, but in real life, you may often need a maximum tolerance of 1%, but you have only an E12 or E24 set of components with 1% tolerance rating available instead of the E96, which would cover all possible values with the tight tolerance. E96 means 96 distinct values per decade, that makes 576 different values for the six decades between 1 and 1M. Even in my professional life as engineer I did not have easy instant access to so many different resistors or capacitors (of course, everything may be ordered). Therefore it is quite handy to construct a required value out of two available components.


There is also another argument for building one single component from two: statistics. Suppose you need a rather precise 101 ohm resistor. From the E96  series, you would probably select a 100 ohm resistor with 1% tolerance. Statistically, most 100 ohm resistors will really have 100 ohm. Some will only have 99.5 ohm, quite as many will probably have 100.5 ohm. The distribution of values will follow a normal distribution, the bell curve. Almost none of the resistors will have 99 or 101 ohm, as that would violate the 1% tolerance spec. If you create the 101 ohm by two resistors from the E12 series with 1% tolerance, you may select 33 ohm and 68 ohm in series to get a nominal value of 101 ohm. Did you gain something? Yes, of course! Now you have a 101 ohm resistor with the center of the bell curve at 101 and a minimum value of 99.99 ohm (33-0.33+68-0.68) and a maximum value of 102.01 ohm (33+0.33+68+0.68). The original 1% tolerance is maintained. A single 100 ohm resistor from the E96 series might in fact have just 99 ohm, which is 2% deviation from the nominal 101 ohm.




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