CHOOSING STANDARD RESISTORS TO MAKE YOUR DESIGN CHEAP(ER)

Choosing an uncommon value of resistance can add a small amount of cost and might lead to issues when you cant source the specific value as you will be left with fewer options. IEC 60063:1963 defines the resistance and tolerance values into a norm, to ease the mass manufacturing of resistors. These standard values are called “preferred values” or “E-series”.

• E6     20% tolerance
• E12   10%
• E24   5% (also available with 1%)
• E48   2%
• E96   1%
• E192 0.5% (also used for resistors with 0.25% and 0.1%).

So what dose this Exx means ??

To understand the E-series sets of preferred values, we will examine the E12 in detail. E12 means that every decade (0.1-1.0, 1-10, 10-100, etc.) is divided in 12 steps on a logarithmic scale. The size of every step is equal to:

Expanding the math

What I want to bring to light in this article is the E24 range should be your preferred band of resistors for all your resistance. even though E24 is primarily a 5% tolerance standard, it also features a 1% band too.

so when trying to select a resistor 22k – 1% or 22.6 – 1% from Digikey result

If the design requires a resistor, which is not in the E24 then only you move into the E96 series. So the idea is even you require a 0.5% resistor, try sticking to the E24 series before going to the E96

And there may be cases, that the design that requires an odd value for some specific applications. They only you start looking into the below table of E192

Lets check for a 22.9k 1% resistor in Digikey, at this particular date (22-3-2022). you can see that the 1% tolerance resistor is not in stock. replacing this with a 0.1% resistor might do the trick temporarily, But that would be overdoing your design and the cost impact is roughly 6 times that using a E24 based value.

[Considering E24 resistor 1 number cost  $ 0.10000, and E192 for 1 number cost 1 : $ 0.59000 ]

Hope you understood the maths behind the resistor series and the importance of sticking to standard values in the lower E24 and E96 range when designing, even though resistors of all values are available in the market.

For everyone who came till the end.

Printable copy of the table above