When designing a circuit or working on a piece of equipment we get used to seeing the same “standard” resistor values. For example, when making a voltage divider the math may say you need a 2341 ohm resistor, but in general we would see that implemented as 2200 ohms or 2700 ohms. Maybe 2320 ohms if it really needed to be close to the calculated value. Why? Like so many other things in engineering, it goes back to a standard from over 70 years ago.

In the early 1950’s electronics were really starting to develop. We had already seen the world’s first programmable electronic computer (ENIAC) and electronics were starting to become a regularly manufactured product. The International Electrotechnical Commission met and decided to standardize resistor values to help make manufacturing and design of electronic circuits more efficient. They came up with the preferred or “E” series values and documented it in IEC 60063: 1963.

While the E-series values may appear somewhat random, they are actually arranged in a clever distribution. Each value is the division of a decade (factor of 10) into a certain number of steps in logarithmic space. For example, the E12 series of resistors divides the decade into 12 steps. We know that 10 raised to the 1/12 power is 1.21, so each resistor is 1.21 times (21%) higher than the last value.

A crucial factor to remember in any manufacturing process is that there is error in everything. In electronics we specify a tolerance from the “ideal” value that our design can tolerate. For components with non-crucial functions a tolerance of 20% may be fine while a precision component may be needed in some instrumentation applications and a tolerance of 0.5% or 0.1% may be selected. The main reason to use loose tolerances is cost savings on components. This does not really change things for small projects or low quality production, but saving a fraction of a cent on each iPhone produced adds up! 

Now, how does tolerance relate to geometric series? Well, since each resistor in the E12 series is 1.21 times the last, it means that the tolerance band of +/-10% negates the need for any values in between those two as the actual value could be anywhere in the range! It’s a really nice way to use the absolute minimum number of values to guarantee you can get practically any resistance assuming a tolerance range. If your design can’t tolerate such a large variation, you need a more precise resistor.

Since everything is based on the “decade” of factors of ten, it also means we can just define a single sequence (say from 1 to 10 ohms or 10 to 100, it doesn’t matter) and then multiply/divide by 10 as needed to get any value we want. Let’s take a look at the E12 values below. The 12 could represent a 0.12, 1.2, 12, 120, 1200, 12000, etc. ohm resistor. Next would be 15 and so on. Easy, right?? 

 

There are the E6, E12, E24, E48, E96, and E192 series values available. Generally with the price of electronics anymore, when we are designing instrumentation we use 0.1% resistors commonly, but also scatter in a few 1% when we can. In my mind, there just isn’t any reason to use more coarse values in scientific equipment as the cost difference is negligible and we aren’t building mass market consumer goods.

Having the E-Series values at your fingertips can be a game-changer in circuit design. Even for a seemingly straightforward circuit like a voltage divider, you need resistors with values that are readily available. That’s precisely why we’ve put together a handy PDF guide for your reference. Click the link below to snag your free copy – perfect for keeping on your computer or printing out for your workbench. Happy designing!

John Leeman
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