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Why the weird numbers? Capacitor values explained.

Written by Anthony Kenny

November 29, 2017

Just like most passive components, capacitors come in standard values. These values are based on a geometric series, commonly referred to as the E series. Although it is possible in practice to produce a component of any capacitance value, manufacturers use the E series of preferred values for convenience of manufacture. 

The preferred number series (E series)

The International Electrotechnical Commission (IEC) defined the preferred number series in 1952 with the aim of enabling manufacturers to produce passive components more easily. The E series defines capacitance and resistance values as well as tolerances for capacitors and resistors. These standard values are published in the IEC standards, IEC 60063:1963. The E series has been adopted by various international standard organizations including the Electrical Industries Association (EIA).

Since the preferred number series uses a logarithmic scale with spaces that are approximately equal, it limits the values of components that manufacturers are required to produce. Moreover, this standardization ensures compatibility of components. As such, engineers are not limited to components from a specific manufacturer. They can source compatible capacitors and resistors from any available manufacturer. 

The E series is derived from progressive ratios of nth root of 10, where n=2ix3 and i is a whole number (0-6). The value n specifies the number of elements in a decade. For instance, the E12 series has 12 elements per decade while the E48 series has 48 elements per decade. 

The E24, E12, and E6 series

The values of the E24 series are obtained by rounding the theoretical numbers of the geometric series (with n=24).It is important to note that some values of the E24 series deviate from the mathematical rule. The values of the E12 series are obtained by omitting every second term of the E24 series. Similarly, the values of the E6 series are obtained by omitting every second term of the E12 series. The table below shows the values for these series.

E61.01.52.23.34.76.8
E121.01.21.51.82.22.73.33.94.75.66.88.2
E241.01.11.21.31.51.61.82.02.22.42.73.0
3.33.63.94.34.75.15.66.26.87.28.29.1

The E192, E96, and E48 series

The E192 series is obtained from the geometric series (with n=192). The theoretical values obtained from the series are rounded to three significant digits. The E96 series is obtained by omitting every second term of the E192 series. Similarly, the E48 series is obtained by omitting every second term of the E96 series. Unlike resistor values, capacitor values are usually specified to two significant digits. It is, therefore, highly uncommon to apply the E48, E96, and E192 series and recommended tolerances to capacitors. In most cases, capacitor manufacturers use tighter tolerances within the E12 & E24 series.

Conclusion

Although it is possible to produce a capacitor of any capacitance value, manufacturers produce capacitors and resistors with standard values. These preferred values are based on a geometric series, commonly known as the E series. For a given series, the E value specifies the number of elements per decade. Standardization of component values enables easier production of components in bulk. Moreover, standardization of capacitor and resistor values enables compatibility of components from different manufacturers. In addition to defining capacitance and resistance, the preferred number series provides recommended tolerances, particularly in regards to resistors.

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