Standard resistor values ... Reasons?

In early days, manufacturers had to make do with approximate values, because they were not able to make resistors with 1 or even 5 percent tolerance.

It was common for them to declare a tolerance of 10 percent (plus or minus). Say they wanted to make a run of 100 ohm values. They would mix up a batch of carbon slurry and pour some resistors.

Their values were not necessarily close to 100 ohms. So they measured each one. If it was within 10 percent of 100 ohms, they labelled it 100 ohms.

If it was 111 ohms or more, they labelled it with the next step up, 120 ohms. Etc.

That is the story I heard about the origin of standard component values.

By sorting them this way, every resistor in a batch could be used.

In fact, there might have been an earlier time when the best tolerance was 20 percent.

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The Electronic Industries Association (EIA), and other authorities, specify standard values for resistors, sometimes referred to as the "preferred value" system.  The preferred value system has its origins in the early years of the last century at a time when most resistors were carbon-graphite with relatively poor manufacturing tolerances.  The rationale is simple - select values for components based on the tolerances with which they are able to be manufactured.  Using 10% tolerance devices as an example, suppose that the first preferred value is 100 ohms.  It makes little sense to produce a 105 ohm resistor since 105 ohms falls within the 10% tolerance range of the 100 ohm resistor. The next reasonable value is 120 ohms because the 100 ohm resistor with a 10% tolerance is expected to have a value somewhere between 90 and 110 ohms. The 120 ohm resistor has a value ranging between 110 and 130 ohms. Following this logic, the preferred values for 10% tolerance resistors between 100 and 1,000 ohms would be 100, 120, 150, 180, 220, 270, 330 and so on (rounded appropriately); this is the E12 series shown in the table below.

The EIA "E" series specify the preferred values for various tolerances.  The number following the "E" specifies the number of logarithmic steps per decade.  The table below is normalized for the decade between 100 and 1,000.  The values in any decade can be derived by merely dividing or multiplying the table entries by powers of 10.  The series are as follows:

E3     50% tolerance (no longer used)
E6     20% tolerance (now seldom used)
E12   10% tolerance
E24     5% tolerance
E48     2% tolerance
E96     1% tolerance
E192   0.5, 0.25, 0.1% and higher tolerances

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While the "E" preferred value lists are the best way to insure one is stocking the optimum number of values for a given tolerance, a word of caution is in order with respect to what is actually available in the marketplace and certain real world practices.  For instance, the E48 list is often used as a stock list for 1% resistors for inventory control (48 values per decade rather than 96), but this practice leaves "holes" or gaps in one's stock not covered by tolerance overlap, an undesirable practice in a prototype lab (less of an issue to the digital designer than to an analog circuit designer).  The use of the E48 list for inventory control of 1% resistors works out well because every value on the E48 list just happens to also appear on the E96 list; the holes are thus symmetrical and easily filled by acquisition of one of the other 48 values per decade being omitted from stock.  However, this is not always the case as can be seen by comparing the E24 and E96 lists.  Nevertheless, many manufacturers make every single value on the E24 list in 1% tolerance even though the practice makes little mathematical sense (think about the obvious tolerance overlap between the 120 and 121 values for instance).  Stocking only the E24 series in 1% will result in less symmetrical holes in stock than the practice of stocking only the E48 series.  In any event, one should be aware of these practices to avoid confusion.

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