Standard component values

Although in theory any resistive or capacitive value can be obtained, in practice manufacturing costs require the use of standard values to maintain consistency in production. In 1952, the International Electrotechnical Commission (IEC) began a process of defining standard values for components known as the E series that is currently updated to IEC 60063:2015. The E series specifies how many values within each linear decade (1–10, 10–100, 100–1000 and so on) are available, and is based on a (broadly) logarithmic scale centred on a magnitude of 3. We recall from chapter 6 that a logarithmic scale is based on orders of magnitude, and for the E series values of 3, 6, 12, 24, 48, 96 and 192 are used to define the number of values available within each linear decade.

For example, the E3 series divides each decade into three values of 1.0, 2.2 and 4.7, whilst the E6 series divides it further into six values of 1.0, 1.5, 2.2, 3.3, 4.7, 6.8. This leads to the following table of common component values (Appendix 3 Table 1):

Although there are far more values available in the E48 and E96 series, these are highly specific (and hence more expensive). A better approach (particularly when beginning in electronics) is to start with the lowest series possible and determine whether a resistor is available that can be used, working upwards if none can be found. This can save a lot of time when performing gain and filter cutoff calculations, where an absolute value may not be needed in practice. An additional factor is the accuracy of the component relative to the value specified, where the table above shows how different series are linked to specific accuracy levels (known as tolerance) for the component. The table shows that while the E6 series gives a tolerance of (+/−20%) the accuracy increases for each series – E24 has a tolerance of (+/−5%), and beyond the listing are E48 (+/−2%) and E96 (+/−1%). As an example, any series could be used for a 2.2kΩ resistor but the actual value could be anywhere between 1760Ω (2200 − 440) to 2640Ω (2200 + 440) for an E6 component. In some cases (particularly digital electronics) the tolerance of the component is not critical to the effective operation of the circuit, so a pull-up resistor that serves to provide a reference voltage does not need to be of an absolute value to allow the microcontroller to distinguish between LOW and HIGH. Nor does a limiting resistor (such as the 1MΩ resistor in parallel with the piezo in the chapter 5 drum trigger project) need to be a of a specific value – it is there to protect other circuit components from damage due to voltage spikes.

Having said this, the need for more accurate component values becomes more obvious in analogue circuits such as amplifiers (to set gain) and filters (to define the cutoff frequency). In cases such as these, an accurate value is crucial to the effective operation of the circuit and so higher-tolerance components are used (which increases costs). In addition, the example questions at the end of chapter 8 show the limitations of working with standard components when a 470Ω resistor is combined with a 1.5μF capacitor in an RC filter to give a cutoff frequency of 226Hz. For audio circuits, the difference of 26Hz from the required value of 200Hz would not normally be important enough to merit a more expensive component unless the specific frequency of 200Hz was being measured (as would happen at much higher frequencies in communications circuits). For this reason, this introductory text works with lower series standard value components when building project circuits to keep costs down. It is important to note that a margin of error has been introduced as a result – a margin that is acceptable for our purposes, but may not be sufficient for more audiophile applications.